So most rebuttals to the argument from reason make the claim that explanation of a mental act of rational inference in terms of particle physics that makes no reference to reasons, and the explanation of that same act in terms of propositional attitudes, intentionality, truth, and laws of logic, are compatible with one another. Given a proper relationship between the explanation given at the physical level, the explanation at the mental level can obtain also, and one needs no more to choose between a mental and a physical explanation than one needs to choose between a biological and a physical explanation.

But let’s take a look at what explanatory compatibility amounts to in different cases. Let us say we have an explanation, at the level of particle physics, for a round peg going into a round hole. In this case, the explanation in terms of physics fails to make mention of pegs or holes, but the physical structure of the peg and the hole, (as well as the force putting the peg into the hole), is what makes the causal transaction possible. It could be pointed out that one could have a peg and a hole made of different types of material, and that the statement “a round peg fits in a round hole” is multiply realizable-it can be a wooden peg, a metal peg, or a plastic peg, etc. But any such peg or hole would have to satisfy a certain structural specification to play a role in a transaction described thus, in particular, it would have to be round.

Another example would be the webbed feet of a duck. The natural webbed feet of a duck will permit the duck to move in the water as well as on land, but perhaps veterinary medicine might be able to provide ducks whose webbed feet have been injured or destroyed with artificial webbing that is molecularly very different form their natural webbing. Nevertheless even though its microstructure is different from natural webbing, its macrostructure is similar, and it is the macrostructure that enables it to do its job. Putting steel in place of webbing will result in the duck not being able function in water as effectively as they currently do. We might call this kind of explanatory compatibility structural compatibility. That is, even though the upper level explanation uses terms which are not found in the lower-level explanations, the structure of lower-level items guarantees that the upper-level explanation will obtain. So, for example, one can describe each brick of a wall without mentioning a wall at all, we know very well that, given the position of the bricks, the wall must exist. Where there is structural compatibility, an ideal scientist, looking at large-scale physical patterns, could know what higher-level compatible explanations could be given.

But explanatory compatibility can be of a different type entirely. Jerry Fodor has argued that monetary exchanges cannot plausibly be analyzed in terms of particle physics, nevertheless no one is inclined to suppose that there is anything nonphysical about money.[13] Once again it is possible to fully describe a monetary transaction from the standpoint of particle physics without mentioning that it is a monetary transaction. But the property “being money” is a matter, not of physical structure, but of human convention. In the United States presenting slips of paper with green ink on them depicting various leaders from American history will result in a person’s receiving goods and services commensurate with the indicated value of those slips of paper. Presenting maroon and gold slips of paper with the pictures of Arizona State University football players will not produce this result, even at the concession stands at Sun Devil Stadium. The structure of money makes no difference whatsoever, what makes a difference is its relation to a background of mental states. What that means is that economic explanation is compatible with physicalism just in case mental explanation is, and if not then not. As Geoffrey Madell puts it:

Given our understanding of human interests and wants, we can understand how those wants can be furthered by the institution of money, by the business of endowing a range of very different items with the same conventional significance. But our understanding of this presupposes the background of human psychology.

So contrary to Fodor, there may be something nonphysical about money, if it turns out that our convention-making capability is something nonphysical. So, although this example is often offered as an example of an ontologically innocuous instance of irreducibility, it is nothing of the sort. Economic exchanges are compatible with physicalism just in case the kinds of mental states listed above are compatible with physicalism, and that’s the very thing we are trying to ascertain at the moment.

We might call the type of explanatory compatibility we are talking about here conventional compatibility. But conventional compatibility between mechanism and reason is not going to help us solve the problem of whether reason is possible in a mechanistic universe, because it leaves open the question of how the convention-maker can be rational in a mechanistic universe.

Computers are often offered as clear and decisive cases where physical and mental explanations can be offered for the activities of undeniably physical systems. But computers are clear cases of conventional compatibility. In the case of the computer, a “full” explanation of its activities can, of course, be offered in physical terms. But the computer has the characteristics it does because reasoners built it to model their own rational thought patterns. So the computer is not a clear example of a system for which the total causal story is clearly mechanistic. As William Hasker points out

Computers function as they do because they have been constructed by human being endowed with rational insight. A computer, in other words, is merely an extension of the rationality of its designers and users; it is no more an independent source of rational thought than a television set is an independent source of news and entertainment.

So the compatibility of mechanism and rationality found in computers hardly shows that the mechanistic world view is compatible with the actuality of human reason. If humans are physical systems, then computer activity can be reconciled with a physicalist view of the universe. If not, then computer states, while themselves physical, rely for their mental characteristics on the existence and intentions of nonphysical entities. But the existence of “mental” states in computers helps not at all in showing that our mental states are or can be physical states, given that the universe is purely physical.

Further, it seems clear enough that conventionally established characteristic cannot be causally relevant. Structure, after all, is what counts in causal transactions. If a baseball breaks a window, this window breaking can be explained both at the level of particle physics and at the level of macro-mechanics. Some properties of the baseball, such as its size and weight, and its velocity, are relevant to the question of whether it will break the window. But other properties are clearly not relevant. If the ball was used in Sandy Koufax’s fourth career no-hitter, this may explain why the owner, (presumably a Dodger fan) would prize it over other baseballs he owns. But it is irrelevant to whether the ball will or will not break the window upon impact. Whether a computer’s activity is interpreted as a chess game or a word-processing program will not affect the actual output of the computer, though it will no doubt affect the input that its users generate. What this means is that if I want to say that I accept naturalism because there are good reasons for believing it, then it cannot be true that my acceptance of naturalism is a matter of conventional compatibility. What is more, conventionally established characteristics of a person are also irrelevant in evolutionary explanation. After all, it cannot be a matter of convention whether I survive long enough to pass on my genes. So any attempt to use evolution to account for the reliability of my belief-producing mechanisms is bound to fail if mental states are conventionally established characteristics.

Yet many materialist theories of mind seem to provide compatibilist accounts of mechanism and purpose which are, in the final analysis, based on conventional compatibility. According to some versions of functionalism, not only is it the case that mental states are multiply realizable, but that those mental states can be realized in just any medium. On this view mental states are to be distinguished by their causal role, and these roles can be played by anything from Swiss cheese to a Cartesian mind. What this means to me is that nothing about physical structure is relevant to whether some particular causal role is played or not played. But unless there is something about the structure of those states that makes them capable of playing the relevant causal role, it must be the case that these states play that role relevant to a set of conventions adopted by thinkers. To illustrate this we can compare two different game devices. A game played with frisbees requires those frisbees to have a certain structure. I can’t play a frisbee game if I replace the frisbee with just anything, such as a manhole cover or a basketball. There may be conventions surrounding a frisbee game, but a frisbee still requires one particular kind of structure. On the other hand, if I am playing chess, and I am missing a pawn, I can get a penny, a button, a half-eaten carrot, or just about anything else to play the role of a pawn. This is because unlike the frisbee, the pawn’s role is purely symbolic, determined by convention. It seems clear from these examples that causal roles can be assigned to physical things with total medium independence just in case those roles are assigned conventionally. If the causal role is not assigned conventionally, then something about the structure of the system is required for it to play the causal role it does.