Theaetetus is primarily concerned with questions of epistemology: how humans acquire knowledge and the degree to which the types of knowledge acquired are trustworthy.
The opening, or frame, of the dialogue, in which Theaetetus is dying from wounds received in a battle, suggests two themes that are dealt with in the main body of the dialogue. The first is that the wise, noble, middle-aged man Theaetetus has been shaped by his excellent early education. The portrayal of him as an intelligent and modest youth learning from Socrates and pursuing elenchus even when the path to knowledge becomes difficult or frustrating shows how knowledge is shaped by character and education.
Next, the theme of the wounds suffered by the middle-aged Theaetetus suggests the later discussion of how knowledge or perception can be distorted by a fever or other maladies of the body. This raises the issue of mind–body dualism and the way the material nature of the human body and its reliance on perception lead to unreliable opinion rather than knowledge.
For Plato, humans acquire information in two ways: through reason (noesis), which reveals the noumena (things known through reason), and sense perception (aisthesis), which reveals phenomena (sensibilia) or aistheta. Because the world is in Heraclitean flux and is constantly becoming, it cannot be truly known, as truth is stable and unchanging. The evidence of the senses thus leads only to opinion rather than knowledge. The phenomena perceived by the senses are only pale imitations of the forms which are noumenal, or understandable by reason. True knowledge can only be attained by reason. Philosophy, like mathematics, treats of the forms and uses reason, and the wise person learns to rely on reason and distrust the sense.