The concept of proper natural science is introduced in the Preface to the Metaphysical Foundations of Natural Science (1786), and it has attracted increasing interest in recent scholarship on Kant’s philosophy of physical science. I shall argue here, however, that it also plays a central role in the critical system as a whole as it develops from the first edition of the Critique of Pure Reason (1781) to the Critique of the Power of Judgement (1790) and beyond. In particular, there is an ongoing entanglement between the physical science of Kant’s day (including, especially, the science of physical chemistry) that eventually extends to all of the central aspects of his critical system – from the necessity of particular empirical causal laws to the relationship between mechanism and teleology and even to Kant’s most developed treatment of the relationship between theoretical and practical reason at the end of his critical period.