In contrast to Heraclitus’s understanding that “all things flow,” Parmenides advocated a fixed and stable reality. In many regards, Parmenides can be understood as the exact opposite of Heraclitus: he participated in his city’s government and had a reputation for making excellent laws for Elea in southern Italy. Whereas Heraclitus was an empiricist, Parmenides was a rationalist, relying on knowledge based on reasoning and inference. Perhaps the only point that both philosophers agreed on was that the universe had always existed and never had to be created. But whereas Heraclitus was the philosopher of flux par excellence, Parmenides wrote of a universe “whole and of a single kind and unshaken and perfect.” For Parmenides, reality is uncreated and timeless, a plenum, or complete fullness of all that is. It is motionless and the same everywhere, like a ball equally balanced in every direction from its center. This view contrasts with Heraclitus’s that reality consists of opposites in balance.

We have about 150 lines of a philosophical work in verse preserved and passed on to us via a transcription by Simplicius, a 6th-century Byzantine philosopher. The poem has a prolog and two sections: “The Way of Truth,” which concerns matters apprehended by reason, and “The Way of Seeming, or Opinion,” which deals with the senses. This second section begins, “Here I end my trustworthy discourse and thought concerning truth; henceforth learn the beliefs of mortal men, listening to the deceitful ordering of my words.” That’s not a great way to sell empiricism! From the titles of these two sections, we can see that part of Parmenides’s philosophical project is going to address the tension between some ultimate reality and appearances, between an ultimate truth and mere opinion. Parmenides is going to criticize those who spend their time thinking about opposites, which, it would seem, are mere appearances that don’t reflect the true unity of reality. He then goes on to condemn any reliance on the senses.

His distinction between reason and the senses is going to lay the foundation of much of the philosophy that comes after him, especially Plato’s. His poem is also one of the first attempts to employ reason and to reason from purely abstract logic to the nature of the world. Parmenides is sometimes called the father of idealism because his description of “The Way of Truth” seems to portray nonmaterial reality, though that may not necessarily be the best way to understand him.

Nevertheless, we need to understand something about both Heraclitus and Parmenides because they set the stage for Plato’s philosophical project, giving him a framework in which to develop his own rational system. Throughout the Republic, Socrates will sometimes allude to these two philosophers, and it becomes clear that he dismisses Heraclitus’s view, falling more into Parmenides’s camp of rationalism. But whereas Parmenides might not be an idealist after all, Plato is very much one.

Idealism is the philosophical view that reality is ultimately in some sense mental or dependent on the mind. According to idealism, the proper objects of knowledge are ideas. We typically contrast idealism with materialism, the philosophical view that reality is composed of material substance with physical characteristics, such as atoms or cells.

Socrates differs from the natural philosophers by moving philosophy away from the natural sciences and physics to the world of the mind. He also changes philosophy’s focus to ethics and virtue, how to live a good life, how to be happy. For Socrates, the project of ethics and knowledge go hand in hand: we have to know what the good is because once we know it, we will be it. If we know what the good is, it becomes impossible to do evil. All wrongdoing, then, has an epistemological basis: knowledge is virtue. No one knowingly does evil. This runs directly counter to the Christian understanding of sin: “Anyone, then, who knows the good he ought to do and doesn’t do it, sins” (James 4:17) or “I do not understand what I do. For what I want to do I do not do, but what I hate I do” (Romans 7:15).