Hans Jonas, Philosophical Essays, “Seventeenth Century and After: The Meaning of the Scientific and Technological Revolution”
IV
The new cosmology called for a new physics but did not provide one itself. It offered a new image of the universe but no explanation of it. It showed, by an ingenious combination of hypothesis, observation, and mathematical construction, how the macrocosmos “looks” and what motions its bodies describe, but not why they do so — i.e., what causes operate in that universe. The major structures of the world system had decisively changed, but nothing in the Copernican system as such, or in Kepler’s refinement of it, or in Bruno’s widening of its perspective to infinity, decided anything on the modus operandi of this revised totality. In brief, the new cosmology presented the shape of things, but by itself gave no account of the working of things. Its acceptance was thus at first left to the powerful appeal of its mathematical, rational, and imaginative qualities. Neither Copernicus nor Kepler could answer physical questions concerning the nature of their universe. Copernicus did not even ask them; Kepler felt the need for them, but for lack of an adequate physics his search for causes could find no satisfactory answer. His laws were descriptive and marvellously accurate in that respect, but not explanatory. His attempted explanations remained groping. He did, however, conceive the idea of a vis motrix (moving force) for his planets emanating from the sun; and he had proclaimed that measurable quantity was the essence of reality, and measurement the key to its secrets. And Bruno had grasped that motion and change rather than immutability were the truth of this universe, and that forces must provide the bond for its scattered multiplicity in unstructured space.
The one thing clear to all was that Aristotelian physics no longer applied to the altered scheme of things. Movements were no longer explained by forms of order; instead, the form of movement had to be explained by the action of forces. But about them the heavens were silent; their spectacle did not tell its secret For the celestial bodies can only be contemplated, not experimented with; and contemplation alone will not disclose the play of forces. Terrestrial mechanics must come to the aid of the celestial spectacle. It could do so because the new homogeneity of nature would extend the findings of any local mechanics into limitless space. The actual development of such a mechanics itself was entirely independent of that of astronomy and is eminently associated with the name of Galileo.
While the most dramatic of Galileo’s many feats concerned his championing of the Copernican cause and the telescopic discoveries that butressed it, his really decisive contribution to the rise of modern science was the laying of the foundations of a science of motion — a general “kinetics.” This involved — apart from a new method of analysis and verification — a radical reframing of the very concept of motion, which — if less spectacular — was no less revolutionary in its long-term results than the reform of cosmology. Let us briefly remember how motion had been understood before.
In Aristotelian physics, motion was subsumed under the ontological category of change. That is to say, locomotion is one species of the genus “change,” namely change of place (as distinct from change of quality, quantity, and substance). “Place” itself, in this view, is something, and for a body to be in a certain place is to be in a certain condition. When it moves to another place, a change occurs, and while it moves, a continuous sequence of changes occurs — as many changes as there are places traversed: which means an infinite number of changes, because of the infinite divisibility of the spatial continuum. But any change, according to the basic principle of causality, requires the operation of a cause as the sufficient reason for the change to occur, whereas the absence of change, the persistence of a given condition, requires no cause. Ergo, any process of motion, being a serial passage from place to place, and thus a series of changes, requires during its whole duration the continual renewal of the motive power, i.e., the constant activity of an agent cause (in modern parlance: a continuous input of energy). [The fuller and more strictly Aristotelian statement of this argument would have to include the following. Each of the changes of which the whole movement is composed involves a passage from potency to act (which is not the same as the passage from one point of space to another: it underlies it). The mobility of every body means the potentiality it possesses of being in a place other than it is. But this is a passive potency whose actualization requires the activity of an agent cause. (This agent cause may be external or internal: it is internal in animate bodies, and in all bodies insofar as they are not in their “natural” place toward which they have an intrinsic tendency.) Thus any process of motion, as it involves a constantly repeated passage from potency to act corresponding to the serial change of place, requires for its sustainment a constant actuator.]
The corollary of this conception of movement as change is that rest is the natural state of a body, in which it will persist unless caused by an active principle to move. This is precisely what the inertia of bodies here means: the concept of inertia is fulfilled in the state of rest only. As applied to motion, it means that without the added supply of motive energy the motion will cease. Motion itself, the opposite of rest, is not a cause; it has a cause. Rest, on the other hand, has no cause: it is its own cause in the absence of any active cause.
I know of no simpler way to state the conceptual revolution in kinetics associated with Galileo (though in historical fact it was prepared by various prior steps), than to say that he removed motion from the category of change and made it understood as a state equivalent, in regard to cause, to the state of rest. Not the principle of causality, or of sufficient reason, as such was changed (let alone discovered), but the subject to which it applies: not the idea that every change must have a sufficient cause, but the idea of what constitutes a change. The import of this innocent-looking intellectual turn is so profound and so fraught with consequences that it calls for some elaboration.
Motion, so it is contended, is as much the “state” of a body as is rest. Its continuation therefore is not a repetitious change but the retention of a given state and, as such, no more requires a cause than does the retention of rest (except if retention of a state be itself counted among the “causes,” as indeed it can, as we shall see). What requires a cause is the change from motion to rest, or from rest to motion, or a change in motion itself. But what is a change in motion? This question can only be answered by reference to what defines the sameness of motion, its unaltered state.
A motion is defined by velocity and direction. Unaltered (uniform) velocity means that equal distances are covered in equal times; unaltered direction means progression in a straight line. Thus unaltered motion means uniform rectilinear motion. And the new proposition is that a moving body will continue to so move unless interfered with by an outside force. The elementary terms in both of those two defining aspects of motion — in velocity and direction — are space and time: both can be so combined in a geometrizing arithmetic that a given motion at a given moment is defined by a determinate quantity made up of these two terms. It is then a change in this quantity which constitutes a change of the motion, and the quantity of the change, again measured in terms of space and time, is a measure of the force that caused it. Thus any increase or decrease in velocity, and any change in direction, betrays the action of a force added to the force that keeps the body on its uniform rectilinear course: but the initial “force” to which the addition is made remains operative in the compound with its own unaltered quantity — i.e., the new motion is a composite of several motions.