The dialogue begins with a philosophical encounter in Athens where the youthful Socrates engages with the aging Eleatic philosophers Parmenides and Zeno. Zeno defends his paradoxes against the multiplicity of things by arguing that assuming the many leads to greater absurdities than assuming the one. Socrates suggests that while visible objects can easily partake of both likeness and unlikeness, or unity and plurality, the true challenge lies in testing whether the abstract ideas themselves harbor such contradictions.
Parmenides then subjects Socrates to a rigorous critique of the theory of Ideas. He questions whether individuals partake of the whole idea or only a part, and points out that the attempt to define absolute ideas leads to an infinite regress. Furthermore, Parmenides argues that if absolute ideas exist apart from the human sphere, they must remain entirely unknown to human consciousness, and divine knowledge cannot encompass human affairs. Despite these immense difficulties, Parmenides insists that abandoning the ideas entirely destroys the capacity for reasoning and philosophy.
To train Socrates for this arduous task, Parmenides demonstrates a complex method of logical exercise. He takes his own hypothesis of the one and meticulously deduces every possible consequence. The investigation splits into two main branches: first, examining what follows if the one exists, considering both its nature as an isolated unit and its relation to being and plurality; second, examining what follows if the one does not exist, both for the one itself and for the remaining things.
Through an intricate web of deductions, Parmenides shows that if the one is, it must simultaneously be and not be, move and rest, be like and unlike, and even be equal and unequal to itself and others. Every positive assertion about the one generates its own negation and paradox. The second part applies the same exhaustive treatment to the things other than the one, demonstrating that they too experience every opposite affection.
The work concludes with the sweeping realization that whether one assumes the one to be or not to be, all things in relation to themselves and each other appear to be and not to be in every possible way. Rather than descending into mere skepticism, the dialogue serves as an indispensable logical purgation. It forces the mind to confront the hidden contradictions within its own foundational concepts, preparing the ground for a more mature and rigorous philosophy.
