Heraclitus, theorized that opposites flow into one another and that the natural world is in a constant state of movement and change.

The sophists were ancient Greek teachers and are known for their focus on the individual and their place in the world. They claimed that we cannot know the truth about the nature of the universe because we cannot tell the difference between what is natural and what is socially induced. They were concerned with ethical matters and believed that morals and societal behavior vary among different contexts (as in city-states and generations). They found that mastering the art of rhetoric—that is, of speaking convincingly—was valuable in their talks and teachings.

Because Plato wrote down Socrates's teachings, it is sometimes difficult to distinguish between their ideas. By what became known as the Socratic method, Socrates would begin by asking questions of his opponent, which he used to expose his opponent's weakness and prove his own argument's validity. Like the sophists, Socrates knew his argument in advance but used the context of a conversation to prove it through rhetoric and logic.

Plato, on the other hand, helped to define a world of abstract ideas with his theory of forms. This theory explains that the reality behind the material world is filled with timeless forms from which everything in the universe descends. He claimed that the sensory world is not lasting; therefore, we cannot look to it for answers. The only way to true knowledge, in Plato's view, is through reason and the permanent and eternal world of ideas.