In his The Modularity of Mind (1983) Fodor posits “transducers” (senses that connect us with the outside world), “input systems,” and “central systems.” Input and central systems are distinguished by the fact that input systems are modular and central systems are not. Modules each have one kind of cognitive material (for example, the visual module), and their information is encapsulated so that they can work very quickly, although they are inaccessible to conscious introspection. One module can be destroyed without impairing the others, as in cases of “aphasia.”

Besides the different sensory systems, language is a module. It should be noted that Fodor does not hesitate to compare his theory with the system of phrenology propounded by Franz Joseph Gall (1758–1828), which is usually taken to be an example of early pseudo-science. The non-modular central systems correspond to thinking and believing and have access to other contents of the mind. Unlike language, the nonmodular central system is not localized.