 Fodor argues that input systems constitute a natural kind, defined as “a class of phenomena that have many scientifically interesting properties over and above whatever properties define the class” (Fodor, 1983, p. 46). He argues for this by presenting evidence that input systems are modular, where modularity is marked by a cluster of psychologically interesting properties—the most interesting and important of these being informational encapsulation, as discussed in §1. In the course of that discussion, we reviewed a representative sample of this evidence, and for present purposes that should suffice. (Readers interested in further details should consult Fodor, 1983, pp. 47–101.)