INTRODUCTION TO THE THEAETETUS

motion. Here Plato distinguishes two kinds of motion—movement in space and change of quality—and asserts that constant motion of the first kind must be accompanied by change, because otherwise the same things would be at the same time both in motion and at rest. This obvious fallacy Plato appears to ascribe to Heracleitus and his school. The result of this discussion is that if nothing is at rest, every answer on whatever subject is equally correct.

The possibility of false opinion is discussed in connexion with the second definition. This part of the dialogue contains many subtle distinctions and interesting comparisons. The errors of memory are illustrated by the wax tablets which, on account of their imperfections, fail to receive and preserve clear impressions from sensible objects, and the confusion of our recollections by the aviary, the possessor of which takes in his hand one bird when he wishes to take another, though all the birds have previously been caught and imprisoned by him.

The third definition is explained in various ways, none of which is found to be satisfactory, and the dialogue closes with its avowed purpose—the complete definition of knowledge—unaccomplished. Nevertheless the rejection of the definitions proposed is a gain in itself, and the dialogue may be said to prepare the way for the acceptance of the theory of ideas. It serves also as an example of the importance of the dialectic method, and shows Plato’s interest in combating the theories of other philosophers.

The Theaetetus contains many interesting similes and comparisons, and is, like the Sophist and the