Aristotle arrived in Athens from the Macedonian city of Stagira at the age of seventeen to study at Plato’s Academy, and he remained there, as student and then as teacher, for twenty years, until Plato’s death in 348 BC. The relationship between the two men was one of the most productive intellectual partnerships in history, and one of the most complicated. Aristotle revered his teacher; he also thought his teacher was fundamentally mistaken on the questions that mattered most. The distance between Plato’s philosophy and Aristotle’s is not merely technical—it reflects a deep difference in philosophical temperament, in what counts as a satisfying explanation, in where one thinks the weight of reality actually rests.
Form in the World
Plato had located the truly real in a separate, transcendent realm of Forms, accessible only to philosophical reason and entirely independent of the messy, changing world of physical objects. Aristotle found this position, for all its elegance, incoherent. To explain why a particular horse exists and is a horse by positing an eternal Form of Horse in a separate realm was, he argued, to multiply explanations rather than to provide one. The Form of Horse was supposed to be what all horses had in common; but what did the Form share with the horses that participated in it? If there were a Form of Horse-thing-in-common, then a third entity would be needed to explain what the Form shared with the horses, and so on to infinity—a regress Aristotle called the “Third Man” argument. The explanatory burden that Plato had tried to discharge by positing separate Forms was simply relocated, not eliminated.
The alternative Aristotle proposed was hylomorphism: the doctrine that every physical substance is a compound of matter (hylê) and form (morphê or eidos). This was not Plato’s Form slightly modified; it was a reconception of where form resides. For Aristotle, the form of a thing is in the thing, not in a separate ontological realm. The form of a bronze statue is the shape and organisation of the bronze; it does not float free of the bronze in some transcendent space. The form of a human being is the specific organisation of the body—what Aristotle called the soul—that makes this particular arrangement of flesh and bone and blood a living, sensing, reasoning creature. Form is not elsewhere; it is here, doing real causal work in the world we can see and touch.
The Four Causes
This reconception of form was the foundation of Aristotle’s account of causation, which he elaborated into a doctrine of four causes—four distinct ways of answering the question “why?” For any given thing or event, a complete explanation will cite a material cause (what it is made of), a formal cause (what kind of thing it is, its structure and essence), an efficient cause (what brought it into being, the agent of change), and a final cause (the end or purpose toward which it tends). A statue is bronze (material), shaped as a human figure (formal), made by a sculptor (efficient), in order to honour a general (final). A child is flesh and bone (material), a human being (formal), generated by its parents (efficient), in order to fulfil the function of a rational animal (final). The insistence on the final cause—the telos, the purpose—is the most distinctively Aristotelian feature of this framework, and the one that generated the greatest subsequent controversy. Aristotle argued that to understand what something is, one must understand what it is for. A heart is not merely a muscular pump; it is an organ whose function is to circulate blood for the sake of the organism’s survival and flourishing. To explain the heart without reference to its purpose is to leave the most important part of the explanation out.
Virtue and the Golden Mean
When Aristotle turned from the structure of the natural world to the structure of the good life, the same teleological orientation shaped his answer. Human beings, like other living things, have a characteristic function. The function of a human being, Aristotle argued in the Nicomachean Ethics, is the activity of the soul in accordance with reason. The good life is the life in which this function is performed well—consistently, over time, in the face of the ordinary difficulties of human existence. Excellence in this activity is what the Greeks called aretê, usually translated as “virtue” but perhaps better rendered as “excellence of character.”
Virtue, for Aristotle, is not a matter of following rules or obeying commandments. It is a matter of habit. We become just by performing just actions, courageous by performing courageous actions, temperate by performing temperate actions. The virtues are not innate, but neither are they purely conventional. They are the stable dispositions that enable a human being to flourish as the kind of creature it is. And because human beings are social creatures whose flourishing depends on living well with others, the virtues are also the qualities that make a good citizen and a good friend.
The most famous and most misunderstood element of Aristotle’s ethics is the doctrine of the mean. Virtue, Aristotle argued, is a mean between two vices—one of excess and one of deficiency. Courage is the mean between cowardice and recklessness. Generosity is the mean between stinginess and prodigality. Temperance is the mean between self-indulgence and insensibility. The mean is not a mediocre average; it is the precise point at which the action or feeling is appropriate to the situation. And because situations differ, the mean is not a fixed rule that can be applied mechanically. It requires judgment, experience, and the cultivated capacity to see what a given circumstance actually calls for. The person who possesses this capacity in full is the person Aristotle calls phronimos—the practically wise human being.
The Systematic Vision
What distinguishes Aristotle from every philosopher who preceded him is not merely the content of his answers but the comprehensiveness of his inquiry. He wrote systematic works on logic, physics, biology, psychology, ethics, politics, rhetoric, and poetics. He founded the Lyceum, a research institution whose members collected data from across the known world and organised it into the first great encyclopaedia of natural and human knowledge. He tutored the young Alexander the Great, though the effect of that education on its pupil remains one of history’s more durable ironies. He died in 322 BC, a year after Alexander, having outlived his most famous student by less than a year.