BOOK II. CH. I. § 7. Matter and Force. $ 8. Vis insita ; Vis inertia; Vis force as inseparable modes of space occupancy, that is, of material or physical active substance, is for us a final inexplicability, and one which compels us to recognise, that it depends upon something or other which we class under the head of real condition, but which we cannot positively conceive. But then that real condition is unknowable in a very different sense from that in which a pure fiction is unknowable. It is thought of, not as a noumenal entity unknowable a parte rei, but as a phenomenal reality, whose esse is percipi, like that of all realities, only that it is one which is not within the reach of human sensitivities. § 8. The force which I have thus attempted to describe I apprehend to be the same as Newton's ris insita, or (same thing again) his vis inertiæ, as impressa. given in the Third Definition in his Principia: "Definitio III. Materia Vis Insita est potentia resistendi, qua corpus unumquodque, quantum in se est, perseverat in statu suo vel quiescendi vel movendi uniformiter in directum."-"The inherent force of matter is a power of resisting, by which every body, as much as in it lies, perseveres in its state, either of rest, or of uniform motion in a straight line." And the remarks on this definition begin as follows: "Hæc semper proportionalis est suo corpori, neque differt quicquam ab Inertia massa, nisi in modo concipiendi. Per inertiam materiæ, fit ut corpus omne de statu suo vel quiescendi vel morendi difficulter deturbetur. Unde etiam vis insita nomine significantissimo Vis inertia dici possit."-"This force is always proportional to the body it inheres in, nor does it differ at all from the inertia of mass, except in our way of conceiving it. By the inertia CH. I. § 8. of matter is brought about, that every body is only BOOK II with difficulty disturbed from its own state of rest or motion. Whence also vis inertia is a most significant name for vis insita.” It is plain from this, in the first place, that Newton conceives the force he speaks of as an inherent and universal property of mass, that is, of every quantum of matter; and, since there is no matter which has not quantity, as an inherent and universal property of matter also. Secondly, since he admits no mass or matter without it, there is consequently for him no such thing as dead or inert mass or matter, as deadness and inertness are popularly apprehended. apprehended. He decisively negatives the common-sense notion of deadness or inertness in matter. The inertia of mass and consequently of matter, which Newton speaks of, is not inertness but force. Inertia itself is force. It is true, as we shall presently see, that this force is known. only by what it does, or as a doing of something. But, inasmuch as it is always inherent in mass, it is not free or pure force without an agent. Mass is the doer or agent, and its whole doing is force. It is here, in fact, from these luminous and accurate words of Newton, that we gather the physicist's ultimate conception of force. It is here, so to speak, that we run it to earth. For it appears here as that which (1) cannot be got rid of out of mass or matter, and (2) is of extreme simplicity, not as yet modified or specified as force of this or that kind, a doing of this or that. The conception of it is applicable not to ponderable matter only, but to matter of any and every kind, say, for instance, the commonly assumed all-per Vis insita; Vis inertiæ; Vis impressa. BOOK II. vading ether, whether it be ponderable or not. CH. I. § 6. Vis insita; Vis inertiæ; Vis impressa. Whatever is conceived of as a resisting solid, or composed of resisting solids, in the geometrical sense of solid, namely three-dimensional, is conceived of as matter. And in whatever is so conceived, force must also be conceived as inherent. Let us now see what the doing of the force consists in. I quote the words next following those already quoted from the remarks on the Third Definition: "Exercet vero corpus hanc rim solummodo in mutatione status sui per rim aliam in se impressam facta; estque exercitium ejus sub diverso respectu et Resistentia et Impetus: resistentia, quatenus corpus ad conservandum statum suum reluctatur vi impressa; impetus, quatenus corpus idem, vi resistentis obstaculi difficulter cedendo, conatur statum ejus mutare."—"Now a body exerts this force only in the change of its own state which is wrought by another force impressed upon it; and its exertion is both resistance and impetus, but in different respects. It is resistance so far as the body relucts against the impressed force so as to maintain its own state; and it is impetus so far as the same body, in yielding with difficulty to the force of an obstacle resisting it, endeavours to change the state of that obstacle."-In other words, the force of inertia is always present, but is exerted only when a force, external to the body or mass in which it is inherent, is in some way brought into relation with that body. Then it is called forth into exertion, and the body re-acts partly in resisting change of its own state, and partly in effecting change on external bodies. In re-action on external bodies its vis insita becomes CH. I. § 8. vis impressa, exerted, namely, on the external body BOOK II. or bodies, just as the action of the external body, which was taken as first acting on it, was vis impressa exerted upon it. The force and its exertion are not the same thing; and yet in one sense they are the same, since the one is the exertion of the other. The force, vis insita which is also vis inertia, is acted upon by a vis impressa, is exerted when so acted upon, and its exertion both as resistance and impetus is vis impressa. The exertion of ris insita is vis impressa. Let us see what Newton says of vis impressa in the next Definition. "Definitio IV. Vis impressa est actio in corpus exercita, ad mutandum ejus statum vel quiescendi vel movendi uniformiter in directum.-Consistit hæc vis in actione sola, neque post actionem permanet in corpore. Perseverat enim corpus in statu omni novo per solam vim inertiæ. Est autem impressa diversarum originum, ut ex Ictu, ex Pressione, ex vi Centripeta."-" Def. IV. Vis impressa is an action exerted upon a body to change its state either of rest or of uniform motion in a straight line. This force consists in the action only, and does not remain in the body after the action. For a body perseveres in every new state by vis inertia alone. Also vis impressa has divers origins, as from a blow, from pressure, from centripetal force." vis § 9. We have, then, these three things, ris insita, vis inertia, and ris impressa. These three are partly the same thing, and partly different things. Vis insita, Newton says, differs from ris inertia only in our way of conceiving it. But what is this Vis insita; Vis inertia Vis impressa. $ 9. Empirical Matter. BOOK II, $ 9. Empirical Matter. difference? It is plainly this. Vis insita belongs to mass, or to matter, when the latter is considered per se, that is, either as matter generally, or as a particular material thing, but in abstraction from its relations to other material things with which it is connected, or may be brought into connection. This same vis insita is vis inertia when we replace it in the relations we had abstracted from, and consider it as acted upon by other bodies external to that in which it is taken as insita. They are thus the same force differently considered. Next as to vis impressa. This is the exertion of ris insita or inertia, when bodies are taken in relation, and acting one upon another. Not a new force, but the exertion of the force previously defined as unexerted. The three things, then, are (1) force inherent in matter simply, vis insita; (2) force inherent in material things acted on by other material things, vis inertia; and (3) force exerted by material things upon one another, vis impressa. Vis inertia is the central or connecting link of the three. A force external to a body could not act upon the body as ris impressa, if there was no force in that body upon which to act, that is, if the body had no vis inertia. But the central link of the three, vis inertia, could not exist unless it were founded in the nature of the body it belongs to, as vis insita. Nor again could it re-act on the body impressing it, nor exert ris impressa or other bodies, unless they, like it, offered something to be acted on, that is, were like it possessed of ris insita and vis inertiæ. But now to come to closer quarters with that cardinal idea of action, activity, agency, or efficiency, |