Sign language and modularity

There are two known language modalities used by humans: the oral-aural modality of spoken languages, and the manual-visual modality of signed languages found in the deaf communities of the world. The coexistence of these two systems raises fundamental questions about language and cognition. Sign language is considered here in the context of a particular theory of cognitive organization, the modularity thesis of Fodor (1983). It is argued that that modularity theory, which is based on biological as well as representational/computational considerations, does not stand up in the face of the existence of sign language. To demonstrate that sign language must be considered to be part of the same cognitive system as spoken language, a formal comparison is drawn at the level of phonology - perhaps the least expected level of linguistic organization because it is the most physiologically bound. It is then argued that despite this similarity, a language module that is stimulus-domain specific and informationally encapsulated could not include sign language. Sign language is shown to be a challenge for the development of a comprehensive theory of language, which, it is suggested, should aim to predict both similarities and differences between natural languages in the two modalities.