Some call this poem-treatise the beginning of metaphysics, and qua metaphysics it is certainly a great leap in thought from the Milesian and Pythagorean schools. By itself it can establish the true bite of pre-Socratic thought, and that of the ancient Greeks as well. The relatively fanciful natures of the popular works of the period, the Republic and the Nicomachean Ethics, can give the impression the Greeks were mere bandiers-about of words, not as far removed as they thought from sophistry. Yet Plato's Parmenides or Aristotle's Metaphysics, neglected by the tourist of philosophy, are no sophistries, but profundities, and profundities developed essentially in response to the crisis caused by Parmenides, and the flood of varying responses to that crisis.

That might overstate the role of this work a little, but it is not wholly untruthful. The poem is split into two parts, the Way of Truth and the Way of Seeming. The first part is the more important one, since it contains Parmenides' metaphysics, and a lot of it has been preserved through quotations by later writers. The second part is scantily preserved, and is where Parmenides describes how the world seems (but not how it is) in a similar fashion to Pythagorean cosmologies. His metaphysics is somewhat simple: Being is One, it is Unity, it does not move, it cannot be split into parts, it does not experience change, it did not come into being nor will it ever pass from being; this is all deduced logically. I read this work in Francis Cornford's Parmenides and Plato, which includes a commentary on both On Nature and Parmenides, and a commentary is especially helpful for filling in those sections which are lost. As well there is some great scholarship on trends in philosophy around that time, tracking the influences on and from both works.