Parmenides had a theory of existence that was based upon the notion that all things that exist "are" and all things that do not exist "are not." For Parmenides, "are not" makes no sense. Therefore, all things that exist "are." That is to say, all things "are," a present tense form of the verb. If they "are" (rather than were, are becoming, or will be), then they always are. Parmenides believed that since all things "always are," then all things are actually changeless, timeless, complete, all made up of the same stuff.
In "The Way of Truth" (Aletheia), from On Nature, most of which has been lost to history, Parmenides writes:
One way only is left to speak of, namely that it is. Along this way are many signs: that what-is is (i) ungenerated and indestructible, (ii) unique, (iii) unmoved and (iv) complete; it never was nor will be, since it is now, all at once one, continuous.
This monotheistic view that everything is the same substance, changeless and timeless does not correspond to the way humans perceive the world. In other words, the change humans see must be an illusion or alternate version formed by our fallible senses. This idea of duality, of appearance and reality, informed Plato's own ideas that there is the phenomenal world that humans experience (appearance) and there is the eternal world of Ideal Forms (reality), a changeless, timeless world of objects in their perfect state.