The first of Theaetetus' three successive definitions of knowledge that knowledge is 'perception' is not finally 'brought to birth' until Socrates has linked it to Protagoras' famous 'man is the measure' doctrine of relativistic truth, and also to the theory that 'all is motion and change' that Socrates finds most Greek thinkers of the past had accepted, and until he has fitted it out with an elaborate and ingenious theory of perception and how it works.