Modularity of Mind is the title of both a book by Jerry Fodor and the philosophy of cognitive sciences it bore. Even though I don't think anyone, Fodor included, truly understands the writing in Modularity of Mind, some the big points of the book were rather impactful. Cognition (hold that thought) is conceived as the sum output of a network of functionally independent input / output modules. The modules are strictly defined by their adherence to nine criteria - most importantly that they make fast, frugal, specific outputs, in response to fixed inputs, in a way that can't be inhibited and that we can't introspect. The 'hold that thought' above is because Fodor argued that what cognitive psychologists are studying is necessarily limited to 'what inputs go where' kinds of questions. 'Central cognition', the juicy stuff that is what we purport to study, is forever veiled by the nature of the module architecture. So really we're all wasting our time! Or something.
Modularity assumptions were transformative to universal grammar theories, early cognitive neuroscience, evolutionary psychology, certain kinds of formalizations of cognition, and other insurgent parts of our field.