In an attempt to illustrate that the soul, much like the city Socrates describes earlier in the book, is partitioned into three parts, Socrates presents his arguments in three logical steps. He firstly establishes the assumption that the same thing cannot undergo opposite things. Then, he demonstrates that the soul must contain at least two parts, namely the appetitive and the rationally calculating. Lastly, he demonstrates that the spirited part must be different from both of those, thereby proving the tri-constituent structure of the soul.

First, Socrates tackles the question of whether we do everything with the whole of our soul or distinct parts by stating a fundamental premise upon which all subsequent arguments are built. This states that the same thing cannot undergo opposite forces or be in opposite states at the same time in the same respect. More concretely, Socrates gives an example of a person not being able to be standing still and moving with respect to the same frame of reference simultaneously. Recognizing the generality of this argument, he then hypothesizes the correctness of the premise by further listing a string of opposites, including dissent versus assent and wishing versus not-wishing.

Building off of the aforementioned premise, Socrates advances his argument by elucidating the distinction between a natural object by itself and an object of a particular sort. For instance, he argues that thirst, in its independent form, causes people to crave drink. However, when a drink with particular attributes, such as a hot drink, is desired, there must exists a different appetite which depends on the addition of those attributes. Observing Glaucon’s lingering confusion, Socrates offers another example in relation to the concept of knowledge. He argues that the natural object of knowledge is of what can be learned by itself but when in practice