Few philosophical disputes have proven as durable as the one between Plato and his most gifted student, who spent some twenty years working alongside him in the Academy. In the first book of the Metaphysics, Aristotle surveys earlier thinkers and then turns on the theory of Forms with unusual sharpness. He does so while still speaking in the first person plural, of the ways in which “we” prove that the Forms exist.

The theory under attack held that wherever many things share a character, such as beautiful bodies, just acts or large objects, there exists a single, unchanging Form in which they all participate. That Form explains why the many are what they are. It alone is fully knowable, while its sensible instances shift and decay.

Aristotle judged this picture both incoherent and idle. He set out his objections in the Metaphysics and in the lost treatise On Ideas, which survives through Alexander of Aphrodisias. These objections did not reject form as such. They were an argument about where form lives, and that relocation reshaped Western metaphysics.

Aristotle gave the argument its name, but its first appearance is in Plato's own Parmenides (132a–b), where the aged Parmenides presses a young Socrates. Take several large things. Because they share a character, the theory posits a single Form, Largeness itself, standing over them. Socrates readily accepts this, since it is the very reason Forms are introduced.

Now consider the large things together with Largeness itself. Platonic language often suggests that a Form possesses its character perfectly. If so, Largeness is itself large, and this new group also shares a character. By the same reasoning, a second Form of Largeness must stand over the whole group. Repeat the step and the regress never ends. Aristotle's version uses man: a particular man, the Form of Man, and a “third man” over both.

In a celebrated 1954 paper, Gregory Vlastos isolated the premises doing the work. The first is One over Many: any set of things that are F has a Form of F over it. The second is Self-Predication: the Form of F is itself F. The third, which Vlastos argued Plato needed but left unstated, is Non-Identity: whatever is F by participating in a Form is not that Form. Taken together, these premises contradict the claim that each character has exactly one Form.

The threat is serious because the theory's appeal lay in uniqueness. A Form was meant to be the one thing that explains the many. If each Form generates an infinite series of further Forms, it explains nothing finally, and knowledge of “the” Form becomes unattainable. Plato left the objection unanswered in the dialogue. Aristotle concluded that it exposed a structural flaw in the theory rather than a puzzle awaiting a clever reply.

Takeaway

An explanation built on the same pattern as the thing it explains will itself need explaining. A regress is often a sign that a principle has been treated as just one more instance of its own kind.

Aristotle's second line of attack is less formal and arguably more damaging. In Metaphysics A.9 he asks what on earth the Forms contribute to sensible things, whether eternal ones or those that come into being and pass away. His answer is blunt: they cause neither movement nor any change in them.

The complaint should be read alongside his account of explanation in Physics II.3. There, a full answer to “why?” may cite four things: the matter, the form, the source of motion and the end. A Form is changeless and separate, so it cannot set anything in motion. The Phaedo had offered the “safe” answer that beautiful things are beautiful by the Beautiful. To Aristotle, this merely restates what needs explaining. It says nothing about how a particular thing came to be beautiful, or why it stopped being so.

He presses further. To call Forms patterns in which things participate is, he writes, to use “empty words and poetic metaphors.” Plato never analyses participation. Worse, the Forms duplicate the world. Aristotle compares the Platonist to someone who cannot count a few objects and supposes the task will be easier once their number has been doubled. A second realm only multiplies what needs explaining.

Against this, Aristotle offers a plain observation: man begets man (Metaphysics Z.8, Λ.3). The cause of a new human being is an existing human being, who already has the relevant form in suitable matter. No separate paradigm is needed. What is actual comes before what is potential, and change is explained by what is already present in nature.

Takeaway

A genuine cause should make a difference to what happens. If a proposed principle would leave every event unchanged whether or not it existed, it may be a description dressed up as an explanation.

It would be a mistake to read Aristotle as a nominalist who did away with form. His own philosophy is full of it. What he rejected was separation, which the Greeks called chōrismos. In Metaphysics M he credits Socrates with seeking universal definitions without making them separate. It was Plato, he says, who took that further step, and that step was the error.

In the Categories, primary substance is the individual: this man, this horse. Species and genera are secondary substances, real only as things said of individuals. The later Metaphysics complicates matters by naming form itself “primary substance” in Book Z. Even there, though, form is always the form of something: the organisation of matter into a functioning whole.

Aristotle's analysis is called hylomorphic, from hylē (matter) and morphē (shape). A bronze sphere is bronze arranged spherically. Neither the bronze nor the sphericity is a sphere on its own. A living body is matter with a soul. In De Anima, the soul is defined as the first actuality of a natural body that potentially has life. Form can be separated in thought and in definition, but not in existence.

This relocation answers both earlier objections. The regress cannot start, because form is not one more thing alongside particulars that would need a further form of its own. It is what makes a particular the kind of thing it is. Form can also explain change, since it is present in the agents and materials that produce new things. Yet Aristotle's unmoved mover, a pure form without matter, shows that the Platonic inheritance never entirely left him.

Takeaway

Where a principle is located shapes what it can do. By moving form out of a separate realm and into things themselves, Aristotle turned an object of contemplation into a means of explaining the natural world.

Aristotle's critique came from inside the Academy, and its targets were ones Plato himself had glimpsed. In the Parmenides, the teacher raises the Third Man against his own theory. The student treated it as decisive.

The result was not the abandonment of form but its naturalisation. Form became the intelligible structure of individual things, reached through observation and definition rather than through recollection of a separate realm.

The dispute still frames contemporary debates. Arguments over whether properties exist apart from their instances or only in them restage it, as do disputes over whether laws of nature govern events or merely describe them. Reading Aristotle against Plato clarifies what is at stake in either answer.