Aristotle's On the Soul exists as his personal discourse on what the soul truly is. He begins the text stating that "knowledge of any kind is a thing to be honoured and prized." Therefore, he admits that knowledge on the soul is priceless. His discussion acknowledges that he is only offering his ideology on what the soul is, which is not what all may believe it to be.

Aristotle goes on to state that to understand the soul one must possess an understanding of Nature and animal life. He sets a very specific expectation regarding his examination of the soul through making the following statement: "aim is to grasp and understand, first its essential nature, and secondly its properties; of these some are taught to be affections proper to the soul itself, while others are considered to attach to the animal owing to the presence within it of soul." Therefore, this provides a summary of what the purpose of On the Soul is.

Essentially, Aristotle admits that defining the soul is "one of the most difficult things in the world." Aristotle continues with the idea that to define the soul one must define what "class" the soul belongs to. That said, after trying to define what class the soul belongs to, one must also define if the soul possesses "parts." This question forces both Aristotle and the reader to consider the "body." Does the soul exist independent of the body, as part of the body, or is it dependent upon the body for survival?

Aristotle decides that the study of the definition of the soul must be grounded solely in that of the "science of Nature." By grounding the study of the soul in the science of Nature, Aristotle gives others a definitive set of rules and standards to use. (He does this based upon the fact that people within different schools of thought will take those specific rules to define the soul.)

Aristotle, in Part II, opens by stating that the ideologies of "our predecessors" are of the utmost importance. By examining what has already been stated, one can "profit by whatever is sound in their suggestions and avoid their error." This allows Aristotle to learn from and internalize what has already been suggested about the soul.

Later in Part II, Aristotle states the one way of defining the soul is through what moves it [the soul]. He goes on to define this movement as being two-fold: the soul itself moves and things are moved by the soul. He also states that growth only comes from the soul.

In Part III, Aristotle also states that "four species of movement-locomotion, alteration, diminution, growth exist." These movements are especially important in the soul, and that the soul must be responsible for moving itself. That said, as with any movement, the movements of the soul possess countermovements (or effects). He goes on to tie the movements of the soul to earthly elements: fire moves the soul upward and the earth moves the soul downward.

Part IV discusses the harmony of the soul. Here, Aristotle discusses the actual movement of the soul. He states that the soul cannot move "in harmony," but moves as it wills itself to. This idea suggests that the soul, although not stated by Aristotle, possesses the ability to think in order to make thoughtful movement. Later in this part, he discusses that the soul is incapable of being destroyed. This gives the soul an almost supernatural and god-like existence.

Finally, in Part V, Aristotle begins to solidify his defining of the soul. Yet, he ventures off this defining through questioning the existence of a soul within "everything." He questions, specifically, "Why has not everything a soul, since everything either is an element, or is formed out of one or several or all of the elements."

In the end, Aristotle leaves readers with no real answer to the question of the soul. He successfully asks and answers questions regarding the soul without defining it in its entirety. Instead, he leaves readers with a single final thought: that when some plants and animals "when divided into segments" go on living, each segment now possesses its own soul. This idea forces readers to come to the conclusion presented in the opening of Aristotle's essay: "To attain any assured knowledge about the soul is one of the most difficult things in the world."