In this dissertation, I provide a new interpretation of Socrates’ treatment of the secret doctrine of Protagoras in the first part of the Theaetetus, promising to make sense of Socrates’ presentation of it, of its lengthy defence, and of its ultimate rejection. According to the proposed interpretation, the secret doctrine is an ontological theory comprising two theses:

(i) Ontological relativity Nothing is in itself one thing.

(ii) Heracliteanism Everything always changes under all respects.

The image of the world following from the secret doctrine and that following from our daily experience of the world, governed by perception, hugely differ. By choosing to take them both seriously, Socrates seeks to reconcile them and thus develops (iii) a theory of perception. Nonetheless, he realizes that (i), (ii) and (iii) form an inconsistent triad and consequently rejects (ii), leaving (i) and (iii) unscathed.