Realism claims that the physical world exists outside of one’s own perception. Think about the words you are reading on this page. The page is filled with lines that your mind then recognizes as letters and words. To someone who never learned to read, these words would only be what they are—an assortment of lines and curves on a page. Realism claims that words are, at their core, lines on a page. Only through the work of our minds do these lines become something more.

The philosophical debate surrounding realism dates back to a break in thought between the ancient Greek philosopher Plato (c. 428–348 BCE) and his prodigal student Aristotle (384–322 BCE). Plato believed two types of worlds existed: a spiritual or mental world and a world of appearance. For Plato, the spiritual world was the only real truth. This world was eternal, unchanging, and could be understood through the reasoning of the mind. The world of appearance was based in the five senses and was unreliable and disorderly. Plato believed these two worlds were entirely independent of one another. Aristotle’s contribution to realism was building upon Plato’s teachings and combining these two worlds.

Aristotle’s metaphysics contributes to the viewpoint of realism by expanding upon the interaction between universals and particulars. A universal is an abstract idea like “education” or “science.” Universals cannot be directly perceived by our senses. Instead, they must be conceptualized as an idea in the mind.

In contrast, a particular is something physically concrete—like a cookie, which one can see, feel, taste, touch, and smell. Plato held the idea that universals exist regardless of the existence of a particular. However, Aristotle argued universals cannot exist on their own and rely on the existence of particulars. Consider this analogy in Bertrand Russell's The History of Western Philosophy:

Suppose I say, “there is such a thing as the game of football,” most people would regard the remark as a truism. But if I were to infer that football could exist without football-players, I should be rightly held to be talking nonsense. Similarly, it would be held, there is such a thing as parenthood, but only because there are parents. (163)

For this example, the game of football is a universal, while the football players are particulars. Football cannot exist without football players.

Aristotle contributed to realism by grounding it in the existence of particulars (i.e., the physical world). Unsurprisingly, Aristotle was the father of the scientific method—a mode of experimentation reliant on careful observation of the physical world. To this day, Aristotle's teachings on realism anchor the fields of science and mathematics.