Theaetetus reminds Socrates and Theodorus that they neglected to examine the opposite of the theory that everything changes, which they agreed to do in chapter 19. This is the theory “that the universe is one and at rest” (66), an idea that was originally developed by the philosopher Parmenides. However, Socrates argues that discussing this theory would be too time consuming and too much of a diversion from the topic of knowledge.

Returning to the question of knowledge, Socrates says that we perceive through the various sense organs. Everything gained though these senses converges on, and is united in, the “mind” (69). The mind then organizes and gives meaning to various sensory stimuli to form a coherent perception. In this way, the mind can consider things given to it by the body—yet it can also consider things by itself. It can form judgments about the “being” of something. This means it can use contrast and calculation to establish the nature of an object in terms of its likeness and difference to other objects, as well as its usefulness and aesthetic qualities. Further, it is the understanding of an object’s “being,” or nature, that constitutes knowledge.