At Quora, someone recently asked me who was the greater influence upon Plato: Socrates or Parmenides? It is a fascinating question, to which I could only answer with some further related questions, a dash of context, and a joke. I enjoyed it all so much, though, that I will reproduce it here. To begin with, there is no good metric of “influence.” You can’t measure influence like inches or Celsius degrees and say X is exercising more of this unidimensional stuff than Y is. Who influenced John Dewey more: Charles Darwin [pictured] or William James? Except that in the Dewey case we have much more information. We know what Darwin’s views on a wide range of subjects were, and what James’ were, and we can work to understand how Dewey’s thought was formed by each. We don’t depend on Dewey to tell us what James said or thought! In the Plato analog, though, we do largely depend on Plato to tell us what Socrates said or thought. There are other sources who help at the edges — Ar...