160. Hence, there are two parts in every natural science; the theoretic, and the experimental. The former is founded on necessary ideas, the latter on contingent facts; the first without the second, would not come down to the real world; the second without the first, would not rise to the regions of science. 161. The natural sciences merit the name of science, in proportion to the quantity of necessary elements which they contain, and the closeness of the connection by which they unite with them contingent facts. But as no natural science can be conceived, without contingent facts, so there is none entirely free from the contingency which they communicate. 162. These observations reveal a great simplicity in the elements of science, and we may push this simplicity much farther, if we recollect what has been said when analyzing the ideas of number and time. 163. The idea of number arises from the idea of being and not-being: the same is also true of the idea of time; therefore, at bottom, these ideas are but one, though presented under different aspects. 164. Hence, all the natural and exact sciences may be reduced to two elements: the intuition of extension, and the general conception of being. Extension is the basis of all sensible intuitions: externally, it is a necessary condition of the relations which we conceive in the corporeal world; internally, it is a perception, without which the sensibility could not represent external objects. The conception of being, is the basis of all conceptions; developed in different ways, it generates the ideas of number and time; and these, combined with extension, constitute the necessary part of all the natural and exact sciences.