Tuesday 4/7

Consciousness IV: The Explanatory Gap




Class Discussion

We will use zoom for class discussion, meeting at the same time as class, but online. Please download the client here for your computer, laptop, or smartphone. You do not need an account, nor do you need to pay for the service. I will text a meeting ID and meeting password to the class via our class GroupMe and via email about 15 minutes prior to class. We will also use the same zoom room for the dedicated office hour immediately following class.

Video Lecture

I don't think we'll need a video lecture for this discussion, but if it turns out I'm wrong, I'll tape and post it after our discussion today.

Ha! I lied. See below.


To be sure everyone was up to speed, we began today by again going through the steps of Kripke's Modal Argument (the Modal Gap), but see the synopsis from last time for details.

Upon rehearsing the Modal Argument against physicalism, we discovered that a crucial step in the argument is the assertion that conceivability suffices for possibility. That is, what we can conceive (an epistemic claim) establishes what is in fact possible (a modal claim). Levine presses on just this point in the reading for today.

Let us grant Kripke Rigid Designation and, with it, the Necessity of (True) Identities. Even with all that machinery, Kripke's argument hinges on the principle that

If X is conceivable, X is possible.

where for Kripke the key step is to claim that

If it is conceivable that pain ≠ c-fibre nerve stimulation, then it is possible that pain ≠ c-fibre nerve stimulation.

Does conceivability suffice for possibility? There are many reasons to think not.

For starters, we seem to be claiming that what we can imagine to be the case is what is in fact possibly the case. Thus a purely epistemic property (imaginability) is supposed to give us metaphysical results (possibility), yet we would be foolish in general--if not arrogant--to think the world so reflects our beliefs.

We might consider, as we did not in our class discussion, the important historical counterexample of the incommensureability of the diagonal of a square to its sides. Note that if we have a square with unit sides, then by the Pythagorean Theorem the diagonal is the square root of 2. But the square root of 2 is an irrational number: It has infinitely many non-repeating decimal places. Thus the diagonal of a square is not some simple ratio of the sides. We conceive of it as being so measurable, and think it possible to do so. The mathematical facts turn out otherwise, showing us that mere conceivability is not a sure guide to possibility.

Consider two further counter-examples to the principle that conceivability suffices for possibility.

  1. Many people claim to conceive of a being that is omnipotent, omniscient, omnibenevolent, and omnipresent, yet there are many reasons to think that these properties are individually and in concert impossible: The Problem of Evil is but one of the many arguments that can be given to conclude that such a being is not, contrary to conceivability, possible.

  2. A more decisive and less controversial counter-example I gave in class is Russell's Paradox. Briefly, in building a rigorous foundation for mathematics, Frege thought the Axiom of (Unrestricted) Abstraction obvious: Given any property, a set can be formed containing all and only the objects having that property. That is, it seems quite conceivable that given the property of redness, we can form the set of all red things. Maybe we can in the case of redness, but we cannot in general form such sets as the Axiom maintains. Consider that the set of horses is not a member of itself: The set of horses is not itself a horse. The set of things that are not horses is, however, a member of itself, since the set of things that are not horses is clearly not itself a horse. So is the set of all sets that are not members of themselves a member of itself or not? If it is, then it isn't; if it isn't, then it is. Thus we have a contradiction showing that the Axiom of Abstraction is in fact impossible, even though it was thought by Frege and others so obvious as to be an axiom.

Perhaps, then, Kripke is mistaken to think he can conceive that pain ≠ c-fibre nerve stimulation. Levine takes Kripke's strong intuition to the contrary to indicate a deeper problem posed by phenomenal consciousness, the Explanatory Gap.

To understand Levine's point, we had to briefly draw on the Philosophy of Science. It is a commonplace that Biology is reducible to Chemistry and Chemistry is reducible to Physics. That is, Biology is fundamentally Chemistry, Chemistry fundamentally Physics. Put another way, we can 'explain' biological facts and laws in terms of (more fundamental) chemical facts and laws, and we can explain chemical facts and laws in terms of (more fundamental) physical facts and laws.

Of course, the vocabulary of Biology is very different from the vocabulary of Chemistry, and the vocabulary of Chemistry is very different from the vocabulary of Physics. So we need Bridge-Laws which connect the vocabulary of one science to the vocabulary of another and explain how a law in one science can ultimately be translated into the laws of a more fundamental science.

All of this is not without philosophical controversy, but it is the standard view of science.

Levine invites us to consider Psychology as a science. Many, if not most, cognitive scientists hold that psychology is ultimately reducible to biology, much as any good physicalist would hold. There are, however, two problems:

  1. We cannot--vis-a-vis Kripke--imagine what bridge laws between psychology and biology would even look like; and,
  2. There don't seem to be any psychological laws which might make psychology a science in the first place.

Thus there seems to be an explanatory gap such that psychological facts about phenomenal consciousness cannot be explained by reducing them to biological facts. For imagine they could be so reduced. Then it should be the case that we could tell from an organism's biological facts what it is like to be that organism, yet this seems precisely to be what we cannot learn, as we discovered early on with Nagel's poor bat.