Distinction in Frege’s philosophy of language, explored in ‘On Concept and Object’ (1892). Frege regarded predicates as incomplete expressions, in the same way as a mathematical expression for a function, such as sine…or log…, is incomplete. Predicates refer to concepts, which themselves are ‘unsaturated’, and cannot be referred to by subject expressions (we thus get the paradox that the concept of a horse is not a concept). Although Frege recognized the metaphorical nature of the notion of a concept being unsaturated, he was rightly convinced that some such notion is needed to explain the unity of a sentence, and to prevent sentences from being thought of as mere lists of names.