A living organism, Kant argues, is a 'natural purpose' — an entity in which each part exists for the sake of every other part and for the whole, while the whole exists for the sake of the parts. This mutual dependence cannot be captured by the purely mechanical model that governs physics: in a machine, parts are assembled by an external designer; in an organism, the parts generate and maintain the whole from within.
Kant does not reject mechanical explanation — he holds that it is constitutive of natural science. But he argues that no advance in mechanics will ever explain, from the inside, why an organism has the structure it does. Teleological concepts are required as a regulative supplement: we must think of the organism as organised for purposes even as we seek mechanical explanations of its parts.
The teleological judgment of nature is, for Kant, an expression of the limits of the human understanding. We cannot help thinking of organisms teleologically, but we cannot claim that teleology is a feature of nature itself — it reflects the way our cognitive faculties must approach certain objects. This restriction anticipates later debates about the autonomy of biology and the relationship between functional and causal explanation.
Natural purpose and teleological judgment are the subjects of the Critique of Teleological Judgment, §§61–91 of the Critique of Judgment.
