چکیده (انگلیسی): Any great philosopher starts to think by reflection on previous philosopher`s thoughts and also Plato is a great philosopher who is founder of new theories in history of philosophy, so we can understand that he had a basic thought that had inspired from previous philosophers. In conclusion, it is obvious that in his theory of ideas, he is affected by three previous philosopher`s theories which were included: 1. Pythagorean`s theory of " things are numbers"; 2. Conception of unique and unchangeable being of parmenides; 3. Theory of definitions and universals related to Socrates. Plato has also used Herkelitos theory of " all of things always change" and Protagoras theory of " man is measure" to establish his epistemological relativism theory and finally in his teleogical explanation of world, he is affected by the Anaxagoras conception of Nous