The article concerns a widely ignored influence of Protagoras on Plato. My objective is to show that generally speaking there would be no Plato’s philosophy if it was not for the thought of Protagoras. The philosopher from Abdera influenced Plato on many levels. I started off with pointing out that Protagoras “gave” young Plato directions towards his crystallised philosophy. Consequently, I argue that Plato created his world of infinitely existing ideas in opposition to Protagoras’s epistemology. Moreover, in order to prove that sophists are not philosophers, later on Plato abandoned his so-called middle philosophy and once again changed his views, forming his late philosophy of five principles.