§ 187. But what has become of the dream of the mathematical physicist? Is the whole system of Newton, that brilliant triumph of the mechanical method, unfounded and dogmatic? It is the logical instability of this body of knowledge, made manifest in the well-founded scepticism of Hume, that rouses Kant to a reëxamination of the whole foundation of natural science. The general outline of his analysis has been developed above. It is of importance here to understand its relations to the problem of Descartes. Contrary to the view of the English philosophers, natural science is, says Kant, the work of the mind. The certainty of the causal relation is due to the human inability to think otherwise. Hume is mistaken in supposing that mere sensation gives us any knowledge of nature. The very least experience of objects involves the employment of principles which are furnished by the mind. Without the employment of such principles, or in bare sensation, there is no intelligible meaning whatsoever. But once admit the employment of such principles and formulate them systematically, and the whole Newtonian order of nature is seen to follow from them. Furthermore, since these principles or categories are the conditions of human experience, are the very instruments of knowledge, they are valid wherever there is any experience or knowledge. There is but one way to make anything at all out of nature, and that is to conceive it as an order of necessary events in space and time. Newtonian science is part of such a general conception, and is therefore necessary if knowledge is to be possible at all, even the least. Thus Kant turns upon Hume, and shuts him up to the choice between the utter abnegation of all knowledge, including the knowledge of his own scepticism, and the acceptance of the whole body of exact science. But with nature thus conditioned by the necessities of thought, what has become of its externality? That, Kant admits, has indeed vanished. Kant does not attempt, as did Descartes, to hold that the nature which mind constructs and controls, exists also outside of mind. The nature that is known is on that very account phenomenal, anthropocentric--created by its cognitive conditions. Descartes was right in maintaining that sense-perception certifies to the existence of a world outside the mind, but mistaken in calling it nature and identifying it with the realm of science. In short, Kant acknowledges the external world, and names it the _thing-in-itself_; but insists that because it is outside of mind it is outside of knowledge. Thus is the certainty of science saved at the cost of its metaphysical validity. It is necessarily true, but only of a conditioned or dependent world. And in saving science Kant has at the same time prejudiced metaphysics in general. For the human or naturalistic way of knowing is left in sole possession of the field, with the higher interest of reasons in the ultimate nature of being, degraded to the rank of practical faith.