Parmenides

"Parmenides is one of Plato's most challenging and interesting dialogues.
By means of a conversation with the aged philosopher Parmenides, Plato conducts a detailed critical examination of a central tenet of his own philosophy, the Theory of Forms. Parmenides then introduces a series of exercises in dialectic centered on the idea of 'the one'. Many scholars contend that this critique and subsequent intellectual exercise is designed to pave the way for a more mature understanding and defense of the Theory of Forms, but it continues to be a subject of much speculation and fascination.
Despite or perhaps because of its complexity, Parmenides is a key work illuminating the later thought of one of the world's most influential philosophers.