The Uniformity and Diversity of Language: Evidence from Sign Language.

Evidence from sign language strongly supports three positions: (1) language is a coherent system with universal properties; (2) sign languages diverge from spoken languages in some aspects of their structure; and (3) domain-external factors can be identified that account for some crucial aspects of language structure -- uniform and diverse -- in both modalities. Assuming that any of these positions excludes the others defeats the purpose of the enterprise.