Walton's major contribution to philosophy is his theory of representation, known as the make-believe theory. In the context of ontology, the same theory is usually referred to as pretence theory, and in the context of representational arts, prop theory. Walton has been working on this philosophical theory since 1973,[11] and it is expounded in his 1990 magnum opus Mimesis as Make -Believe: On the Foundations of the Representational Arts.[12] The theory is a development of Ernst Gombrich's sketched ideas concerning the relationship between toys and art, presented in his famous essay 'Meditations on a Hobby Horse', which Walton has described as having been “largely ignored” by most of philosophy of art.[13]