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Why did Aristotle reject Plato’s separate realm of Forms?

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Marble bust of Aristotle with a beard and draped robe, showing detailed facial features and wavy hair, set against a dark background. — Aristotle Aristotle, detail of a Roman copy (2nd century bce) of a Greek alabaster portrait bust (c. 325 bce); in the Museo Nazionale Romano, Rome.

Aristotle rejected Plato’s theory of a separate, suprasensible realm of Forms because he believed it failed to explain the reality of the physical world, particularly how things come into existence and undergo change.12

While Plato argued that particulars derive their nature by "participating" in eternal, immutable Forms existing outside of space and time, Aristotle contended that this merely introduced new entities equal in number to those already requiring explanation, effectively doubling the problem rather than solving it.12

Instead, Aristotle proposed that forms are immanent—meaning they exist only within matter as intrinsic principles of a substance. In his hylomorphic framework, form and matter are inseparable components of a composite whole. For Aristotle, the form of a thing is not an external ideal but the internal structure that makes a substance what it is, allowing him to focus on the "here and now" and the biological processes of growth and change.314