Plato’s metaphysics grew out of that of Parmenides, together with a strong feel for Heraclitus’s account of the physical world as a world of incessan… - Catherine Rowett

Besides the ‘how many?’ question, Empedocles seems to be answering two other ancient questions: ‘How did the world come to be as it now is?’ and ‘How did it come to have the creatures that it now has?’. [...] His answers are subtle and intricate.

In the Protagoras Socrates persuades Protagoras that goodness is identical with pleasure. He advocates a form of hedonism. In the Gorgias, Callicles espouses hedonism and Socrates refutes him. Socrates gets Callicles to admit that, after all, some pleasures are not good. [...] So Socrates holds contradictory views on pleasure in the Protagoras and the Gorgias.

Even if Melissus’s analysis of the concept of existence is faulty, his procedure is very interesting. He challenges the data of sense experience by appealing to conceptual truths, facts about what a certain predicate (here ‘true’) must entail. These facts seem to escape the need to appeal to sense experience. We check up what is true about being true by examining our notion of being true, not by checking any things in the external world. So the argument seems to find a way of challenging the value of sense experience without begging the question. Melissus casts doubt on the senses by privileging the logical grammar of the word ‘true’. But, we might ask, did we learn how to use the word ‘true’ without relying on the senses?