Aristotle's Criticisms of Plato's Tripartite Soul

John F. Finamore (University of Iowa)

In his Republic and Timaeus, Plato famously argues that the human soul is composed of three parts: the rational, spirited, and irrational. His pupil Aristotle in his De Anima argued against his former teacher, raising the issue on four separate occasions: 402b1-5, 411b5-30, 413b13-32, 432a22-b7. I propose to examine these four passages, outline Aristotle's arguments, and show why Aristotle considered the matter of such importance.

Like Plato, Aristotle believed that the soul was incorporeal and spread through the body. For him, however, the soul was the body's form, that is, the soul activated the body and made it alive. As such, it was a single entity with multiple powers. Although he was not above using the terms moria and dynameis interchangeably, he was nevertheless aware of the distinction between psychic parts and psychic powers, and he developed a philosophical distinction between the terms that would endure for generations of philosophers who came after him.

Aristotle creates two kinds of argument against Platonic tripartition. The first concerns the possibility of the soul being divided. Aristotle questions how a soul divided into parts can be said to be unified. He rejects the notion that the body can unify a divided soul, but he also argues that if a higher soul is claimed to unify the soul, then this higher soul is the soul philosophers are looking for and they should begin with it. If that soul is not properly unified, then the search could well go on to infinity, he claims. Further, Aristotle says, if the soul were to be divided, it would require more than three parts, for there are other powers of soul, such as nourishment, perception, imagination, and desire. The Platonic design cannot easily subsume at least the last three of these into its tripartite schema since perception and imagination may be rational or irrational and since desire would be housed in all three parts.

Second, Aristotle questions whether the tripartite structure can easily fit empirical evidence. Some plants and insects, when bisected, continue living. Aristotle notes that the whole soul is divided, but not into its three would-be constituent parts. Each segment contains the whole soul, as it were with all three parts. There is no evidence that attests to the division into three parts at all.

Aristotle's division of the soul into powers avoids these difficulties. The soul, as the form of the natural body that is fitted with appropriate organs to receive and use the soul, is uniform. It simply actualizes in a different manner in different organs. The Aristotelian unified soul would also explain the phenomenon of the bisected plants and insects, the soul remaining with all its powers in each segment. Here again, the still unified soul actualizes the potentialities in the organs.

Aristotle's concept of soul is different from Plato's. He is not as concerned with immortality. Indeed, although the problematic active intellect is immortal, it seems to have little or no identity with the human being to whom it was temporarily attached. Rather, Aristotle is concerned with the functioning of the soul in all its various powers in the body (whether a human, animal, or vegetative body). Thus tripartism, which for Plato explained the nature of the rational part's struggle with the lower parts and the effect of that struggle on the soul's immortality, not only does not fit the facts for Aristotle but it prevents philosophers from understanding the true nature and functions of the soul.