As the effect of this extreme scepticism, the surpassing of empiricist and abstractionist Logic had to be begun with the restoration of that Logic itself (because that which does not exist cannot be surpassed), that is to say, with the demonstration, against Hume, that the exact science of nature is possible. Such is the principal task of the _Critique of Pure Reason_, which contains the Logic of the natural and mathematical sciences, thought no longer by an empiricist, but by a philosopher who has surpassed empiricism and recognized that the concepts of experience presuppose the human intellect, which originally constructed them. Leibnitz had already travelled this road, when in a polemic against Locke he maintained that reflection to which Locke appealed, referred back to the innate ideas: for if reflection (he said) is nothing but "_une attention à ce qui est en nous et les sens ne nous donnent point ce que nous portons déjà avec nous,_" how can it ever be denied "_qu'il y est beaucoup d'inné en nous, puisque nous sommes, pour ainsi dire, innés à nous mêmes? Peut-on nier qu'il y ait eu nous être, unité, substance, durée, changement, action, perception, plaisir et mille autres objets de nos idées intellectuelles?_"[13] The _New Essays,_ in which theses and other similar themes were developed, remained for a time unedited, but appeared opportunely in 1765 to fecundate German thought, and acted upon Kant, together with English empiricism and scepticism, the latter giving the problem and the former almost an attempt at a solution. But the innate ideas of Leibnitz are profoundly transformed in the Kantian concept of the _category,_ which is the formal element and really exists only in the very act of judgment, which it effects. Mathematics are thus secured in their possession, no longer by means of the primary qualities of Locke, but because they arise from the _a priori_ forms of intuition, space and time. The natural sciences are also secured, because the concepts of them are constituted by means of the categories of the intellect, on the data of experience. In other words, mathematical and natural science have value, in so far as they are a necessary product of the spirit. [Sidenote: _The limits of science and Kantian scepticism._] But a limitation of value due also to Kant, accompanies this theoretic reinforcement of exact science. That science is necessary, because produced by the categories; but the categories cannot develop their activity except upon the data of experience; so that exact science is limited to experience, and whenever it makes the attempt to surpass it, it becomes involved in antinomies and paralogisms and gesticulates in the void. Science moves among phenomena and can never penetrate beyond them and attain to the "Thing in itself." [Sidenote: _The limits cf science and Jacobi._]