The ability hypothesis is a defense of physicalism, and it is most commonly discussed as a response to the knowledge argument. Physicalism is the view that the world is entirely physical--or, to put it another way, that all of the physical facts about the world are all of the facts about the world. Against physicalism, the knowledge argument argues that one can know all of the physical facts without knowing all of the facts. This is most popularly discussed in terms of Mary, a super-scientist who comes to learn all of the physical facts about color vision without ever seeing a color. She lives her life confined to a black-and-white room. Yet, intuition tells us that, even though she knows all of the physical facts, she does not know what it is like to see colors. When she leaves her black-and-white room and sees colors for the first time, she learns what it is like to see colors. Thus, she learns new facts--facts which obviously cannot be physical facts, because Mary had already learned all of the physical facts from inside her black-and-white room. Thus, physicalism is false. All of the physical facts are not all of the facts.
According to the ability hypothesis, Mary does not gain new factual knowledge, but only a new understanding of color vision which is constituted by certain abilities--namely, the abilities to remember, recognize, and imagine colors. These new abilities constitute phenomenal knowledge, which is what we mean when we talk of "what it is like to x." The point is that this sort of knowledge is not factual; rather, it is non-propositional know-how.
The knowledge argument and the ability hypothesis are both actively debated among professional philosophers. For example, Sam Coleman recently published a paper arguing that the ability hypothesis is irrelevant and is best forgotten. However, I think I did a pretty good job of disputing Mr. Coleman at PhilPapers.
Another discussion of the knowledge argument has begun at PhilPapers, and I have been defending the ability hypothesis yet again. I recently submitted the following content (with minor adjustments), in which I summarize my views and make a case for the ability hypothesis. My main goal here is to explain why I think the ability hypothesis is important. But first I want to get a clearer picture of the knowledge argument.
First, let's consider Dennett's claim that the knowledge argument is just an "intuition pump," and not a plausible argument. His main point of contention is with the premise that Mary has all of the physical facts. Dennett does not reject this premise; he just says that we cannot use it to draw any conclusions, because we have no idea what it means. It's hard to argue with Dennett here, though many have certainly tried. Ultimately, as Dennett says, it just comes down to competing intuitions.
In trying to understand these competing intuitions, I find it helpful to distinguish between abstract and concrete physical facts. This distinction can probably be construed in a number of ways, but for our purposes, I think it can be put this way: Abstract physical facts are theoretical facts which define relationships between scientifically discoverable entities, whereas concrete physical facts describe particular entities, events or processes. Abstract facts are used to make predictions about concrete events. This is why we say science is a fundamentally predictive activity.
Now, having all of the abstract facts means one can make every possible correct prediction about the concrete physical facts. (If we allow for quantum indeterminacy, then having all of the abstract facts does not equate to predictive infallibility.) A person can have all of the abstract physical facts without having all of the concrete physical facts. The knowledge argument stipulates that Mary learns a completed physics (or a completed science, if we do not want to be reductionists about, for example, biology). It follows that Mary has theoretically unlimited predictive powers. However, it does not follow that she has practically unlimited predictive powers. So there may of necessity be physical facts which she cannot ever know. A physicalist has no problem with Mary becoming master of a completed science and still having an indefinite number of physical facts to learn. Thus, the fact that she learns new facts when she leaves her black-and-white room is not a problem for physicalism.
Furthermore, we need not suppose that there is such a thing as "all of the concrete physical facts." Indeed, it is reasonable to suppose that nobody could ever have all of the concrete facts, because that would require omniscience and the inability to learn from new experiences. If Mary had all of the concrete facts, she could never be surprised by any concrete event. She could not think about what to do, because she would already know what she was going to do. Such a person is inconceivable. A being who could not learn from an experience would not be a person under any common sense of the term. So we should promptly reject the premise that Mary can have all of the concrete facts.
Finally, a physicalist need not assume that it is possible to have all of the abstract physical facts. Physicalism does not require the possibility of a completed science, though I grant that many physicalists are open to it.
In sum, we can reject the knowledge argument on the grounds that it is formulated against an impoverished version of physicalism. It is, in effect, a straw man argument.
This is not the end of the knowledge argument, however. We just need to more clearly get at its motivation. To better indicate the driving intuition behind the knowledge argument, I propose the following reformulation: Mary does not learn every physical fact while inside her black-and-white room. Rather, she learns any arbitrarily large set of physical facts about color vision. She learns some vast, though incomplete, set of abstract and concrete facts about color vision. Yet, our intuition still tells us that Mary will not learn what it is like to see colors until she leaves her room. No matter what she learns while insider her room, she will not have a certain sort of phenomenal knowledge. Thus, phenomenal knowledge cannot be a physical fact.
Dennett's objection does not hold against this revised knowledge argument. Yet, our other responses are still on the table. On the one hand, we might still suppose that Mary learns physical facts which can only be learned by direct acquaintance with color vision. Or, we might suppose that the knowledge gained by Mary's color experiences is not propositional knowledge, but non-propositional know-how.
I wonder what sense there is in claiming that there are facts which can only be known through direct acquaintance. The notion of "fact" seems to entail discursive learnability. At least, some philosophers have supposed as much. See, for example, Ryle (1949, Chapter 9). However, perhaps we can adopt a notion of "fact" which does not entail discursive learnability. This is probably an open question, though we should like some explanation for why some factual knowledge is not discursively learnable. More generally, the question is this: Why should any knowledge only be learnable through direct acquaintance, and not description?
Remember that Lewis (1990) shows that the knowledge argument can be formulated against dualism just as easily as it can be formulated against physicalism. Indeed, we can reformulate the knowledge argument this way: Mary does not simply learn physical facts while insider her black-and-white room. She learns every discursively learnable fact--or, let's say she learns any arbitrarily large number of discursively learnable facts. We need not specify that they are physical facts. Yet, our intuition still tells us that, no matter what facts she learns, she will only learn what it is like to see colors when she leaves that room. Thus, phenomenal knowledge is not discursively learnable, be it factual or non-factual, physical or non-physical. If we say that Mary cannot learn the right phenomenal knowledge while insider her black-and-white room, we must ask why that knowledge is not discursively learnable. It is not enough to say that it is not physical, because that does not explain anything. Even a dualist (or an anti-physicalist) needs some answer to the knowledge argument. This is where the value of the ability hypothesis is found.
Consider the relationship between knowledge and experience. As we gain new experiences, we are directly acquainted with new phenomena. Direct acquaintance gives us phenomenal knowledge. According to the ability hypothesis, this knowledge consists in capacities which underlie our descriptive understanding of the world. In other words, the reason why phenomenal knowledge is not discursively learnable is because it is a set of capacities which are more primitive than our descriptive understanding of the world and experience. I think Ryle (1949) and Wittgenstein (1953) showed rather well that factual knowledge relies upon a sort of knowing which is not factual, but which is manifest in our abilities to follow rules and procedures. The fact that we can follow rules and procedures for identifying colors means that our understanding of color vision is not wholly propositional, or factual. To put it another way, the fact that we can follow rules about how to identify colors is evidence that we have an understanding of color vision which does not itself depend upon factual knowledge--and which itself cannot be discursively learned. (As I noted in a post in another thread, Ruth Millikan may also be counted as a supporter of this view, even if she never to my knowledge explicitly remarked on the ability hypothesis.)
In conclusion, the ability hypothesis is called upon to explain why phenomenal knowledge is not discursively learnable. This explanation is required for dualists (or, more generally, non-physicalists) as well as for physicalists. Furthermore, the ability hypothesis accounts for the intuition motivating the knowledge argument without abandoning physicalism. The ability hypothesis is a physicalist doctrine, because it defines phenomenal knowledge wholly in relation to physical processes, events and capacities. Therefore, in accounting for the intuition motivating the knowledge argument, the ability hypothesis offers explanatory power in favor of physicalism.
ReferencesGilbert Ryle (1949/2002). The Concept of Mind. Hutchinson and Co.
David Lewis (1990). "What Experience Teaches." In William G. Lycan (ed.), Mind and Cognition. Blackwell.
Ludwig Wittgenstein (1953/1968). Philosophical Investigations. New York,Macmillan.