In Theaetetus Plato plunges into epistemology, the theory of knowledge. What is knowledge? How do we know things? How do we really know what reality is? and similar questions are the topics of discussion.
The dialog opens with a flashback to Socrates, at the end of his life shortly before his trial, talking to the geometer Theodorus about his current pupils. Theodorus tells Socrates that one, Theaetetus, shows particular promise. The youth arrives on the scene and Socrates decides to test him dialectically. After some preliminary banter, he asks Theatetus to define knowledge, Theaetetus initially starts by listing different kinds of knowledge such as geometry, calculation, shoe making, and others. After some prompting from Socrates, he states that knowledge is perception. Socrates accepts this provisionally but points out that different observers perceive things differently. Does this mean that knowledge is relative? This begins a section which introduces the debate between objectivism and subjectivism which has been a perennial question of Western philosophy. According to most of the pre-Socratic philosophers, including Protagoras, Heracleitus, and Empedocles, reality is not only relative but in a state of constant flux, so knowledge about reality is also relative and mutable.This argument is the precursor not only to later Epicurean theories of the universe, such as we read about in Lucretius, but also foreshadows both postmodern theories of subjectivism and much of modern physics including thermodynamics, relativity, and quantum theory. For example, the passage,
Yes, Theaetetus, and there are plenty of other proofs which will show that motion is the source of what is called being and becoming, and inactivity of not-being and destruction; for fire and warmth, which are supposed to be the parent and guardian of all things, are born of movement and of friction, which is a kind of motion;–is not this then the origin of fire?
Expresses at least the glimmerings of an intuitive understanding of the laws of thermodynamics.
Socrates leads Theaetetus to explore the implications of these ideas further. He also introduces the point that mad men and dreamers see things that are not there, so perception does not necessarily lead to true knowledge. This struck me as interesting, since at the same time I was reading Hermann Hesse’s novel Steppenwolf, in which the main character certainly seems to gain knowledge from events which occur only in his own mind. Be that as it may, however, Socrates’ arguments are more than enough to force Theaetetus to rescind his definition of knowledge as perception.
Socrates then steers the dialog into a discussion of the nature of memory. What about a man who has obtained true knowledge but then forgets or makes mistakes based on that knowledge?
At this point, Theaetetus needs a little help and, at the urging of Theodorus, begins to argue the opposite position. Among other things, he mentions the point that while all men might perceive things differently, wiser men and those with reason and specialized training in the subject at hand, are more likely to perceive correctly and gain true knowledge.
Socrates has another trick to spring, though, when leads Theaetetus to explore the question “What is perception?” and points out that sensory inputs are meaningless until processed by the mind (soul). He then asks about abstract concepts which can not be perceived with the senses. Theaetetus soon agrees that “[K]nowledge does not consist in impressions of sense, but in reasoning about them in that only, and not in the mere impression, truth and being can be attained[.]”
If knowledge is actually a sort of opinion, though, what about false opinions? This line of inquiry leads them back into an exploration of memory, since failure of memory is one possible cause of false opinions. This leads them back into deep waters. Socrates then points out that they have not adequately defined the verb “to know”, nor the concept of “false opinion”. With these points clarified he leads the boy on another discussion in which they define knowledge as “a way of understanding something by understanding it’s component parts.”
From this position, Socrates introduces yet a third conception of knowledge, understanding things by understanding the differences between them. But after discussing this, they conclude,
But how utterly foolish, when we are asking what is knowledge, that the reply should only be, right opinion with knowledge of difference or of anything! And so, Theatetus, knowledge is neither sensation nor true opinion, nor yet definition and explanation accompanying and added to true opinion?”
Since the definition itself includes the word “knowledge” and is thus circular. However, even though they have failed to provide the definition for which they were seeking, they are better for the attempt itself. Socrates then leaves for his indictment before the king archon, possibly never to see Theaetetus again.
Never mentioned explicitly, yet ever present in the dialog, is Plato’s theory of ideas. Plato believed that even though the material world is indeed mutable, imperfect, and subjective, everything in it partakes of perfect ideals or ideas which are unchanging and objective. Perhaps Socrates doesn’t bring up the theory of ideas because he wants to see if Theatetus will discover the concept on his own. Perhaps Plato is simply trying to keep the dialog to a manageable size and complexity. Or perhaps the state of epistemology conveyed in the dialog is actually as far as Socrates ever got on the subject, and this particular dialog is meant to express the master’s views without Plato’s later additions.