The aim of this thesis is to discover the cognitive role of Plato's metaphors for the gods and the soul; that is, what part they play in the expression and elucidation of his philosophical theories and arguments. I maintain that these metaphors are primarily illustrative and persuasive; that they serve both to illuminate certain concepts and ideas and to persuade the reader into accepting them as true. But I also believe that in very particular circumstances, that is, in the formulation of radically new theories, Plato uses metaphors to develop and to express ideas for which no literal terms already exist. In Part I the cognitive role of metaphor is discussed. Three modern views (the Epistemic, Non-Informative and Illustrative) are set out and compared both to Aristotle's account of metaphor and to Plato's comments on the philosophical significance of figurative language. Part II opens with a discussion of the role of metaphor in theology, embracing both modern perspectives and the views of Plato. The various groups of metaphors for the gods are examined and three passages from the dialogues are analysed in detail, showing how metaphors can function as very effective instruments of persuasion. In Part III the debate turns to Plato's metaphors for the soul. First I argue against the view that Plato's soul myths provide, or were intended to provide, special cognitive access to the soul. Second 1 examine the significance of metaphors for the immortality of the soul, the soul/body relationship and for the inner nature of the tripartite soul. Finally I offer a new interpretation of the development and rhetorical significance of the metaphor of spiritual pregnancy in the Symposium.