Theaetetus: On Knowledge

What is it to know something? The various attempts to answer this question in Plato’s Theaetetus sets the scene the subsequent struggle to find agreement in a controversy over ‘epistemology’ that continues today.


  • Six weekly small-group sessions
  • Read a set passage before the evening talk and discussion
  • Not for beginners—take Reading Plato before doing this course


This class will run again subject to demand. Make a request, watch this space for details or subscribe to our blog.


Finding a translation of Plato’s Theaetetus

Finding a translation in eBooks is easy. For the Penguin Paperback eBook (ISBN: 9780140444506), see here for options. The scholarly Greek/English LOEB translation is 100 years old, out of copyright, and so available for free in PDF (e.g., here).

Finding hardcopy in Australian bookstores is not so easy. The Penguin paperback (ISBN: 9780140444506) is now available from Readings at our request. Otherwise, try fishpond or Book Depository. For the Oxford translation (ISBN 9780199646166), try fishpond or booktopia.

Course outline

First Session | Framing the question | page 144 to 151e

The scene opens at a gymnasium where Socrates and the geometer Theodorus are chatting. The question of epistemology is immediately raised when Theodorus’s prize student Theaetetus arrives.

What does Socrates mean when claiming the role of midwife?

Second Session | Knowledge is perception | page 151e to 161a

The famous sophist Protagoras claimed that ‘man is the measure of all things’ and that what we each perceive in the senses is what we know.

How does Socrates relate this doctrine to his doctrine of ‘being and becoming’?

Third Session | Is reason your master or your slave? | page 161a to 177c

The investigation of the doctrine ‘to perceive is to know’ leads to the view that perceptions are what is real. In finding this view contradictory, the very methodology of investigation comes into question.

How does the philosophical attitude to argument compare to that of the advocate or lawyer?

Fourth Session | Between Heraclitus and Parmenides | page 177c to 187a

The conversation turns from the epistemology of Protagoras to the doctrine attributed to Heraclitus that all is in a constant state of flux. A cryptic reference to Plato’s developments on the scepticism of Parmenides (explicit in his Parmenides and Sophist) leads back to a discussion of perception and how formal opposites are involved at a very elementary level.

What are the formal opposites introduced by Socrates and where have we come across them before?

Fifth Session | True and false opinion | page 187b to 201c

Perhaps knowledge is opinion that is judged to be true, true to what is. But this raises the question not only of how to make this judgement, but about the very nature of opinion that is false.

On what basis does Socrates deny that there can even be false opinion?

Sixth Session | Justified true belief | page 201c to 210

Knowledge might be true belief that is justified by a rational account (logos). But then that which is elementary to knowledge might be the very foundation of truth and yet too elementary to be expressed in an account. Moreover, if an adequate account of something is to present its distinguishing characteristics, then this would only be saying what must already have been found as distinctive of the thing before making this rational justification.

The finding of the difference of things, their distinguishing characteristic, will be what the Platonic ‘method of division’ is all about. This method is presented in the following dialogue, the Sophist, but can you see how ‘making the right distinctions’ has already come to the fore at the end of this famously inconclusive dialogue?