The object of this paper is to demonstrate an intimate coherence of thought on the problems of flux and language between the Cratylus, Theaetetus and Timaeus, and as a consequence to salvage the traditional view that the Theaetetus and Timaeus both propound versions of Plato's own theory of perception. This undertaking will involve grappling with views which were given their most forceful expression by Owen, but which have frequently been adopted and marshalled afresh since his classic article. The burden of these views is that the Cratylus and Theaetetus are engaged in analytical work which is more sophisticated than anything in the Timaeus; that this is supported by a number of points on which they are demonstrably correcting the Timaeus (and other earlier works); and that the similarities on the subject of perception, for example, are to be explained on the grounds that Plato in the Theaetetus is concerned to expose the essential incoherence of the basis on which a theory such as that of the Timaeus rests. It will be desirable, therefore, to examine first the passages on which this argument is principally based.