151. The idea of time being the perception of the order between being and not-being, this relation, considered in general, belongs to the pure intellectual order. The transition to experience is realized in the same manner as in other general and indeterminate conceptions. 152. It is necessary to make a distinction between pure ideal time and empirical time: pure ideal time is the relation between being and not-being, considered in the greatest generality and the most complete indeterminateness; empirical time is the same relation subjected to a sensible measure. 153. To measure this succession, three things are necessary, and their union forms the idea of empirical time. They are, first, the pure idea of being and not-being, or of change; secondly, the application of this idea to a sensible phenomenon, as, for example, the solar motion; and thirdly, the idea of number applied to the determining of the changes of this phenomenon. 154. We thus conceive why empirical time implies a true necessity, and is the object of science. Of the three elements which compose it, the first is a metaphysical idea, the second, a mathematical idea, and the third, a fact of observation, to which these ideas are applied. If this fact be not real, it must, at least, be possible, in order to save the necessity of the calculation which is based upon it. 155. There is a close relation between the idea of time and the principle of contradiction. Each is explained by the other, yet this is not a vicious circle. The principle of contradiction consists in the mutual exclusion of being and not-being, and the idea of time is the perception of the order between being and not-being. Analysis must therefore lead to a part which is identical in both, to the comparison of the ideas of being and not-being. 156. Without the idea of time, memory would be impossible; consequently also, the unity of consciousness. CHAPTER XIX. A GLANCE AT THE IDEAS OF SPACE, NUMBER, AND TIME. 157. We may now mark out and determine with perfect exactness the necessary elements which form the object of the natural and exact sciences. This is not only curious, but highly important; for it presents under the simplest aspect, an immense field of knowledge, the limits of which expand, as we advance; so that, it is impossible to assign a limit to progress. 158. Space, number, and time, are the three elements of all the natural and exact sciences. All else contained in them pertains to mere experience, to the order of contingent facts, which involve no necessity, and cannot strictly be the objects of science. 159. Universal arithmetic is founded on the idea of numbers, geometry on that of space, and the idea of time places us in communication with the sensible world, so as to determine the relations of its phenomena. These phenomena are isolated contingent facts, and cannot become the object of science, until subjected to the general ideas of space, number, and time.