when it launches into theory, it is bound to adapt its be- havior to the general form of practice. However high it raay rise, it must be ready to fall back into the field of action, and at once to get on its feet. This would not be possible for it, if its rhythm differed absolutely from that of action itself. Now action, we have said, proceeds by leaps. To act is to re-adapt oneself. To know, that is to eay, to foresee in order to act, is then to go from situation to situation, from arrangement to rearrangement. Science may consider rearrangements that come closer and closer to each other; it may thus increase the number of moments that it isolates, but it always isolates moments. As to what happens in the interval between the moments, science is no more concerned with that than are our common in- telligence, our senses and our language: it does not bear on the interval, but only on the extremities. So the cine- matographical method forces itself upon our science, as it did already on that of the ancients.
Wherein, then, is the difference between the two sciences? We indicated it when we said that the ancients reduced the physical order to the vital order, that is to say, laws to genera, while the modems try to resolve genera into laws. But we have to look at it in another aspect, which, more- over, is only a transposition, of the first. Wherein consists the difference of attitude of the two sciences toward change? We may formulate it by saying that ancient science ihiTikx ii knows its object sufficiently when it has noted of il some privileged moments, whereas modem science considers the object at any moment whatever.
The forms or ideas of Plato or of Aristotle correspond to privileged or salient moments in the history of things — those, in general, that have been fixed by language. They are supposed, like the childhood or the old age of a li\-ing being, to characterize a period of which they ejcpresa the
Science
quintessence, all the rest of this period being filled by the passage, of no interest in itKcIf, from one form to another fonn. Take, for instance, a falling body. It was thought that we got near enough to the fact when we characterized it as a whole: it was a movement downward; it was the tendency toward a centre; it was the natural movement of a body which, separated from tiie earth to which it be- longed, was now going to find its place again. They noted, then, the final term or culminating point (rlXoa, <fa/i^) and set it up as the essential moment: this moment, that language has retained in order to express the whole of the fact, sufBeed also for science to characterize it. In the physics of Aristotle, it is by the concepts "high" and "low," spontaneous displacement and forced displacement, own place and strange place, that the movement of a body shot into space or falling freely ia defined. But Galileo thought there was no essential moment, no pri-v-ileged instant. To study the falling body is to consider it at it matters not what moment in its course. The true science of gravity is that which will determine, for any moment of time what- ever, the position of the body in spsice. For this, indeed, signs far more precise than those of language are required. We may say, then, that our physics differs from that of the ancients chiefly in the indefinite breaking up of time. For the ancients, time comprises as many undivided periods as our natural perception and our language cut out in it successive facts, each presenting a kind of individuality. For that reason, each of these facts admits, in their view, of only a total definition or description. If, in describing I it, we are led to distinguish phases in it, we have several I facts instead of a single one, several undivided periods in- [ stead of a single period; but time is always supposed to be I divided into determinate periods, and the mode of division [ to be forced on the mind by apparent crises of the real,
loal^^^
comparable to that of puberty, by the apparent release of a new fomi. — For a Kepler or a Galileo, on the contrary, time is not divided obj ectively in one way or another by the matter that Blls it. It has no natural articulations. We can, we ought to, divide it as we please. All moments count. None of them has the right to set itself up as a moment that represents or dominates the others. And, consequently, we know a change only when we are able to determine what it is about at any one of its moments.
The difference is profound. In fact, in a certain aspect it 13 radical. But, fronti the point of view from which we are regarding it, it is a difference of degree rather than of kind. The human mind has passed from the first kind of knowledge to the second through gradual perfecting, simply by seeking a higher precision. There is the same relation between these two sciences as between the noting of the phases of a movement by the eye and the much more complete recording of these phases by instantaneous pho- tography. It is the same cinematographical mechanism in both cases, but it reaches a precision in the second that it cannot have in the first. Of the gallop of a horse our eye perceives chiefiy a characteristic, essential or rather sche- matic attitude, a form that appears to radiate over a whole period and so fill up a tune of gallop. It is this attitude that sculpture has fixed on the frieze of the Parthenon. But instantaneous photography isolates any moment; it puts them all in the same rank, and thus the gallop of a horse spreads out for it into as many successive attitudes as it wishes, instead of massing itself into a single attitude, which is supposed to flash out in a privileged moment and to iUuminate a whole period.
From this original difference flow all the others. A
science that considers, one after the other, undivided periods
of duration, sees nothing but phases succeeding phases,
fonns replacing fonns; it is content with a qualitative de- scription of objects, which it hkens to organized beinga. But when we seek to know what happeas within one of these periods, at any moment of time, we are aiming at something entirely different. The changes which are pro- duced from one moment to another are no longer, by the hypothesis, changes of quality; they are quantilative vari- ations, it may be of the phenomenon itself, it may be of its elementary parts. We were right then to say that modem science is distinguishable from the ancient in that it applies to magnitudes and proposes first and foremost to measure them. The ancients did indeed trj- experiments, and on the other hand Kepler trieti no experiment, in the proper sense of the word, in order to discover a law which is the very type of scientific knowledge as we understand it. What distinguishes modem science is not that it is experi- mental, but that it experiments and, more generaUy, worka only with a view to measure.
For that reason it is right, again, to say that ancient science applied to concepts, while modem science seeks laws — constant relations between variable magnitudes. The concept of circularity was sufficient to Aristotle to define the movement of the heavenly bodies. But, even with the more accurate concept of elliptical form, Kepler did not think he had accounted for the movement of planets. He bad to get a law, that is to say, a constant relation be- tween the quantitative variations of two or several elements of the planetary movement.
Yet these are only consequences — differences that follow from the fundamental difference. It did happen to the ancients accidentally to experiment with a view to measur- ing, as also to discover a law axpressing a constant relation between magnitudes. The principle of Archimedes is a true experimental law. It takes into account three variable
magnitudes: the volume of a body, the density of the liquid in which the body is immersed, the vertical pressure that is being exerted. And it states indeed that one of these three terms is a function of the other two.
The essential, originaJ difference must therefore be sought elsewhere. It is the same that we noticed first. The science of the ancients is static. Either it considers in block the change that it studies, or, if it divides the change into periods, it makes of each of these periods a block in its turn: which amounts to saying that it takes no account of time. But modern science has been built up around the discoveries of Galileo and of Kepler, which immediately furnished it with a model. Now, what do the laws of Kepler say? They lay down a relation between the areas described by the heliocentric radius-vector of a planet and the time employed in describing them, a relation between the longer axis of the orbit and the lime taken up by the course. And what was the principle discovered by Galileo? A law which connected the space traversed by a falling body with the time occupied by the fall. Furthermore, in what did the first of the great transformations of geometry in modem times consist, if not in introducing — in a veiled form, it is true — time and movement even in the considera- tion of figures? For the ancients, geometry was a purely static science. Figures were given to it at once, completely finished, like the Platonic Ideas, But the essence of the Cartesian geometry (although Descartes did not give it this form) was to regard every plane curve as described by the movement of a point on a movable strajght line which is displaced, parallel to itself, along the axis of the abscissae — the displacement of the movable straight line being supposed to be uniform and the abscissa thus be- coming representative of the time. The curve is then defined if we can state the relation connecting the space
rr.i
travereed on the movable straight line to the time employed in traversing it, that is, if we are able to indicate the po- sition of the movable point, on the straight line which it traverses, at any moment whatever of its course. This relation is just what we call the equation of the curve. To substitute an equation for a figure consists, therefore, in seeing the actual position of the moving points in the tra- cing of the curve at any moment whatever, instead of re- garding this tracing all at once, gathered up in the unique moment when the curve has reached its finished state.
Such, then, was the directing idea of the refonn by which both the science of nature and mathematics, which serves as its instrument, were renewed. Modern science is the daughter of astronomy; it has come down from heaven to earth along the inclined plane of Galileo, for it is through Galileo that Newton and his successors are connected with Kepler. Now, how did the astronomical problem present itself to Kepler? The question was, knowing the respective positions of the planets at a given moment, how to calculate their positions at any other moment. So the same question presented itself, henceforth, for every material system. Each material point became a rudimentary planet, and the main question, the ideal problem whose solution would yield the key to all the others was, the positions of these elements at a particular moment being ^ven, how to de- termine their relative positions at any moment. No doubt the problem cannot be put in these precise terms except in very simple cases, for a schematized reality; for we never know the respective positions of the real elements of matter, supposing there are real elements; and, even if we knew them at a given moment, the calculation of their positions at another moment would generally require a mathematical effort surpassing human powers. But it is enough for us to know that these elements might be known,
tliat their present positions might be noted, and that a
superhuman intellect might, by submitting these data to
mathematical operations, determine the positions of the
elements at any other moment of time. This con\-iction
is at the bottom of the questions we put to ourselves on
the subject of nature, and of the methods we employ to
solve them. That is why every law in static form seems
to us as a provisional instalment or as a particular \iew
of a dynamic law which alone would give us whole and
definitive knowledge.