A traditional view maintains that thought, while expressed in language, is non-linguistic in nature and occurs in non-linguistic beings as well. I assess this view against current theories of the evolutionary design of human grammar. I argue that even if some forms of human thought are shared with non-human animals, a residue remains that characterizes a unique way in which human thought is organized as a system. I explore the hypothesis that the cause of this difference is a grammatical way (...) of structuring semantic information, and I present evidence that the organization of grammar precisely reflects the organization of a specific mode of thought apparently distinctive of humans. Since there appears to be no known non-grammatical structuring principle for the relevant mode of thought, I suggest that grammar is that principle, with no independent ?Language of Thought? needed. (shrink)