In this paper I want to present a practical descriptive approach to the semantics of grammatical categories, especially of the binary type involving two forms only. In doing so, I hope to be able to attract the attention of linguists concerned with the structure of a comprehensive semantic theory of human language. Substitutional relations of a grammatical kind (as opposed to syntactic and lexical relations) are too often neglected in textbooks on modern semantics. For example, in Ruth Kempson's otherwise excellent introduction to semantic theory (Kempson, 1977), there is no mention of the semantics of grammatical categories at all. In my view, not only must such Substitutional relations be accommodated within a total theory of semantics – even on a narrow definition of the discipline – but they may provide important insights into the nature of meaning which will affect some of the current suppositions in semantic theory. In particular I shall attempt to shed light on the role of ‘subjectivity’ – a notion which is becoming increasingly difficult to ignore and which challenges the very common restriction among semanticists of the scope of semantics to just a truth-conditional component.