Theaetetus, who we know from the opening scenes of the dialogue is a young man of rare courage, rises to the challenge. At once, he asserts that a person who knows something perceives that he knows it, and so ‘as it appears at present (hōs ge nuni phainetai), knowledge is nothing other than perception’ (151e). We should note here that Theaetetus sensibly qualifies his statement – it may ‘appear’ to be the case at this stage of the debate, but it may not persist in appearing that way. Still, as we know it takes confidence to make a positive statement in front of Socrates, so we are cheering him on!

Indeed, Socrates also cheers and encourages the young man: ‘Good! Excellent, my boy (Eȗ ge kai gennaiōs, ō paȋ)! That is the way one ought to speak out.’ True to form, though, he warns that they need to examine Theaetetus’s suggestion to see if it should prove to be a genuine production – gonimon, ‘offspring’ – or just wind – anemaȋon.

Socrates recalls that the sophist Protagoras – a prominent intellectual who features in other Platonic dialogues – used to give a similar definition of knowledge, expressed differently: ‘For he says somewhere that man is “the measure of all things, of the existence of the things that are and the non-existence of the things that are not”’ (152a). In other words, for each person, things are as they appear (phainetai once more) to him or her. But can this be the case?

After all, as Socrates goes on to describe, sometimes one person in a group will feel cold when the wind blows, while another does not. I was at a lunch the other day: two of the people at our table were feeling hot, growing red in the face, and complaining that they were sure the air-conditioning hadn’t been turned on – when, if anything, I felt a little on the cool side. How can this be? How can the same ambient temperature appear (phainetai) so differently to different people? Can the experience of one or more people be a guide to how things really are?

As Socrates says, we can either say that the wind is in itself cold or not cold – or we can accept Protagoras’s statement, that it’s cold for the person who feels cold, and not for the person who does not. Theaetetus cautiously agrees (152b) – and his caution turns out to be well justified, for now Socrates begins to show that he thinks the Protagorean position cannot be correct.

In fact, Socrates declares, maybe the great sophist only uttered it to the masses, while telling the truth in secret to his students (152c). (These students, let’s remember, or rather their parents, will have paid good money to be taught by Protagoras – whereas Socrates readily presents his views for free.) Theaetetus is taken aback: ‘Why, Socrates, why do you say that (Pȏs dē, ō Sōkrates, toȗto legeis)?’

Because, Socrates explains, if – as Protagoras’s position seems to commit us to – everything is as it appears, then the same thing will be both light and heavy; both large and small. ‘Nothing whatever is one, either a particular thing or of a particular quality; but it is out of movement and motion and mixture with one another that all those things become (gignetai) which we wrongly say “are” (panta ha dē phamen eȋnai) – wrongly, because nothing ever is, but is always becoming’ (152d-e). Many philosophers and poets have suggested the same thing. Is this what Protagoras means?

Again, with trepidation, Theaetetus assents. Seemingly in support of such a reading of Protagoras, Socrates goes on to give examples of things which only exist, or are only preserved, as appears, through motion. Fire is created through movement and friction; animals come into being in a similar way. Health is compromised by inactivity, preserved by exercise; and the soul itself benefits from learning and practice, but without practice and study, it learns nothing and forgets what it learned before (153a-c).

So far, so plausible – but we know enough about Socrates by now to know that this is unlikely to be the full story. As so often in Plato’s dialogues, the worldview of a prominent sophist is going to be dissected, and will probably be found wanting in one or another respect. Like Theaetetus, we have a feeling that we have a ringside seat to a master class in analysis – and we’re cautious about committing to a view just yet. The state of things ‘as it appears at present’ may be both short-lived and misleading!