Because of the unique syntactic behavior of the English modal auxiliaries (e.g., lack of complementation), many generative grammarians have argued for the position that they are categorially distinct from verbs. This study addresses the resulting historical question of how English modals such as may and must split away from the verb system. Dr. Nagle proposes two discrete changes in the underlying grammar whose gradual surface ramifications appear to be syntactic drift. He further argues that whether a change's surface output proceeds gradually or rapidly depends on the strength and form of language learners' inferential decisions. Two such cognitive decisions underlie the gradual-then-rapid surface drift of the English modals.