This thesis attempts to explain Aristotle's conception of the self-mover (introduced in Physics VIII. 4-6) by analysing, in particular, the relationship between the locomotive faculty of the soul and the sumphuton pneuma. Aristotle's theory of self-motion calls for resolutions to three major problems: (a) how is self-motion to be explained without denying the existence of the first mover, i.e. the ultimate cause of the motions of all sublunary beings? (b) how is the self-motion of the living being different from the natural motion of the non-living being? and (c) what is the relationship between the unmoved moving part and the moved part of the self-mover (identified as the soul and the body)? Chapter I discusses (i) some potential problems that Aristotle faces in maintaining the theory of self-motion as a part of his overall theory of natural change, (ii) the characteristics and the relationships of the internal parts of the self-mover, and (iii) the reason for identifying the parts with the soul and the body. Chapter II turns to examine modem views on Aristotle's conception of the soul-body relationship, focusing on the functionalist interpretation of it as entailing compositional plasticity, viz. the view that the same psychological state may be realised by several different material states. Chapter III examines what psychological capacities are necessary for the arousal of animal locomotion and what their interrelationships are, whereas Chapter IV argues against Nussbaum's claim that Aristotle maintains that phantasia is an absolutely necessary capacity for an animal to arouse locomotion. Chapter V analyses the locomotive faculty and its relationship with the sumphuton pneuma. On the basis of this examination, this thesis ascribes to Aristotle the following claims: (al) that all natural beings have natures for initiating their own motions, which cannot be merely brought about by the external mover, (bl ) that self-motion is differentiated from natural motion in that, although both depend on external conditions, the former, unlike the latter, also depends on the internal condition of the mover, and (CI) that psychological capacities can be realised only in the pneuma and in nothing else.