This doctoral dissertation aims to give a comprehensive and contextual account of Aristotle’s theory of metaphor. The dissertation is organized around the central claim that Aristotle’s definition of metaphor in Chapter 22 of the Poetics, as well as his discussion of it in Book III of the Rhetoric, commit him to what I call a vertical theory of metaphor, rather than to a horizontal one. Horizontal theories of metaphor assert that ‘metaphor’ is a word that has been transferred from a literal to a figurative sense; vertical theories of metaphor, on the other hand, assert that ‘metaphor’ is the transference of a word from one thing to another thing. In addition to the introduction and conclusion, the dissertation itself has five chapters. The first chapter sketches out the historical context within which the vertical character of Aristotle’s theory of metaphor becomes meaningful, both by (a) giving a rough outline of Plato’s critical appraisal of rhetoric and poetry in the Gorgias, Phaedrus, Ion, and Republic, and then (b) showing how Aristotle’s own Rhetoric and Poetics should be read as a faithful attempt to reform both activities in accordance with the criteria laid down by Plato in these dialogues. The second and third chapters elaborate the main thesis and show how Aristotle’s texts support it, by painstakingly reconstructing the relevant passages of the Poetics, Rhetoric, On Interpretation, Categories and On Sophistical Refutations, and resolving a number of interpretive disputes that these passages raise in the secondary literature. Finally, the fourth and fifth chapters together pursue the philosophical implications of the thesis that I elaborate in the first three, and resolve some perceived contradictions between Aristotle’s theory of metaphor in the Poetics and Rhetoric, his prohibition against the use of metaphors in the Posterior Analytics, and his own use of similes and analogical comparisons in the dialectical discussions found in the former text, the De Anima and the later stages of his argument in the Metaphysics. In many ways, the most philosophically noteworthy insight uncovered by my dissertation is the basic consideration that, for Aristotle, all metaphors involve a statement of similarity between two or more things – specifically, they involve a statement of what I call secondary resemblance, which inheres to different degrees of imperfection among things that are presumed to be substantially different, as opposed to the primary and perfect similarities that inhere among things of the same kind. The major, hitherto unnoticed consequence I draw from this insight is that it is ultimately the philosopher, as the one who best knows these secondary similarities, who is implicitly singled out in Aristotle’s treatises on rhetoric and poetry as being both the ideal poet and the ideal orator, at least to the extent that Aristotle holds the use of metaphor to be a necessary condition for the mastery of both pursuits. This further underscores what I argue in the first chapter is the inherently philosophical character of the Poetics and the Rhetoric, and shows the extent to which they demand to be read in connection with, rather than in isolation from, the more ‘central’ themes of Aristotle’s philosophical system.