112. Time is not only conceived as a general order of change, or as a relation of being and not-being; but also as something fixed, which can be measured with exactness. Thus, before the creation of the world, we conceive not only an abstract order, or time, but a time composed of years, of centuries, or some other terms. But this, if we closely examine it, is only an idea in which we conceive the phenomena of experience under a general view, taking them out of actuality and contemplating them in the sphere of possibility. Neither the years nor the centuries existed when there was nothing by which they could be measured. If we imagine a sort of vague line of duration prolonged to infinity, abstracting it from the measure and the object measured, we become the sport of our imagination, and are entangled in contradictions from which it is difficult to extricate ourselves. 113. The pure and abstract idea of time admits no measure; it is a mere relation of being and not-being. The measure is possible only when the idea of time is combined with the phenomena of experience. Subject as we are to change, and situated amid beings as changeable as ourselves, we should certainly fall into the greatest confusion of our ideas, if in this ebb and flow of external as well as internal existences which appear to us, we had not the greatest facility in referring them to fixed measures, which are the thread that guides us in this labyrinth of continual variations. 114. Two things are required for this measure: first, a suitable phenomenon, and secondly, the idea of number. The common idea of time which serves for the ordinary purposes of life of these three elements: the pure idea of time, or the relation of being and not-being; secondly, a suitable phenomenon to which we apply this pure idea; and thirdly, the numeration of the changes of this phenomenon. Apply this observation to all the measures of time, and you will find these three elements always sufficient, but always indispensable also. 115. From this we deduce the necessity of time, even considered empirically; for it involves two ideas, the one metaphysical, and the other mathematical, applied to a fact. The metaphysical idea is the relation of being and not-being; the mathematical idea is number; and the fact is the sensible phenomenon, as, for example, the solar, or human motion. Metaphysics and arithmetic take charge of the absolute certainty; the fact observed answers for the experimental certainty; and as, on the other hand, this phenomenon is supposed to be certain, because, in case it were necessary we could abstract it from the reality, and attend only to the possibility; it follows that time, even considered empirically, may become the object of the exact sciences.