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.