Dialogue of the middle Platonic period on the topic of knowledge.

Dramatic Context

The dialogue Theaetetus is divided into two narrative frames. In the first, the dramatic date is 391 B.C. just after the battle of Corinth. The interlocutors, Terpsion and Eucleides, both from Megara, meet on the outskirts of Athens. After a short exchange, they decide to walk to Euclides’ house to discuss his account of a philosophical discussion that took place between the mathematician Theodorus, his young student Theaetetus and Socrates. The discussion was recorded in the form of a ‘book’ by Eucleides who, although not present for the original discussion, was informed about the details of it through a series of subsequent meetings with Socrates. The account is read out to Terpsion and Eucleides by a slave of the latter – a unique device in the Platonic corpus. The philosophical discussion between Socrates, Theodorus and Theaetetus - the second narrative frame- takes place in 399 the day that Socrates makes his way to the King-archon’s court to be informed of the accusations levelled against him by Meletus. It is of great significance that the ‘book’ is written in the form of direct dialogue as opposed to a narrative, giving the impression that it takes place ‘right here and right now’.

Characters, structure, content

The Theaetetus follows the typical structure of the Socratic dialogues. In the proemium, the characters and the topic under discussion are introduced. The interlocutors are the mathematician Theodorus from Cyrene, a friend and close associate of the sophist Protagoras, and the young Athenian geometer Theaetetus. The opening of the discussion introduces a form of question that is typical in the aporetic dialogues: ‘What is X?’ – the X in this case being knowledge. In what follows, the interlocutors investigate and ultimately reject a series of definitions of knowledge. In the dialectical game that takes place, Socrates’ interlocutors become gradually aware of their inability to give am adequate definition of knowledge, which leads them ultimately to aporia. Further, by moving from apparently simpler definitions to more complex ones, the level of ignorance among the interlocutors is reduced. At the end of the discussion, they all agree to meet the following day and Socrates makes his way to the king archon’s court.

The definitions of Knowledge

The first answer to the ‘small aporia’ expressed by Socrates regarding the question ‘what is knowledge?’, consists in the enumeration of various kinds of expertise such as geometry, astronomy etc. Socrates, by harkening back to the requirements definition as they emerge in the early dialogues, claims that the examples given are not a. necessary, because what they are searching for is not a simple enumeration of kinds of knowledge, but rather what knowledge actually is, and b. sufficient, because the interlocutors presuppose the definition of knowledge. A definition must, according to Socrates, be general and cannot be just a list of specific instances which can go on endlessly.

What follows is the famous midwife reference. Socrates likens himself to a midwife who helps his interlocutors give birth to ideas (148e7-151d7). Theaetetus, with Socrates’ assistance, gives three definitions of knowledge, which are all refuted. During the course of examining the three definitions, the interlocutors consider the Heracletean theory of flux, the Protagorean theory of relativism according to which ‘man is the measure of all things’ and ‘what appears true to me, is true for me’, the possibility of false beliefs, and the relation between parts and wholes.

According to the first definition, ‘knowledge is sense-perception’ (151e-187a). Socrates claims that this definitions entails two claims: a. Protagorean relativism , b. Heracletean flux. This definition comes under scrutiny and through a long and complicated argument is refuted. This refutation relies on the assumption that certain qualities like colour or temperature can be apprehended only by the senses, while other qualities such a similarity, difference, number and Being – the koina - are grasped not by our senses but by our minds. Socrates argues that one must grasp Being in order to grasp truth and without truth, one cannot have knowledge. Therefore, because Being is not an object of perception but rather of thought, knowledge is to be found in the rational activity of the mind and not in the conveyances of the senses.

The second definition proposed is that ‘knowledge is true belief (doxa) (187b-201c). Aiming to clarify what a true belief is, the interlocutors engage in a thorough examination of false belief. This investigation considers a series of arguments that an empiricist might give in support of the existence of false belief. The interlocutors fail to explain how it is possible to hold a belief and at the same time to be mistaken about it. Further, the second definition is ultimately rejected by examining various contexts in which true belief is not sufficient for knowledge.

The third proposed definition of knowledge as ‘true belief with logos’ is close to what contemporary philosophers refer to as ‘justified true belief’ (201d-210a). The examination of the definition is divided into two parts. In the first part, Socrates and Theaetetus examine which subjects are knowable. They consider first the simple elements of which we cannot predicate anything, letters are an example of these, and then they examine the case of composite subjects, such as syllables. The need to justify true belies leads the interlocutors to the examination of the various meanings of logos. All of them are problematic, mainly due to the theory of Dream. The main criticism concerns understanding logos to be a sign or differentness, which allows them to draw a clear distinction between various objects. Understanding logos in this way leads to a vicious circle, because having a true belief requires some form of distinction. Thus, some form of knowledge must be presupposed if they are to reach a definition of knowledge itself.


The dialogue Theaetetus is the main Platonic dialogue on the topic of knowledge, although a definition is never given. The third proposed definition reveals all the difficulties we come across in the attempt to answer the question ‘what is knowledge?’. Even if we consider the option of knowledge as a justified true belief, we fail to answer the question whether it is justified true belief regarding the explanation or whether it is knowledge. Most scholars understand the dialogue as a criticism of empiricism. At the same time, the dialogue is a typical aporetic dialogue, in which the interlocutors gradually understand that it impossible to talk about knowledge, before reaching a definition. Finally, the Theaetetus is an example of a philosophical work that shows philosophy in action, but also its limits.

Author: Eleni Kaklamanou
  • Cornford, F.M. Plato's Theory of Knowledge. London: Routledge, 1935.
  • Sedley, D. The Midwife of Platonism. Oxford: Oxford University Press, 2004.
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