Parmenides of Elea thought extensively about the nature of reality. He argued that there are only two paths for inquiry: either what-is is, or what-is-not is. He concludes that what-is is necessary. Indeed, it is a contradiction to say that what-is-not is. Moreover, he says that one cannot know what-is-not because it does not exist. Contradictions defy the rules of reason, and so Parmenides states that the contradictory path cannot be investigated.
Logically, Parmenides has presented a disjunction with tautological necessity. He presents what is and its opposite, and then says that either one or the other is true. This disjunction is necessarily true because if either of his propositions is false, then its opposite is true, in which case the disjunction is true. In order to show that what-is is the only valid option, he assumes that we can know things which are. He then proceeds to state that what-is-not is not knowable. If things which are can be known and what-is-not is not knowable, then what-is-not is not. This leads to the conclusion that what-is and not what-is-not are true. In other words, what-is is necessary.
Next, he states, "Just one story of a route is still left: that it is. On this there are signs very many, that what-is is ungenerated and imperishable, a whole of a single kind, unshaken, and complete" (Curd, 59). Being, because it is the substance of all that is, could not have come from something else which exists because causes precede effects, and being must be coextensive with anything which is. Anything which could generate being would participate in it, and so what-is must be ungenerated. He also says, "Now was it ever, nor will it be, since it is now, all together one, holding together" (Curd, 59). This seems to contradict experience, but it follows from the logic of his argument. Consequently, either Parmenides' propositions are false, or our experience deceives us about the nature of reality. Perhaps this is why he says, "do not let habit, rich in experience, compel you along this route to direct an aimless eye and an echoing ear and tongue, but judge by reasoning the much-contested examination spoken by me" (Curd, 59).
Hence Parmenides provides a well-reasoned and non-contradictory account of being. He shows that it is necessary and ungenerated. However, his account implies that human experience obscures the true nature of reality. For Parmenides, the world may not be as it seems. Nonetheless, Parmenides concludes that the question of is or is not has been decided by necessity, and that it is not possible to think that non-being is.