Sentences that are ungrammatical and yet intelligible are instances of what I call perfectly thinkable thoughts. I argue that the existence of perfectly thinkable thoughts is revealing in regard to the question of why syntactic islands should exist. If language is an instrument of thought as understood in the biolinguistics tradition, then a uniquely human subset of thoughts is generated in narrow syntax, which suggests that island constraints cannot be rooted in narrow syntax alone and thus must reflect interface conditions (...) imposed on the output of the computational system and its mapping to external systems. (shrink)