On Nature (the poem of Parmenides)

A philosophical poem in Greek hexameters, traditionally titled On Nature, of which about a hundred and sixty lines survive, mostly through later quotations. In the proem a young man is carried by chariot, guided by the daughters of the Sun, to the gates of Night and Day, where a goddess welcomes him and promises to teach him both the truth and the opinions of mortals, in which there is no true reliance. The first part, the Way of Truth, offers two paths of inquiry: that it is, and that it is not. The second cannot even be thought or spoken, for thinking and being are the same. From this the goddess argues that what is cannot come into being or pass away, is undivided, unchanging and complete, like a well-rounded sphere. Mortals, who mix being and not-being, wander two-headed. The second part, the Way of Opinion, gives a cosmology built on light and night, which survives only in fragments.

It is often called the first sustained argument in Western philosophy, and its claim that what is cannot change set the problem that Plato, Aristotle and the atomists all had to answer.