 However, carrying this further may lead to absurdity, since externalism about empty concepts can suggest that a concept’s form is sensitive to the environment (Boghossian 1997; 1998). Suppose, as is plausible, that empty concepts have a descriptive form—so that (e.g.) the unicorn concept is understood as the concept “horse with a horn.” For most externalists, this contrasts with our concept of water which does not have a descriptive form, but is rather atomic. If that is correct, then Dry Oscar expresses a descriptive concept when using ‘water’, whereas Oscar expresses an atomic one. In which case, the environmental difference creates a difference in form as well as content. But that is strange, since form seems to be a purely syntactic matter, fixed entirely by what’s in the head.