are enabled to place it on the same level as the unchangeableness of a body at rest; and to see that it cannot change its direction or velocity, unless there be some external condition of change. And yet, although this is a principle which seems so far contradictory to experience that it is never really exemplified, there being no real motions that are uniform and rectilinear, it is nevertheless a principle which was experimentally established, and was not even conceived until experiment had suggested it. I mean, that the principle was not conceived as a truth of general reach until it came out as the generalized result of experiment.

14. There have been many debates on this point. Eminent philosophers have held, and some still hold, that this and other axioms are à priori, and independent of experience, because they cannot be experimentally demonstrated. But in this argument it is forgotten that what experiment discloses respecting the Sensible has only to be carried into the Extra-sensible to form the legitimate axiom of Experience, when these extra-sensibles become the rational equivalents of sensibles; otherwise no universal truth could be experiential, since every experiment must be particular. It is said that the axioms are given in the form of all experiences; but this is equivocal. The elements must be given in the experiences if they are to be abstracted from the experiences; but the axioms are assuredly not present in experiences in their abstract shape as conceptions. We do not deduce the facts of motion from the laws of motion, but elicit the laws from the facts. We do not begin our observations with abstractions. In the case immediately under notice, it is notorious that, up to the time of Galileo, this principle of the uniform persistence of motion, so far from being conceived as an abstract truth, was not even suspected, so occupied were men's minds with the concrete

truths which seemed to contradict it. Only a wider induction from more precise observations led to its conception; and although it is no longer quite true to say, "that we cannot know otherwise than by induction and experiment that the velocity communicated to a body will not become slower and slower of itself, and finally cease," - not quite true, because now that the conception has been formed, it can be presented to the mind in terms which make it self-evident (such is the potency of conceptions),† -yet we know historically that the evidence of experiment, which showed that as the obstacles were diminished the motion became more and more uniform and rectilinear, had to be generalized and extended before the abstract truth could be revealed.

15. And what did it reveal? The indestructibility of energy: that is to say, the motion which was diminished by the obstacles it overcame was energy because it overcame them; and when these were no longer opposing it, the motion continued undiminished, the energy of that motion being virtual, not actual, abstract capability of doing work, if there were only work to be done.‡ Unless we assign this inalienable property of doing work

* POISSON, Mécanique, § 113. In the first edition, § 183, Poisson is somewhat more guarded in expression, and says nothing of the body getting slower of itself, but only that it will get slower by the obstacles it meets.

It is remarked by Lagrange that Galileo, although he discovered the principle of the composition of forces, failed to see its application to all cases of equilibrium (Mécanique Analytique, p. 13). This application, so evident to us, was not evident to the great Galileo; yet it was given in all cases of equilibrium, quite as decisively as the indestructibility of force is "given" in all experiences of force. Both the one and the other are conceptions to which Experience slowly leads, and which Reflection afterwards confirms.

"Dans l'état d'équilibre la force n'a pas d'exercice actuel; elle ne produit qu'une simple tendance au mouvement; mais on doit toujours la mesurer par l'effet qu'elle produirait si elle n'était pas arrêtée.” — LAGRANGE, Mécanique Analytique, p. 1.

to every particle of matter, as a virtual, which becomes actual when in relation to some other particle, we must either deny the axiom of indestructibility of Force, or declare Force to be something wholly independent of Matter, sui generis, not only in conception, but in reality.

16. Pressure arises in obstructed Motion, and all bodies, we are compelled to conclude, are either actually or potentially moving, their motion when arrested by counter-motion still persisting in pressure. Not only do we know that our planet is whirling round the sun, and that the sun with its satellites moves with immense rapidity through space, we also know that even in the bodies said to be at rest every molecule is vibrating, though not passing beyond the limit of oscillation. Not only is this deducible from the conception of pressure as the dynamic of Mass, it is also inductively reached. For since our planet is presumably never for two consecutive instants in the same part of space, no single molecule can for two consecutive instants be in the same relation to the sun; its temperature must therefore vary. We cannot see this, cannot measure it by Sense, but we see it by the eye of Science. In some remarkable examples we may even approximate to it by Sense. Thus, by the aid of excessively delicate instruments, the astronomer Cæsaris showed that the walls of the Milan observatory, seemingly so fixed and moveless, were subject to periodic oscillations, due to the varying action of the sun; and Pictet found that a metallic rod fixed in an upright position became shorter, owing to the slow downward movement of its molecules, subject to the pull of gravitation. We may say, therefore, "that absolute Rest nowhere exists. in Nature"; all that exists is "the condition of equilibrium, in which a point experiences no change of motion.” *

* THOMSON AND TAIT, Natural Philosophy, I. 179, 182; POISSON, Mécanique, I. 231.

Therefore Motion, and change in the direction of Motion, Pressure, and change in the differences of Pressure, constitute the dynamic aspect of Existence. Assuming the energy of a molecule to be a constant quantity, indestructible, the sum of all these energies must be constant, and the only variation can be in their directions, i. e. their differences of pressure. We see in electricity how effects depend on difference of Potentials. Dissimilar metals in contact produce currents; nay, it is so even when there is only difference of temper in the same metal (as soft and hard iron, or brass).

17. It follows from this, that the mathematical fiction. which makes Matter inert, and Force an external cause of change, is strictly consequent. But while it is logical to consider a force as external to the system on which it operates, since no system can operate on itself, a serious speculative error arises if the artificial nature of the distinction is overlooked, the error, namely, of personifying an abstraction, and creating an entity as the Agent apart from the Activity, a Cause which is not the effect.

FORCE AS CAUSE OF CHANGE.

18. When Force is defined, "that which causes or tends to cause a change of motion," if we ask, What is that? we are told by one very numerous class, that it is "what lies beyond human ken"; by another class we are referred to "the condition of the change."

19. I must here anticipate the conclusion respecting the nature of Cause, which will be established in Chap. II., and say briefly that Cause is the condensed expression of the factors of any phenomenon, the Effect being the fact itself. Cause is the group of conditions which pass into the effect, ideally distinguishable from the product, but not really separable. In cause and effect there are not two things, one preceding the other, but two as

pects of one phenomenon successively viewed. The effect is the effectum, the causatum, the procession of the cause.* The two things which may be said to co-operate are the two related terms of the operation; but we must not isolate these terms, and consider the one to be cause or antecedent, the other effect or consequent: since isolated, the terms lose all causal significance, and related, the one is not the product of the other, but both must co-operate in the causal relation.

If this statement excites the reader's opposition, he is requested to suspend all further consideration of the present topic until he has meditated on what is expounded in Chap. II.; or else he must take it for granted, and see how it applies in the following argument. All that need here be added is, that every cause is a plural, the symbol of complex conditions, co-operant factors.

20. Thus viewed, what shall we say to the wearisomely iterated statement that man can know nothing of Force, because he can never know causes, only effects? Of course, if we have personified the distinction, and made cause something different from effect, existing apart, and creating the effect by a mysterious legerdemain, it is clear that we cannot know what we have thus banished from the region of knowledge; we know the effects, and cause is said to be something not these. Gravity, we are told, is the unknown cause by the action of which bodies fall to the earth if unsupported. We know the fact, the effect; and that is all. Now I say we know just so much of the cause as we know of the effect, since the cause is not one thing and its effect another. The fall of the bodies is the gravitation of the bodies. If you ask, What causes this gravitation? the answer may be, The differential relations of pressure; and if you ask, What causes

*"Toda la realidad del efecto ha de estar virtual in causa. - JAIME BALMES, Escritos Póstumos, Barcelona, 1850, p. 270.