the Conservation of Force. 83 



stand these images in themselves, nor the objects from which 

 they proceed j but we see that under the same circumstances the 

 same images always appeal*, and this regularity of their appear- 

 ance permits us to determine them beforehand. We construct 

 formulae, either verbal or in mathematical symbols, and name the 

 principles from which these are deduced Laws of Nature. 



We have motion incessantly before us, partly as the motion 

 of masses, and partly as the motion of the ultimate particles of 

 matter among themselves, in the form of heat ; or when the rays 

 of this heat excite our organs of vision, in the form of light. 

 We have so studied the circumstances under which these motions 

 vary and succeed each other, that we can predict their mode of 

 appearance, their intensity and their direction; and we have 

 found that the formulae which serve for this purpose assume 

 their simplest form, when we ascribe to the ultimate particles of 

 matter attributes which we name forces, the magnitude of which 

 we make dependent on mass and distance. No one can prove 

 that these attributes have a real existence, in the strict sense of 

 the word, that they exist in the things themselves, and beyond 

 the domain of human thought. From the experience of the 

 senses, we know no other real source of motion than other mo- 

 tion, and so on, till the links of the chain disappear from our 

 sight : but we help ourselves by assuming causes which we name 

 forces, with which we deal, and as experience proves, with good 

 effect. They exist actually in the domain of thought, and have 

 there their full justification : whether they would also have an 

 existence supposing no thinking being were to have had an ex- 

 istence, is a question which man cannot decide, because, beyond 

 the sphere of his thought, knowledge has for him no existence. 

 The upshot of all natural investigations is to seize upon that 

 which comes immediately within the sphere of our perceptions, 

 and to unfold that which, on account of its magnitude as regards 

 either time or space, is withdrawn from immediate perception, by 

 instruments, by experiments, or by induction, and finally to dis- 

 cover a connexion between the collective phsenomena which shall 

 be in harmony with our laws of thought. From this point of 

 view, and only from this, Newton's doubt as to the force of attrac- 

 tion acting at a distance is, I believe, to be regarded. He has 

 shown by his acts how deeply he felt the necessity of giving to 

 his law tlie form which he has given to it, because in this way alone 

 could it appear as the immediate expression of the facts; but he 

 went further, and wished to form a conception of the forces which 

 were the results of iiis own logical abstraction, and here he encoun- 

 tered difficulties. We represent to ourselves with perfect clearness 

 things and motions which lie without the domain of our senses, 

 and we need only the constructive understanding, to foi'ux and 

 build up the conception. We figure to ourselvca the motion of 



G3 



