86. In order to form determinate ideas of the parts of space, we must take a measure and refer to it: to form an idea of the parts of time we also need a measure. The measure of space is the extension of some body which we know: the measure of time is some series of changes which we know. To measure space we seek for fixed things, as far as possible; for the want of something better, men have recourse to the parts of the body, the hand, the foot, the yard, and the pace, which give an approximate, if not an exact measure. The exact sciences having advanced, they have taken for their measure the forty-millionth part of the meridian of the earth: time is measured by the motion of the celestial bodies, by the diurnal motion, the lunar, solar, and sidereal year. 87. The idea of number is necessary in order to determine space and compare its different parts: the same idea is necessary in the same manner to time. The discrete quantity explains the continuous. CHAPTER XII. RELATIONS OF THE IDEA OF TIME TO EXPERIENCE. 88. If time is nothing distinct from things, how does it happen that we conceive it in the abstract, independently of things themselves? How does it happen that it presents itself to us as an absolute being, subject to no transformation or motion, while within it every thing is moved and transformed? If it is a subjective fact, why do we apply it to things? If it is objective, why is it mingled with all our perceptions? Because it contains a necessity sufficient to be the object of science. The idea of time, whatever it may be, seems prior to all perception of transformation, the consciousness of all internal acts included. It is impossible for us to know any of these things, unless time serves as a receptacle in which we may place our own changes and those of others. 89. The idea of time is not the result of observation; for in that case it would be the expression of a contingent fact, and could not be the principle of science. We measure time with the same exactness as we do space, and it is one of the most fundamental ideas of the exact sciences, in so far as they have any application to the objects of nature. 90. It might seem to follow from this that the idea of time is innate in our mind; and that it is prior to all ideas, and even sensations; for both are necessarily involved in successive duration. 91. The necessity of the idea of time seems to prove that time is independent of transitory things; in this case we are obliged to convert it into a purely subjective fact, or else to grant it an objective reality, independent of that which is changeable. By the former we destroy it; by the latter we make it an attribute of the divinity. To deny time is to deny the light of the sun; to raise it to the rank of an attribute of divinity is to admit change in an immutable being. If we make it purely subjective, we deny it; if objective, we make it divine: is there no middle way?