Aristotle was Plato’s student. The three greatest philosophers ever, then, came one after another: Socrates, Plato, Aristotle.

It is absolutely impossible to overstate Aristotle’s genius. He wrote about meteorology, biology, science, logic, astronomy, philosophy, and even veterinary science. He was so good that for 2000 years he was known as “The Philosopher”, as if there were only one.

In this book, Aristotle discusses how a person should live.

The differences between Aristotle and Plato are quite striking. While Plato refers to an other-worldly, heavenly realm (the realm of The Forms), Aristotle says, in short, the good life is whatever good people do. Obviously, this is a bit circular (how do good people know what to do?). It is also just about right on the money if you ask me.

Aristotle is generally like that: a close-enough kind of guy. He knows that the perfect is the enemy of the good, and he is commonly taught as the first scientist or “empiricist”. Empiricists are people who look to the external world to discover truth.

Most people are empiricists now, so it may be helpful to consider some examples that throw empiricism into doubt. Imagine I asked you to tell me how many degrees are in the interior angles of a triangle. How would you go about it?

As an empiricist, you might get a protractor and measure a triangle. But this really doesn’t answer the question; among many other problems, you’ve only told me about one triangle, not all triangles. You’ll also never measure to exactly 180º, so you should say that triangles have only approximately that many.

A rationalist (the opposite of an empiricist) says that counting or measuring is uncertain or worse. There are ways to prove, without doubt, that all triangles have exactly 180º. And amazingly, astonishingly, incredibly, the proof can be done entirely in one’s own mind, without even drawing or measuring any actual triangles. The truth is found only through deductive reasoning.

The Basic Works of Aristotle (Modern Library Classics)Descartes, Plato and Kant (whom we’ll see later) are rationalists; they believe in a priori truth: questions are tested within the mind. Aristotle and Mill (later) are empiricists; they believe in a posteriori truth: questions are tested by experiment.

In this reading, Aristotle tells us what the good life is. Unsurprisingly, he looks outward to see what good people do. To Aristotle, the question of the good life, is that eternal question of philosophy: what is the meaning of life. He thinks that we have a purpose, and when we fulfil it, we are living well.

Aristotle died when he tried to swim two directions at once. I’m not kidding.