This paper comparatively examines Plato and Aristotle's discussions of the doctrine that the soul is a harmonia. With regard to the former, Simmias' argument against the immortality of the soul and Socrates' response to it in Phaedo are scrutinized, on the basis of which it is argued that the association of the doctrine with the Pythagoreans in the dialogue is not the report of a historical fact, but a product of Plato's sophisticated artistry. The insufficiency of Socrates' response to Simmias' challenge is readmitted. At the same time, however, it is argued that this problem stems from a fundamental limitation of the very project of explaining the soul in terms of the concept of harmonia. With regard to Aristotle's treatment of the doctrine of harmonia, an illuminating difference between his discussions in On the Soul and in Eudemus is brought out and it is suggested that the latter is connected to his arguments concerning the difference between substance and quality in Cartegories.