 Cummins (1997) argues that causal theories of mental content are incompatible with the fact that one’s perception of objects in the physical environment is typically mediated by a theory. His argument proceeds in two stages. In one stage, he argues that, on a causal theory, for each primitive “X” there must be some bit of machinery or mechanism that is responsible for detecting Xs. But, since a finite device, such as the human brain, contains only a finite amount of material, it can only generate a finite number of primitive representations. Next, he observes that thought is productive—that it can, in principle, generate an unbounded number of semantically distinct representations. This means that to generate the stock of mental representations corresponding to each of these distinct thoughts, one must have a syntactically and semantically combinatorial system of mental representation of the sort found in a language of thought (LOT). More explicitly, this scheme of mental representation must have the following properties: