In his philosophical work, Grayling connected solutions to the problem of scepticism in epistemology with the questions about assertibility and the problem of meaning in the philosophy of language and logic. A principal theme in his work is that considerations of metaphysics, which relate to what exists, has to be kept separate from the two connected questions of the relation of thought to its objects in the variety of domains over which thought ranges, and the mastery of discourses about those domains, where a justificationist approach is required.