Parmenides was an ancient Greek philosopher born in Elea (a city in southern Italy) in 510 BC. He is best known for discussing the concept of being. What exists and what does not exist, and what can we say about it? He was opposed to the writings of Heraclitus, who believed that the elements of the world are constantly changing into one another, instead putting forth that there is only one thing that exists and that is reality itself. He is famous for his use of deductive logic (the practice of figuring out what conclusions logically follow from a statement) and for his view of the senses as deceptive.

Like Heraclitus, Parmenides was an aristocrat. But unlike Heraclitus who didn’t like his fellow citizens and eventually went off to live in the mountains, Parmenides was well respected in Elea and wrote laws for the city that were in use for hundreds of years. He was also respected for how he lived, morally and contemplatively, which became known as the “Parmenidean life.” He wrote poetically in his work On Nature to show others what he believed was divine wisdom.

Parmenides discusses what he calls the Way of Truth and the Way of Opinion. The foundation of the Way of Truth is that “it is.” Parmenides realized that while many things are doubtful, no one can doubt that reality exists. “What is, is.” In philosophy, this is known as the principle of identity. Likewise, if “what is, is” then “what isn’t, isn’t.” We cannot say anything about that which does not exist, because it is nothing. The only thing we can talk about is that which exists, and all we can say of it is that it exists.

Parmenides believed that whatever we can think of must have existence. Because we can think of reality, but cannot think of unreality, reality exists and unreality does not exist. While a mermaid might not exist, our thought of a mermaid means that the “thought of a mermaid” exists. In this way, he maintains philosophical positions based on a rational process of logical deduction (that which follows from an initial statement). The reason people make mistakes, following the Way of Opinion rather than the Way of Truth, is that they think things are true based on what they experience. While practical, he sees the senses as untrustworthy for understanding the truth.

Parmenides believed that what is could never not have been. This is because something that exists could not have come from something that does not exist. Likewise, something which exists cannot become nonexistent. In this way, he understood the world to be eternal. As such, he denies the possibility of the world having been created (cosmogony). He also denies the possibility of change (i.e. becoming) because how can something become something else when things are whatever they are and are not whatever they are not?

In this way, Parmenides comes to understand that there is but one truth: “it is.” Reality is one thing. Reality cannot be divided into separate things, like elements, because how can reality be more than one thing? Anaximenes, the Milesian philosopher, believed air to be the fundamental principle of the world and that change was accounted for through the rarefaction and condensation of air into the other elements (earth, water, fire). But Parmenides denies the possibility that something could become finer or dense, because how could something be more or less of itself? What is, is, no more no less. Change is an illusion, the Way of Opinion obtained through the senses.

Parmenides did appreciate the practical value of “non-ultimate” truths. He felt the senses can help us to be successful in life, they simply cannot provide us with knowledge of the ultimate truth of reality. In this way, Parmenides critiques the cosmologies (“studies of the world”) put forth by the philosophers before him including Thales, Anaximander, Anaximenes, and Heraclitus. His use of deductive reasoning would continue in the thoughts of his student Zeno and his famous paradoxes.