The notion "Subject", as distinct from the notion "NP", designates a 'grammatical function' rather than a 'grammatical category'. It is, in other words, an inherently relational notion. We say, in traditional terms, that in (1) ['sincerity may frighten the boy'] 'sincerity' is an NP (not that it is the NP of the sentence), and that it is (functions as) the 'subject-of' the sentence (not that it is a Subject). Functional notions like "Subject", "Predicate" are to be sharply distinguished from categorial notions such as "Noun Phrase", "Verb", a distinction that is not to be obscured by the occasional use of the same term for notions of both kinds.
- Chomsky (1969), Pag. 68