Metaphysical view according to which every natural body consists of two intrinsic principles, one potential (namely, primary matter) and one actual (namely, substantial form).
It was the central doctrine of Aristotle 's philosophy of nature. He based his argument for hylomorphism chiefly on the analysis of change. If a being changes (e.g., from being cold to being hot), something permanent must exist that remains throughout the change; in addition, there must be an actual principle that differentiates the earlier from the later state. The permanent principle is matter, the actual principle form.