From a philosophical point of view, the so-called "Theory of Ideas" is the central topic in Plato’s dialogues; however in Plato's dialogues Plato's "perhaps only direct argument for the existence of forms [ideas] is in Timaeus 510D-E” (Charles Kahn).The purpose of the article is to analyse this argument and to answer the question: why did Plato in the "Timaeus" defend the existence of the Ideas despite the objections in the "Parmenides"? He defended it again because the latent presupposition of the apories in the "Parmenides", the substantial view of sensibles, is removed through the introduction of space as “substantialized extension”.
First (I) it is shown that Plato remained, in dialogues, like the "Sophist" and "Politicus", faithful to the “Theory of Ideas” despite his criticism in the "Parmenides". The common theme in the trilogy of the "Theaetetus", "Sophist" and "Politicus" is to refute relativism by showing that any relativism presupposes something absolute, that is, something like Platonic Ideas. The second part of the paper (II) examines closely the logical structure of the argument for the existence of Platonic Ideas in the "Timaeus" (51d3-52a7). The third part (III) shows how this argument can avoid the criticism of the Platonic ideas in the "Parmenides". In this criticism sensibles are treated as substantial entities. But, as the "Timaeus" shows, sensibles are not substantial entities but merely qualities, namely qualities of space, which is the only substance beside the Ideas.