Kant’s attempt to save metaphysics and causality from Hume

TL;DR

  • Hume sort of wrecked metaphysics. This inspired Kant to try and save it.
  • Hume thought that terms were only meaningful insofar as they were derived from experience.
  • We never actually experience necessary connections between events, we just see correlations. So Hume thought that the idea of causality as necessary connection is empty and confused, and that all our idea of causality really amounts to is correlation.
  • Kant didn’t like this. He wanted to PROTECT causality. But how??
  • Kant said that metaphysical knowledge was both a priori and substantive, and justified this by describing these things called pure intuitions and pure concepts.
  • Intuitions are representations of things (like sense perceptions). Pure intuitions are the necessary preconditions for us to represent things at all.
  • Concepts are classifications of representations (like “red”). Pure concepts are the necessary preconditions underlying all classifications of representations.
  • There are two pure intuitions (space and time) and twelve pure concepts (one of which is causality).
  • We get substantive a priori knowledge by referring to pure intuitions (mathematics) or pure concepts (laws of nature, metaphysics).
  • Yay! We saved metaphysics!

 

(Okay, now on to the actual essay. This was not originally written for this blog, which is why it’s significantly more formal than my usual fare.)

 

***

 

David Hume’s Enquiry Into Human Understanding stands out as a profound and original challenge to the validity of metaphysical knowledge. Part of the historical legacy of this work is its effect on Kant, who describes Hume as being responsible for [interrupting] my dogmatic slumber and [giving] my investigations in the field of speculative philosophy a completely different direction.” Despite the great inspiration that Kant took from Hume’s writing, their thinking on many matters is diametrically opposed. A prime example of this is their views on causality.

Hume’s take on causation is famously unintuitive. He gives a deflationary account of the concept, arguing that the traditional conception lacks a solid epistemic foundation and must be replaced by mere correlation. To understand this conclusion, we need to back up and consider the goal and methodology of the Enquiry.

He starts with an appeal to the importance of careful and accurate reasoning in all areas of human life, and especially in philosophy. In a beautiful bit of prose, he warns against the danger of being overwhelmed by popular superstition and religious prejudice when casting one’s mind towards the especially difficult and abstruse questions of metaphysics.

But this obscurity in the profound and abstract philosophy is objected to, not only as painful and fatiguing, but as the inevitable source of uncertainty and error. Here indeed lies the most just and most plausible objection against a considerable part of metaphysics, that they are not properly a science, but arise either from the fruitless efforts of human vanity, which would penetrate into subjects utterly inaccessible to the understanding, or from the craft of popular superstitions, which, being unable to defend themselves on fair ground, raise these entangling brambles to cover and protect their weakness. Chased from the open country, these robbers fly into the forest, and lie in wait to break in upon every unguarded avenue of the mind, and overwhelm it with religious fears and prejudices. The stoutest antagonist, if he remit his watch a moment, is oppressed. And many, through cowardice and folly, open the gates to the enemies, and willingly receive them with reverence and submission, as their legal sovereigns.

In less poetic terms, Hume’s worry about metaphysics is that its difficulty and abstruseness makes its practitioners vulnerable to flawed reasoning. Even worse, the difficulty serves to make the subject all the more tempting for “each adventurous genius[, who] will still leap at the arduous prize and find himself stimulated, rather than discouraged by the failures of his predecessors, while he hopes that the glory of achieving so hard an adventure is reserved for him alone.”

Thus, says Hume, the only solution is “to free learning at once from these abstruse questions [by inquiring] seriously into the nature of human understanding and [showing], from an exact analysis of its powers and capacity, that it is by no means fitted for such remote and abstruse questions.”

Here we get the first major divergence between Kant and Hume. Kant doesn’t share Hume’s eagerness to banish metaphysics. His Prolegomena To Any Future Metaphysics and Critique of Pure Reason are attempts to find it a safe haven from Hume’s attacks. However, while Kant might not be similarly constituted to Hume in this way, he does take Hume’s methodology very seriously. He states in the preface to the Prolegomena that “since the origin of metaphysics as far as history reaches, nothing has ever happened which could have been more decisive to its fate than the attack made upon it by David Hume.” Many of the principles which Hume derives, Kant agrees with wholeheartedly, making the task of shielding metaphysics even harder for him.

With that understanding of Hume’s methodology in mind, let’s look at how he argues for his view of causality. We’ll start with a distinction that is central to Hume’s philosophy: that between ideas and impressions. The difference between the memory of a sensation and the sensation itself is a good example. While the memory may mimic or copy the sensation, it can never reach its full force and vivacity. In general, Hume suggests that our experiences fall into two distinct categories, separated by a qualitative gap in liveliness. The more lively category he calls impressions, which includes sensory perceptions like the smell of a rose or the taste of wine, as well as internal experiences like the feeling of love or anger. The less lively category he refers to as thoughts or ideas. These include memories of impressions as well as imagined scenes, concepts, and abstract thoughts. 

With this distinction in hand, Hume proposes his first limit on the human mind. He claims that no matter how creative or original you are, all of your thoughts are the product of “compounding, transposing, augmenting, or diminishing the materials afforded us by the senses and experiences.” This is the copy principle: all ideas are copies of impressions, or compositions of simpler ideas that are in turn copies of impressions.

Hume turns this observation of the nature of our mind into a powerful criterion of meaning. “When we entertain any suspicion that a philosophical term is employed without any meaning or idea (as is but too frequent), we need but enquire, From what impression is that supposed idea derived? And if it be impossible to assign any, this will serve to confirm our suspicion.

This criterion turns out to be just the tool Hume needs in order to establish his conclusion. He examines the traditional conception of causation as a necessary connection between events, searches for the impressions that might correspond to this idea, and, failing to find anything satisfactory, declares that “we have no idea of connection or power at all and that these words are absolutely without any meaning when employed in either philosophical reasonings or common life.” His primary argument here is that all of our observations are of mere correlation, and that we can never actually observe a necessary connection.

Interestingly, at this point he refrains from recommending that we throw out the term causation. Instead he proposes a redefinition of the term, suggesting a more subtle interpretation of his criterion of meaning. Rather than eliminating the concept altogether upon discovering it to have no satisfactory basis in experience, he reconceives it in terms of the impressions from which it is actually formed. In particular, he argues that our idea of causation is really based on “the connection which we feel in the mind, this customary transition of the imagination from one object to its usual attendant.”

Here Hume is saying that humans have a rationally unjustifiable habit of thought where, when we repeatedly observe one type of event followed by another, we begin to call the first a cause and the second its effect, and we expect that future instances of the cause will be followed by future instances of the effect. Causation, then, is just this constant conjunction between events, and our mind’s habit of projecting the conjunction into the future. We can summarize all of this in a few lines:

Hume’s denial of the traditional concept of causation

  1. Ideas are always either copies of impressions or composites of simpler ideas that are copies of impressions.
  2. The traditional conception of causation is neither of these.
  3. So we have no idea of the traditional conception of causation.

Hume’s reconceptualization of causation

  1. An idea is the idea of the impression that it is a copy of.
  2. The idea of causation is copied from the impression of constant conjunction.
  3. So the idea of causation is just the idea of constant conjunction.

There we have Hume’s line of reasoning, which provoked Kant to examine the foundations of metaphysics anew. Kant wanted to resist Hume’s dismissal of the traditional conception of causation, while accepting that our sense perceptions reveal no necessary connections to us. Thus his strategy was to deny the Copy Principle and give an account of how we can have substantive knowledge that is not ultimately traceable to impressions. He does this by introducing the analytic/synthetic distinction and the notion of a priori synthetic knowledge.

Kant’s original definition of analytic judgments is that they “express nothing in the predicate but what has already been actually thought in the concept of the subject.” This suggests that the truth value of an analytic judgment is determined by purely the meanings of the concepts in use. A standard example of this is “All bachelors are unmarried.” The truth of this statement follows immediately just by understanding what it means, as the concept of bachelor already contains the predicate unmarried.  Synthetic judgments, on the other hand, are not fixed in truth value by merely the meanings of the concepts in use. These judgments amplify our knowledge and bring us to genuinely new conclusions about our concepts. An example: “The President is ornery.” This certainly doesn’t follow by definition; you’d have to go out and watch the news to realize its truth.

We can now put the challenge to metaphysics slightly differently. Metaphysics purports to be discovering truths that are both necessary (and therefore a priori) as well as substantive (adding to our concepts and thus synthetic). But this category of synthetic a priori judgments seems a bit mysterious. Evidently, the truth values of such judgments can be determined without referring to experience, but can’t be determined by merely the meanings of the relevant concepts. So apparently something further is required besides the meanings of concepts in order to make a synthetic a priori judgment. What is this thing?

Kant’s answer is that the further requirement is pure intuition and pure concepts. These terms need explanation.

Pure Intuitions

For Kant, an intuition is a direct, immediate representation of an object. An obvious example of this is sense perception; looking at a cup gives you a direct and immediate representation of an object, namely, the cup. But pure intuitions must be independent of experience, or else judgments based on them would not be a priori. In other words, the only type of intuition that could possibly be a priori is one that is present in all possible perceptions, so that its existence is not contingent upon what perceptions are being had. Kant claims that this is only possible if pure intuitions represent the necessary preconditions for the possibility of perception.

What are these necessary preconditions? Kant famously claimed that the only two are space and time. This implies that all of our perceptions have spatiotemporal features, and indeed that perception is only possible in virtue of the existence of space and time. It also implies, according to Kant, that space and time don’t exist outside of our minds!  Consider that pure intuitions exist equally in all possible perceptions and thus are independent of the actual properties of external objects. This independence suggests that rather than being objective features of the external world, space and time are structural features of our minds that frame all our experiences.

This is why Kant’s philosophy is a species of idealism. Space and time get turned into features of the mind, and correspondingly appearances in space and time become internal as well. Kant forcefully argues that this view does not make space and time into illusions, saying that without his doctrine “it would be absolutely impossible to determine whether the intuitions of space and time, which we borrow from no experience, but which still lie in our representation a priori, are not mere phantasms of our brain.”

The pure intuitions of space and time play an important role in Kant’s philosophy of mathematics: they serve to justify the synthetic a priori status of geometry and arithmetic. When we judge that the sum of the interior angles of a triangle is 180º, for example, we do so not purely by examining the concepts triangle, sum, and angle. We also need to consult the pure intuition of space! And similarly, our affirmations of arithmetic truths rely upon the pure intuition of time for their validity.

Pure Concepts

Pure intuition is only one part of the story. We don’t just perceive the world, we also think about our perceptions. In Kant’s words, “Thoughts without content are empty; intuitions without concepts are blind. […] The understanding cannot intuit anything, and the senses cannot think anything. Only from their union can cognition arise.” As pure intuitions are to perceptions, pure concepts are to thought. Pure concepts are necessary for our empirical judgments, and without them we could not make sense of perception. It is this category in which causality falls.

Kant’s argument for this is that causality is a necessary condition for the judgment that events occur in a temporal order. He starts by observing that we don’t directly perceive time. For instance, we never have a perception of one event being before another, we just perceive one and, separately, the other. So to conclude that the first preceded the second requires something beyond perception, that is, a concept connecting them.

He argues that this connection must be necessary: “For this objective relation to be cognized as determinate, the relation between the two states must be thought as being such that it determines as necessary which of the states must be placed before and which after.” And as we’ve seen, the only way to get a necessary connection between perceptions is through a pure concept. The required pure concept is the relation of cause and effect: “the cause is what determines the effect in time, and determines it as the consequence.” So starting from the observation that we judge events to occur in a temporal order, Kant concludes that we must have a pure concept of cause and effect.

What about particular causal rules, like that striking a match produces a flame? Such rules are not derived solely from experience, but also from the pure concept of causality, on which their existence depends. It is the presence of the pure concept that allows the inference of these particular rules from experience, even though they postulate a necessary connection.

Now we can see how different Kant and Hume’s conceptions of causality are. While Hume thought that the traditional concept of causality as a necessary connection was unrescuable and extraneous to our perceptions, Kant sees it as a bedrock principle of experience that is necessary for us to be able to make sense of our perceptions at all. Kant rejects Hume’s definition of cause in terms of constant conjunction on the grounds that it “cannot be reconciled with the scientific a priori cognitions that we actually have.”

Despite this great gulf between the two philosophers’ conceptions of causality, there are some similarities. As we saw above, Kant agrees wholeheartedly with Hume that perception alone is insufficient for concluding that there is a necessary connection between events. He also agrees that a purely analytic approach is insufficient. Since Kant sees pure intuitions and pure concepts as features of the mind, not the external world, both philosophers deny that causation is an objective relationship between things in themselves (as opposed to perceptions of things). Of course, Kant would deny that this makes causality an illusion, just as he denied that space and time are made illusory by his philosophy.

Of course, it’s impossible to know to what extent the two philosophers would have actually agreed, had Hume been able to read Kant’s responses to his works. Would he have been convinced that synthetic a priori judgments really exist? If so, would he accept Kant’s pure intuitions and pure concepts? I suspect that at the crux of their disagreement would be Kant’s claim that math is synthetic a priori. While Hume never explicitly proclaims math’s analyticity (he didn’t have the term, after all), it seems more in line with his views on algebra and arithmetic as purely concerning the way that ideas relate to one another. It is also more in line with the axiomatic approach to mathematics familiar to Hume, in which one defines a set of axioms from which all truths about the mathematical concepts involved necessarily follow.

If Hume did maintain math’s analyticity, then Kant’s arguments about the importance of synthetic a priori knowledge would probably hold much less sway for him, and would largely amount to an appeal to the validity of metaphysical knowledge, which Hume already doubted. Hume also would likely want to resist Kant’s idealism; in Section XII of the Enquiry he mocks philosophers that doubt the connection between the objects of our senses and external objects, saying that if you “deprive matter of all its intelligible qualities, both primary and secondary, you in a manner annihilate it and leave only a certain unknown, inexplicable something as the cause of our perceptions – a notion so imperfect that no skeptic will think it worthwhile to contend against it.”

Deriving the Lorentz transformation

My last few posts have been all about visualizing the Lorentz transformation, the coordinate transformation in special relativity. But where does this transformation come from? In this post, I’ll derive it from basic principles. I saw this derivation first probably a year ago, and have since tried unsuccessfully to re-find the source.  It isn’t the algebraically simplest derivation I’ve seen, but it is the conceptually simplest. The principles we’ll use to derive the transformation should all seem extremely obvious to you.

So let’s dive straight in!

The Lorentz transformation in full generality is a 4D matrix that tells you how to transform spacetime coordinates in one inertial reference frame to spacetime coordinates in another inertial reference frame. It turns out that once you’ve found the Lorentz transformation for one spatial dimension, it’s quite simple to generalize it to three spatial dimensions, so for simplicity we’ll just stick to the 1D case. The Lorentz transformation also allows you to transform to a coordinate system that is both translated some distance and rotated some angle. Both of these are pretty straightforward, and work the way we intuitively think rotation and translation should work. So I’ll not consider them either. The interesting part of the Lorentz transformation is what happens when we translate to reference frames that are co-moving (moving with respect to one another). Strictly speaking, this is called a Lorentz boost. That’s what I’ll be deriving for you: the 1D Lorentz boost.

So, we start by imagine some reference frame, in which an event is labeled by its temporal and spatial coordinates: t and x. Then we look at a new reference frame moving at velocity v with respect to the starting reference frame. We describe the temporal and spatial coordinates of the same event in the new coordinate system: t’ and x’. In general, these new coordinates can be any function whatsoever of the starting coordinates and the velocity v.

Screen Shot 2018-12-09 at 10.31.11 PM.png

To narrow down what these functions f and g might be, we need to postulate some general relationship between the primed and unprimed coordinate system.

So, our first postulate!

1. Straight lines stay straight.

Our first postulate is that all observers in inertial reference frames will agree about if an object is moving at a constant velocity. Since objects moving at constant velocities are straight lines on diagrams of position vs time, this is equivalent to saying that a straight path through spacetime in one reference frame is a straight path through spacetime in all reference frames.

More formally, if x is proportional to t, then x’ is proportional to t’ (though the constant of proportionality may differ).

Screen Shot 2018-12-09 at 10.41.03 PM.png

This postulate turns out to be immensely powerful. There is a special name for the types of transformations that keep straight lines straight: they are linear transformations. (Note, by the way, that the linearity is only in the coordinates t and x, since those are the things that retain straightness. There is no guarantee that the dependence on v will be linear, and in fact it will turn out not to be.)

 These transformations are extremely simple, and can be represented by a matrix. Let’s write out the matrix in full generality:

Screen Shot 2018-12-09 at 10.45.02 PM.png

We’ve gone from two functions (f and g) to four (A, B, C, and D). But in exchange, each of these four functions is now only a function of one variable: the velocity v. For ease of future reference, I’ve chosen to name the matrix T(v).

So, our first postulate gives us linearity. On to the second!

2. An object at rest in the starting reference frame is moving with velocity -v in the moving reference frame

This is more or less definitional. If somebody tells you that they had a function that transformed coordinates from one reference frame to a moving reference frame, then the most basic check you can do to see if they’re telling the truth is verify that objects at rest in the starting reference frame end up moving in the final reference frame. And again, it seems to follow from what it means for the reference frame to be moving right at 1 m/s that the initially stationary objects should end up moving left at 1 m/s.

Let’s consider an object sitting at rest at x = 0 in the starting frame of reference. Then we have:

Screen Shot 2018-12-09 at 10.52.06 PM.png

We can plug this into our matrix to get a constraint on the functions A and C:

Screen Shot 2018-12-09 at 10.54.59 PM.png

Great! We’ve gone from four functions to three!

Screen Shot 2018-12-09 at 10.56.02 PM.png

3. Moving to the left at velocity v and to the right at the same velocity is the same as not moving at all

More specifically: Start with any reference frame. Now consider a new reference frame that is moving at velocity v with respect to the starting reference frame. Now, from this new reference frame, consider a third reference frame that is moving at velocity -v. This third reference frame should be identical to the one we started with. Got it?

Formally, this is simply saying the following:

Screen Shot 2018-12-09 at 11.01.36 PM.png

(I is the identity matrix.)

To make this equation useful, we need to say more about T(-v). In particular, it would be best if we could express T(-v) in terms of our three functions A(v), B(v), and D(v). We do this with our next postulate:

4. Moving at velocity -v is the same as turning 180°, then moving at velocity v, then turning 180° again.

Again, this is quite self-explanatory. As a geometric fact, the reference frame you end up with by turning around, moving at velocity v, and then turning back has got to be the same as the reference frame you’d end up with by moving at velocity -v. All we need to formalize this postulate is the matrix corresponding to rotating 180°.

Screen Shot 2018-12-09 at 11.07.28 PM.png

There we go! Rotating by 180° is the same as taking every position in the starting reference frame and flipping its sign. Now we can write our postulate more precisely:

Screen Shot 2018-12-09 at 11.09.47 PM

Screen Shot 2018-12-09 at 11.10.44 PM.png

Now we can finally use Postulate 3!

Screen Shot 2018-12-09 at 11.11.56 PM

Doing a little algebra, we get…

Screen Shot 2018-12-09 at 11.12.42 PM.png

(You might notice that we can only conclude that A = D if we reject the possibility that A = B = 0. We are allowed to do this because allowing A = B = 0 gives us a trivial result in which a moving reference frame experiences no time. Prove this for yourself!)

Now we have managed to express all four of our starting functions in terms of just one!

Screen Shot 2018-12-09 at 11.18.23 PM.png

So far our assumptions have been grounded by almost entirely a priori considerations about what we mean by velocity. It’s pretty amazing how far we got with so little! But to progress, we need to include one final a posteriori postulate, that which motivated Einstein to develop special relativity in the first place: the invariance of the speed of light.

5. Light’s velocity is c in all reference frames.

The motivation for this postulate comes from mountains of empirical evidence, as well as good theoretical arguments from the nature of light as an electromagnetic phenomenon. We can write it quite simply as:

Screen Shot 2018-12-09 at 11.43.23 PM

Plugging in our transformation, we get:

Screen Shot 2018-12-09 at 11.43.28 PM

Multiplying the time coordinate by c must give us the space coordinate:

Screen Shot 2018-12-10 at 3.27.16 AM

And we’re done with the derivation!

Summarizing our five postulates:

Screen Shot 2018-12-10 at 12.37.23 AM.png

And our final result:

Screen Shot 2018-12-10 at 3.29.09 AM.png

Pushing anti-anthropic intuitions

A stranger comes up to you and offers to play the following game with you: “I will roll a pair of dice. If they land snake eyes (i.e. they both land 1), you give me one dollar. Otherwise, if they land anything else, I give you a dollar.”

Do you play this game?

Here’s an intuitive response: Yes, of course you should! You have a 35/36 chance of gaining $1, and only a 1/36 chance of losing $1. You’d have to be quite risk averse to refuse those odds.

What if the stranger tells you that they are giving this same bet to many other people? Should that change your calculation?

Intuitively: No, of course not! It doesn’t matter what else the stranger is doing with other people.

What if they tell you that they’ve given this offer to people in the past, and might give the offer to others in the future? Should that change anything?

Once again, it seems intuitively not to matter. The offers given to others simply have nothing to do with you. What matters are your possible outcomes and the probabilities of each of these outcomes. And what other people are doing has nothing to do with either of these.

… Right?

Now imagine that the stranger is playing the game in the following way: First they find one person and offer to play the game with them. If the dice land snake eyes, then they collect a dollar and stop playing the game. Otherwise, they find ten new people and offer to play the game with them. Same as before: snake eyes, the stranger collects $1 from each and stops playing, otherwise he moves on to 100 new people. Et cetera forever.

We now ask the question: How does the average person given the offer do if they take the offer? Well, no matter how many rounds of offers the stranger gives, at least 90% of people end up in his last round. That means that at least 90% of people end up giving over $1 and at most 10% gain $1. This is clearly net negative for those that hand over money!

Think about it this way: Imagine a population of individuals who all take the offer, and compare them to a population that all reject the offer. Which population does better on average?

For the population who takes the offer, the average person loses money. An upper bound on how much they lose is 10% ($1) + 90% (-$1) = -$.80. For the population that reject the offer, nobody gains money or loses It either: the average case is exactly $0. $0 is better than -$.80, so the strategy of rejecting the offer is better, on average!

This thought experiment is very closely related to the dice killer thought experiment. I think of it as a variant that pushes our anti-anthropic-reasoning intuitions. It just seems really wrong to me that if somebody comes up to you and offers you this deal that has a 35/36 chance of paying out you should reject it. The details of who else is being offered the deal seem totally irrelevant.

But of course, all of the previous arguments I’ve made for anthropic reasoning apply here as well. And it is just true that the average person that rejects the offer does better than the average person that accepts it. Perhaps this is just another bullet that we have to bite in our attempt to formalize rationality!

Clarifying self-defeating beliefs

In a previous post, I mentioned self-defeating beliefs as a category that I am confused about. I wrote:

How should we reason about self defeating beliefs?

The classic self-defeating belief is “This statement is a lie.” If you believe it, then you are compelled to disbelieve it, eliminating the need to believe it in the first place. Broadly speaking, self-defeating beliefs are those that undermine the justifications for belief in them.

Here’s an example that might actually apply in the real world: Black holes glow. The process of emission is known as Hawking radiation. In principle, any configuration of particles with a mass less than the black hole can be emitted from it. Larger configurations are less likely to be emitted, but even configurations such as a human brain have a non-zero probability of being emitted. Henceforth, we will call such configurations black hole brains.

Now, imagine discovering some cosmological evidence that the era in which life can naturally arise on planets circling stars is finite, and that after this era there will be an infinite stretch of time during which all that exists are black holes and their radiation. In such a universe, the expected number of black hole brains produced is infinite (a tiny finite probability multiplied by an infinite stretch of time), while the expected number of “ordinary” brains produced is finite (assuming a finite spatial extent as well).

What this means is that discovering this cosmological evidence should give you an extremely strong boost in credence that you are a black hole brain. (Simply because most brains in your exact situation are black hole brains.) But most black hole brains have completely unreliable beliefs about their environment! They are produced by a stochastic process which cares nothing for producing brains with reliable beliefs. So if you believe that you are a black hole brain, then you should suddenly doubt all of your experiences and beliefs. In particular, you have no reason to think that the cosmological evidence you received was veridical at all!

I don’t know how to deal with this. It seems perfectly possible to find evidence for a scenario that suggests that we are black hole brains (I’d say that we havealready found such evidence, multiple times). But then it seems we have no way to rationally respond to this evidence! In fact, if we do a naive application of Bayes’ theorem here, we find that the probability of receiving any evidence in support of black hole brains to be 0!

So we have a few options. First, we could rule out any possible skeptical scenarios like black hole brains, as well as anything that could provide anyamount of evidence for them (no matter how tiny). Or we could accept the possibility of such scenarios but face paralysis upon actually encountering evidence for them! Both of these seem clearly wrong, but I don’t know what else to do.

A friend (whose blog Compassionate Equilibria you should definitely check out) left a comment in response, saying:

I think I feel somewhat less confused about self-defeating beliefs (at least when considering the black hole brain scenario maybe I would feel more confused about other cases).

It seems like the problem might be when you say “imagine discovering some cosmological evidence that the era in which life can naturally arise on planets circling stars is finite, and that after this era there will be an infinite stretch of time during which all that exists are black holes and their radiation.” Presumably, whatever experience you had that you are interpreting as this cosmological evidence is an experience that you would actually be very unlikely to have given that you exist in that universe and as a result shouldn’t be interpreted as evidence for existing in such a universe. Instead you would have to think about in what kind of universe would you be most likely to have those experiences that naively seemed to indicate living in a universe with an infinity of black hole brains.

This could be a very difficult question to answer but not totally intractable. This also doesn’t seem to rule out starting with a high prior in being a black hole brain and it seems like you might even be able to get evidence for being a black hole brain (although I’m not sure what this would be; maybe having a some crazy jumble of incoherent experiences while suddenly dying?).

I think this is a really good point that clears up a lot of my confusion on the topic. My response ended up being quite long, so I’ve decided to make it its own post.

 

*** My response starts here ***

 

The key point that I was stuck on before reading this comment was the notion that this argument puts a strong a priori constraint on the types of experiences we can expect to have. This is because P(E) is near zero when E strongly implies a theory and that theory undermines E.

Your point, which seems right, is: It’s not that it’s impossible or near impossible to observe certain things that appear to strongly suggest a cosmology with an infinity of black hole brains. It’s that we can observe these things, and they aren’t actually evidence for these cosmologies (for just the reasons you laid out).

That is, there just aren’t observations that provide evidence for radical skeptical scenarios. Observations that appear to provide such evidence, prove to not do so upon closer examination. It’s about the fact that the belief that you are a black hole brain is by construction unmotivateable: this is what it means to say P(E) ~ 0. (More precisely, the types of observations that actually provide evidence for black hole brains are those that are not undermined by the belief in black hole brains. Your “crazy jumble of incoherent experiences” might be a good example of this. And importantly, basically any scientific evidence of the sort that we think could adjudicate between different cosmological theories will be undermined.)

One more thing as I digest this: Previously I had been really disturbed by the idea that I’d heard mentioned by Sean Carroll and others that one criterion for a feasible cosmology is that it doesn’t end up making it highly likely that we are black hole brains. This seemed like a bizarrely strong a priori constraint on the types of theories we allow ourselves to consider. But this actually makes a lot of sense if conceived of not as an a priori constraint but as a combination of two things: (1) updating on the strong experiential evidence that we are not black hole brains (the extremely structured and self-consistent nature of our experiences) and (2) noticing that these theories are very difficult to motivate, as most pieces of evidence that intuitively seem to support them actually don’t upon closer examination.

So (1) the condition that P(E) is near zero is not necessarily a constraint on your possible experiences, and (2) it makes sense to treat cosmologies that imply that we are black hole brains as empirically unsound and nearly unmotivateable.

Now, I’m almost all the way there, but still have a few remaining hesitations.

One thing is that things get more confusing when you break an argument for black hole brains down into its component parts and try to figure out where exactly you went wrong. Like, say you already have a whole lot of evidence that after a finite length of time, the universe will be black holes forever, but don’t yet know about Hawking radiation. So far everything is fine. But now scientists observe Hawking radiation. From this they conclude that black holes radiate, though they don’t have a theory of the stochastic nature of the process that entails that it can in principle produce brains. They then notice that Hawking radiation is actually predicted by combining aspects of QM and GR, and see that this entails that black holes can produce brains. Now they have all the pieces that together imply that they are black hole brains, but at which step did they go wrong? And what should they conclude now? They appear to have developed a mountain of solid evidence that when put together (and combined with some anthropic reasoning) straightforwardly imply that they are black hole brains. But this can’t be the case, since this would undermine the evidence they started with.

We can frame this as a multilemma. The general reasoning process that leads to the conclusion that we are black hole brains might look like:

  1. We observe nature.
  2. We generate laws of physics from these observations.
  3. We predict from the laws of physics that there is a greater abundance of black hole brains than normal brains.
  4. We infer from (3) that we are black hole brains (via anthropic reasoning).

Either this process fails at some point, or we should believe that we are black hole brains. Our multilemma (five propositions, at least one of which must be accepted) is thus:

  1. Our observations of nature were invalid.
  2. Our observations were valid, but our inference of laws of physics from them was invalid.
  3. Our inference of laws of physics from our observations were valid, but our inference from these laws of there being a greater abundance of black hole brains than normal brains was invalid.
  4.  Our inference from the laws of there being a greater abundance of black hole brains from normal brains was valid, but the anthropic step was invalid.
  5. We are black hole brains.

Clearly we want to deny (5). I also would want to deny (3) and (4) – I’m imagining them to be fairly straightforward deductive steps. (1) is just some form of skepticism about our access to nature, which I also want to deny. The best choice, it looks like, is (2): our inductive inference of laws of physics from observations of nature is flawed in some way. But even this is a hard bullet to bite. It’s not sufficient to just say that other laws of physics might equally well or better explain the data. What is required is to say that in fact our observations don’t really provide compelling evidence for QM, GR, and so on.

So the end result is that I pretty much want to deny every possible way the process could have failed, while also denying the conclusion. But we have to deny something! This is clearly not okay!

Summing up: The remaining disturbing thing to me is that it seems totally possible to accidentally run into a situation where your best theories of physics inevitably imply (by a process of reasoning each step of which you accept is valid) that you are a black hole brain, and I’m not sure what to do next at that point.

The Anthropic Dice Killer

Today we discuss anthropic reasoning.

The Problem

Imagine the following scenario:

A mad killer has locked you in a room. You are trapped and alone, with only your knowledge of your situation to help you out.

One piece of information that you have is that you are aware of the maniacal schemes of your captor. His plans began by capturing one random person. He then rolled a pair of dice to determine their fate. If the dice landed snake eyes (both 1), then the captive would be killed. If not, then they would be let free.

But if they are let free, the killer will search for new victims, and this time bring back ten new people and lock them alone in rooms. He will then determine their fate just as before, with a pair of dice. Snake eyes means they die, otherwise they will be let free and he will search for new victims.

His murder spree will continue until the first time he rolls snake eyes. Then he will kill the group that he currently has imprisoned and retire from the serial-killer life.

Now. You become aware of a risky way out of the room you are locked in and to freedom. The chances of surviving this escape route are only 50%. Your choices are thus either (1) to traverse the escape route with a 50% chance of survival or (2) to just wait for the killer to roll his dice, and hope that it doesn’t land snake eyes.

What should you do?

 

 

 

(Think about it before reading on)

 

 

 

A plausible-sounding answer

Your chance of dying if you stay and wait is just the chance that the dice lands snake eyes. The probability of snake eyes is just 1/36 (1/6 for each dice landing 1).

So your chance of death is only 1/36 (≈ 3%) if you wait, and it’s 50% if you try to run for it. Clearly, you are better off waiting!

But…

You guessed it, things aren’t that easy. You have extra information about your situation besides just how the dice works, and you should use it. In particular, the killing pattern of your captor turns out to be very useful information.

Ask the following question: Out of all of the people that have been captured or will be captured at some point by this madman, how many of them will end up dying? This is just the very last group, which, incidentally, is the largest group.

Consider: if the dice land snake eyes the first time they are rolled, then only one person is ever captured, and this person dies. So the fraction of those captured that die is 100%.

If they lands snake eyes the second time they are rolled, then 11 people total are captured, 10 of whom die. So the fraction of those captured that die is 10/11, or ≈ 91%.

If it’s the third time, then 111 people total are captured, 100 of whom die. Now the fraction is just over 90%.

In general, no matter how many times the dice rolls before landing snake eyes, it always ends up that over 90% of those captured end up being in the last round, and thus end up dying.

So! This looks like bad news for you… you’ve been captured, and over 90% of those that are captured always die. Thus, your chance of death is guaranteed to be greater than 90%.

The escape route with a 50% survival chance is looking nicer now, right?

Wtf is this kind of reasoning??

What we just did is called anthropic reasoning. Anthropic reasoning really just means updating on all of the information available to you, including indexical information (information about your existence, age, location, and so on). In this case, the initial argument neglected the very crucial information that you are one of the people that were captured by the killer. When updating on this information, we get an answer that is very very different from what we started with. And in this life-or-death scenario, this is an important difference!

You might still feel hesitant about the answer we got. After all, if you expect a 90% chance of death, this means that you expect a 90% chance for the dice to land snake eyes. But it’s not that you think the dice are biased or anything… Isn’t this just blatantly contradictory?

This is a convincing-sounding rebuttal, but it’s subtly wrong. The key point is that even though the dice are fair, there is a selection bias in the results you are seeing. This selection bias amounts to the fact that when the dice inevitably lands snake-eyes, there are more people around to see it. The fact that you are more likely than 1/36 to see snake-eyes is kind of like the fact that if you are given the ticket of a random concert-goer, you have a higher chance of ending seeing a really popular band than if you just looked at the current proportion of shows performed by really popular bands.

It’s kind of like the fact that in your life you will spend more time waiting in long lines than short lines, and that on average your friends have more friends than you. This all seems counterintuitive and wrong until you think closely about the selection biases involved.

Anyway, I want to impress upon you that 90% really is the right answer, so I’ll throw some math at you. Let’s calculate in full detail what fraction of the group ends up surviving on average.

Screen Shot 2018-08-02 at 1.16.15 AM

By the way, the discrepancy between the baseline chance of death (1/36) and the anthropic chance of death (90%) can be made as large as you like by manipulating the starting problem. Suppose that instead of 1/36, the chance of the group dying was 1/100, and instead of the group multiplying by 10 in size each round, it grew by a factor of 100. Then the baseline chance of death would be 1%, and the anthropic probability would be 99%.

We can find the general formula for any such scenario:

Screen Shot 2018-08-02 at 4.54.30 AM.png

IF ANYBODY CAN SOLVE THIS, PLEASE TELL ME! I’ve been trying for too long now and would really like an analytic general solution. 🙂

There is a lot more to be said about this thought experiment, but I’ll leave it there for now. In the next post, I’ll present a slight variant on this thought experiment that appears to give us a way to get direct Bayesian evidence for different theories of consciousness! Stay tuned.

What do I find conceptually puzzling?

There are lots of things that I don’t know, like, say, what the birth rate in Sweden is or what the effect of poverty on IQ is. There are also lots of things that I find really confusing and hard to understand, like quantum field theory and monetary policy. There’s also a special category of things that I find conceptually puzzling. These things aren’t difficult to grasp because the facts about them are difficult to understand or require learning complicated jargon. Instead, they’re difficult to grasp because I suspect that I’m confused about the concepts in use.

This is a much deeper level of confusion. It can’t be adjudicated by just reading lots of facts about the subject matter. It requires philosophical reflection on the nature of these concepts, which can sometimes leave me totally confused about everything and grasping for the solid ground of mere factual ignorance.

As such, it feels like a big deal when something I’ve been conceptually puzzled about becomes clear. I want to compile a list for future reference of things that I’m currently conceptually puzzled about and things that I’ve become un-puzzled about. (This is not a complete list, but I believe it touches on the major themes.)

Things I’m conceptually puzzled about

What is the relationship between consciousness and physics?

I’ve written about this here.

Essentially, at this point every available viewpoint on consciousness seems wrong to me.

Eliminativism amounts to a denial of pretty much the only thing that we can be sure can’t be denied – that we are having conscious experiences. Physicalism entails the claim that facts about conscious experience can be derived from laws of physics, which is wrong as a matter of logic.

Dualism entails that the laws of physics by themselves cannot account for the behavior of the matter in our brains, which is wrong. And epiphenomenalism entails that our beliefs about our own conscious experience are almost certainly wrong, and are no better representations of our actual conscious experiences than random chance.

How do we make sense of decision theory if we deny libertarian free will?

Written about this here and here.

Decision theory is ultimately about finding the decision D that maximizes expected utility EU(D). But to do this calculation, we have to decide what the set of possible decisions we are searching is.

EU confusion

Make this set too large, and you end up getting fantastical and impossible results (like that the optimal decision is to snap your fingers and make the world into a utopia). Make it too small, and you end up getting underwhelming results (in the extreme case, you just get that the optimal decision is to do exactly what you are going to do, since this is the only thing you can do in a strictly deterministic world).

We want to find a nice middle ground between these two – a boundary where we can say “inside here the things that are actually possible for us to do, and outside are those that are not.” But any principled distinction between what’s in the set and what’s not must be based on some conception of some actions being “truly possible” to us, and others being truly impossible. I don’t know how to make this distinction in the absence of a robust conception of libertarian free will.

Are there objectively right choices of priors?

I’ve written about this here.

If you say no, then there are no objectively right answers to questions like “What should I believe given the evidence I have?” And if you say yes, then you have to deal with thought experiments like the cube problem, where any choice of priors looks arbitrary and unjustifiable.

(If you are going to be handed a cube, and all you know is that it has a volume less than 1 cm3, then setting maximum entropy priors over volumes gives different answers than setting maximum entropy priors over side areas or side lengths. This means that what qualifies as “maximally uncertain” depends on whether we frame our reasoning in terms of side length, areas, or cube volume. Other approaches besides MaxEnt have similar problems of concept dependence.)

How should we deal with infinities in decision theory?

I wrote about this here, here, here, and here.

The basic problem is that expected utility theory does great at delivering reasonable answers when the rewards are finite, but becomes wacky when the rewards become infinite. There are a huge amount of examples of this. For instance, in the St. Petersburg paradox, you are given the option to play a game with an infinite expected payout, suggesting that you should buy in to the game no matter how high the cost. You end up making obviously irrational choices, such as spending $1,000,000 on the hope that a fair coin will land heads 20 times in a row. Variants of this involve the inability of EU theory to distinguish between obviously better and worse bets that have infinite expected value.

And Pascal’s mugging is an even worse case. Roughly speaking, a person comes up to you and threatens you with infinite torture if you don’t submit to them and give them 20 dollars. Now, the probability that this threat is credible is surely tiny. But it is non-zero! (as long as you don’t think it is literally logically impossible for this threat to come true)

An infinite penalty times a finite probability is still an infinite expected penalty. So we stand to gain an infinite expected utility by just handing over the 20 dollars. This seems ridiculous, but I don’t know any reasonable formalization of decision theory that allows me to refute it.

Is causality fundamental?

Causality has been nicely formalized by Pearl’s probabilistic graphical models. This is a simple extension of probability theory, out of which naturally falls causality and counterfactuals.

One can use this framework to represent the states of fundamental particles and how they change over time and interact with one another. What I’m confused about is that in some ways of looking at it, the causal relations appear to be useful but un-fundamental constructs for the sake of easing calculations. In other ways of looking at it, causal relations are necessarily built into the structure of the world, and we can go out and empirically discover them. I don’t know which is right. (Sorry for the vagueness in this one – it’s confusing enough to me that I have trouble even precisely phrasing the dilemma).

How should we deal with the apparent dependence of inductive reasoning upon our choices of concepts?

I’ve written about this here. Beyond just the problem of concept-dependence in our choices of priors, there’s also the problem presented by the grue/bleen thought experiment.

This thought experiment proposes two new concepts: grue (= the set of things that are either green before 2100 or blue after 2100) and bleen (the inverse of grue). It then shows that if we reasoned in terms of grue and bleen, standard induction would have us concluding that all emeralds will suddenly turn blue after 2100. (We repeatedly observed them being grue before 2100, so we should conclude that they will be grue after 2100.)

In other words, choose the wrong concepts and induction breaks down. This is really disturbing – choices of concepts should be merely pragmatic matters! They shouldn’t function as fatal epistemic handicaps. And given that they appear to, we need to develop some criterion we can use to determine what concepts are good and what concepts are bad.

The trouble with this is that the only proposals I’ve seen for such a criterion reference the idea of concepts that “carve reality at its joints”; in other words, the world is composed of green and blue things, not grue and bleen things, so we should use the former rather than the latter. But this relies on the outcome of our inductive process to draw conclusions about the starting step on which this outcome depends!

I don’t know how to cash out “good choices of concepts” without ultimately reasoning circularly. I also don’t even know how to make sense of the idea of concepts being better or worse for more than merely pragmatic reasons.

How should we reason about self defeating beliefs?

The classic self-defeating belief is “This statement is a lie.” If you believe it, then you are compelled to disbelieve it, eliminating the need to believe it in the first place. Broadly speaking, self-defeating beliefs are those that undermine the justifications for belief in them.

Here’s an example that might actually apply in the real world: Black holes glow. The process of emission is known as Hawking radiation. In principle, any configuration of particles with a mass less than the black hole can be emitted from it. Larger configurations are less likely to be emitted, but even configurations such as a human brain have a non-zero probability of being emitted. Henceforth, we will call such configurations black hole brains.

Now, imagine discovering some cosmological evidence that the era in which life can naturally arise on planets circling stars is finite, and that after this era there will be an infinite stretch of time during which all that exists are black holes and their radiation. In such a universe, the expected number of black hole brains produced is infinite (a tiny finite probability multiplied by an infinite stretch of time), while the expected number of “ordinary” brains produced is finite (assuming a finite spatial extent as well).

What this means is that discovering this cosmological evidence should give you an extremely strong boost in credence that you are a black hole brain. (Simply because most brains in your exact situation are black hole brains.) But most black hole brains have completely unreliable beliefs about their environment! They are produced by a stochastic process which cares nothing for producing brains with reliable beliefs. So if you believe that you are a black hole brain, then you should suddenly doubt all of your experiences and beliefs. In particular, you have no reason to think that the cosmological evidence you received was veridical at all!

I don’t know how to deal with this. It seems perfectly possible to find evidence for a scenario that suggests that we are black hole brains (I’d say that we have already found such evidence, multiple times). But then it seems we have no way to rationally respond to this evidence! In fact, if we do a naive application of Bayes’ theorem here, we find that the probability of receiving any evidence in support of black hole brains to be 0!

So we have a few options. First, we could rule out any possible skeptical scenarios like black hole brains, as well as anything that could provide any amount of evidence for them (no matter how tiny). Or we could accept the possibility of such scenarios but face paralysis upon actually encountering evidence for them! Both of these seem clearly wrong, but I don’t know what else to do.

How should we reason about our own existence and indexical statements in general?

This is called anthropic reasoning. I haven’t written about it on this blog, but expect future posts on it.

A thought experiment: imagine a murderous psychopath who has decided to go on an unusual rampage. He will start by abducting one random person. He rolls a pair of dice, and kills the person if they land snake eyes (1, 1). If not, he lets them free and hunts down ten new people. Once again, he rolls his pair of die. If he gets snake eyes he kills all ten. Otherwise he frees them and kidnaps 100 new people. On and on until he eventually gets snake eyes, at which point his murder spree ends.

Now, you wake up and find that you have been abducted. You don’t know how many others have been abducted alongside you. The murderer is about to roll the dice. What is your chance of survival?

Your first thought might be that your chance of death is just the chance of both dice landing 1: 1/36. But think instead about the proportion of all people that are ever abducted by him that end up dying. This value ends up being roughly 90%! So once you condition upon the information that you have been captured, you end up being much more worried about your survival chance.

But at the same time, it seems really wrong to be watching the two dice tumble and internally thinking that there is a 90% chance that they land snake eyes. It’s as if you’re imagining that there’s some weird anthropic “force” pushing the dice towards snake eyes. There’s way more to say about this, but I’ll leave it for future posts.

Things I’ve become un-puzzled about

Newcomb’s problem – one box or two box?

To almost everyone, it is perfectly clear and obvious what should be done. The difficulty is that these people seem to divide almost evenly on the problem, with large numbers thinking that the opposing half is just being silly.

– Nozick, 1969

I’ve spent months and months being hopelessly puzzled about Newcomb’s problem. I now am convinced that there’s an unambiguous right answer, which is to take the one box. I wrote up a dialogue here explaining the justification for this choice.

In a few words, you should one-box because one-boxing makes it nearly certain that the simulation of you run by the predictor also one-boxed, thus making it nearly certain that you will get 1 million dollars. The dependence between your action and the simulation is not an ordinary causal dependence, nor even a spurious correlation – it is a logical dependence arising from the shared input-output structure. It is the same type of dependence that exists in the clone prisoner dilemma, where you can defect or cooperate with an individual you are assured is identical to you in every single way. When you take into account this logical dependence (also called subjunctive dependence), the answer is unambiguous: one-boxing is the way to go.

Summing up:

Things I remain conceptually confused about:

  • Consciousness
  • Decision theory & free will
  • Objective priors
  • Infinities in decision theory
  • Fundamentality of causality
  • Dependence of induction on concept choice
  • Self-defeating beliefs
  • Anthropic reasoning

More on quantum entanglement and irreducibility

A few posts ago, I talked about how quantum mechanics entails the existence of irreducible states – states of particles that in principle cannot be described as the product of their individual components. The classic example of such an entangled state is the two qubit state

Screen Shot 2018-07-17 at 8.03.53 PM

This state describes a system which is in an equal-probability superposition of both particles being |0 and both particles being |1. As it turns out, this state cannot be expressed as the product of two single-qubit states.

A friend of mine asked me a question about this that was good enough to deserve its own post in response. Start by imagining that Alice and Bob each have a coin. They each put their quarter inside a small box with heads facing up. Now they close their respective boxes, and shake them up in the exact same way. This is important! (as well as unrealistic) We suppose that whatever happens to the coin in Alice’s box, also happens to the coin in Bob’s box.

Now we have two boxes, each of which contains a coin, and these coins are guaranteed to be facing the same way. We just don’t know what way they are facing.

Alice and Bob pick up their boxes, being very careful to not disturb the states of their respective coins, and travel to opposite ends of the galaxy. The Milky Way is 100,000 light years across, so any communication between the two now would take a minimum of 100,000 years. But if Alice now opens her box, she instantly knows the state of Bob’s coin!

So while Alice and Bob cannot send messages about the state of their boxes any faster than 100,000 years, they can instantly receive information about each others’ boxes by just observing their own! Is this a contradiction?

No, of course not. While Alice does learn something about Bob’s box, this is not because of any message passed between the two. It is the result of the fact that in the past the configurations of their coins were carefully designed to be identical. So what seemed on its face to be special and interesting turns out to be no paradox at all.

Finally, we get to the question my friend asked. How is this any different from the case of entangled particles in quantum mechanics??

Both systems would be found to be in the states |00 and |11⟩ with equal probability (where |0⟩ is heads and |1⟩ is tails). And both have the property that learning the state of one instantly tells you the state of the other. Indeed, the coins-in-boxes system also has the property of irreducibility that we talked about before! Try as we might, we cannot coherently treat the system of both coins as the product of two independent coins, as doing so will ignore the statistical dependence between the two coins.

(Which, by the way, is exactly the sort of statistical dependence that justifies timeless decision theory and makes it a necessary update to decision theory.)

I love this question. The premise of the question is that we can construct a classical system that behaves in just the same supposedly weird ways that quantum systems behave, and thus make sense of all this mystery. And answering it requires that we get to the root of why quantum mechanics is a fundamentally different description of reality than anything classical.

So! I’ll describe the two primary disanalogies between entangled particles and “entangled” coins.

Epistemic Uncertainty vs Fundamental Indeterminacy

First disanalogy. With the coins, either they are both heads or they are both tails. There is an actual fact in the world about which of these two is true, and the probabilities we reference when we talk about the chance of HH or TT represent epistemic uncertainty. There is a true determinate state of the coins, and probability only arises as a way to deal with our imperfect knowledge.

On the other hand, according to the mainstream interpretation of quantum mechanics, the state of the two particles is fundamentally indeterminate. There isn’t a true fact out there waiting to be discovered about whether the state is |00⟩ or |11⟩. The actual state of the system is this unusual thing called a superposition of |00⟩ and |11⟩. When we observe it to be |00⟩, the state has now actually changed from the superposition to the determinate state.

We can phrase this in terms of counterfactuals: If when we look at the coins, we see that they are HH, then we know that they were HH all along. In particular, we know that if we had observed them a moment later or earlier, we would have gotten H with 100% certainty. Give that we actually observed HH, the probability that we would have observed HH is 100%.

But if we observe the state of the particles to be |00⟩, this does not mean that had we observed it a moment before, we would be guaranteed to get the same answer. Given that we actually observed |00⟩, the probability that we would have observed |00⟩ is still 50%.

(A project for some enterprising reader: see what the truths of these counterfactuals imply for an interpretation of quantum mechanics in terms of Pearl-style causal diagrams. Is it even possible to do?)

Predictive differences

The second difference between the two cases is a straightforward experimental difference. Suppose that Alice and Bob identically prepare thousands of coins as we described before, and also identically prepare thousands of entangled particles. They ensure that the coins are treated exactly the same way, so that they are guaranteed to all be in the same state, and similarly for the entangled pairs.

If they now just observe all of their entangled pairs and coins, they will get similar results – roughly half of the coins will be HH and roughly half of the entangled pairs will be |00⟩. But there are other experiments they could run on the entangled pairs that would give difference answers than 

The conclusion of this is that even if you tried to model the entangled pair as a simple probability distribution similar to the coins, you will get the wrong answer in some experiments. I described what these experiments could be in this earlier post – essentially they involve applying an operation that takes qubits in and out of superposition.

So we have both a theoretical argument and a practical argument for the difference between these two cases. They key take-away is the following:

According to quantum mechanics an entangled pair is in a state that is fundamentally indeterminate. When we describe it with probabilities, we are not saying “This probabilistic description is an account of my imperfect knowledge of the state of the system”. We’re saying that nature herself is undecided on what we will observe when we look at the state. (Side note: there is actually a way to describe epistemic uncertainty in quantum mechanics. It is called the density matrix, and is completely different from the description of superpositions.)

In addition, the most fundamental and accurate probability description for the state of the two particles is one that cannot be described as the product of two independent particles. This is not the case with the coins! The most fundamental and accurate probability description for the state of the two coins is either 100% HH or 100% TT (whichever turns out to be the case). What this means is that in the quantum case, not only is the state indeterminate, but the two particles are fundamentally interdependent – entangled. There is no independent description of the individual components of the system, there is only the system as a whole.