Contemporary linguistics has established 3 results: 1. Sign languages, used by Deaf communities throughout the word, are full-fledged languages that share typological properties among themselves and also with spoken languages. 2. Sign languages have the same 'logical spine' as spoken languages, but sometimes they make the logical structure of sentences far more explicit than is the case in spoken language. A salient case concerns logical variables, which are covert in spoken language but are realized overtly in sign language by way of positions in signing space. 3. But in addition, sign languages have rich iconic possibilities, including at their logical core. For instance, logical variables can simultaneously function as simplified iconic representations of their denotations. By contrast, iconic possibilities exist but are limited in the spoken modality.
Should we conclude (from 2 and 3) that sign languages are more expressive than spoken languages, since they have the same logical spine but richer iconic possibilities? For the comparison to be complete, one must re-integrate into spoken language semantics the study of co-speech gestures, which have rich iconic capabilities. But we will argue that even when sign language is compared to speech-plus-gestures, sign languages have an entire class of expressive possibilities that spoken languages mostly lack.