I just happened to have the SEP article for Parmenides [1] open. It makes the claim that Plato was strongly influenced by Socrates, Pythagoras and Parmenides.

I just happened to have the SEP article for Parmenides [1] open. It makes the claim that Plato was strongly influenced by Socrates, Pythagoras and Parmenides. It isn't easy to understand Parmenides and there are conflicting interpretations of his arguments. But he seems to be one of the first philosophers to connect that which can be thought (or understood) with that which is real through the abstract concept of "being" (ontology). [2] In some sense, mathematics is just a formalization of the attempt to capture the totality of "what may be conceived of by thought". 1. https://plato.stanford.edu/entries/parmenides/ https://plato.stanford.edu/entries/parmenides/ 2. https://en.wikipedia.org/wiki/Ontology#History https://en.wikipedia.org/wiki/Ontology#History (as an aside, one of the reasons we even have the thought of Parmenides is because Simplicius wrote out a large excerpt of his work in one of his own books)