Prof. Challis on Newton's "Foundation of all Philosophy" 285 



a priori conclusion relative to the fundamental idea of force. 

 For if all force be in reality pressure, a substance which presses 

 must be as universally diffused as force itself. With respect to 

 the quality of the pressing body, observation and experience 

 leave us no room to doubt that it must be fluid, because, 

 assuming its existence, it is matter of experience that all sen- 

 sible bodies move readily through it. Again, because all known 

 fluids press more the more they are compressed, the same 

 law must be predicated of the sether. We may even assume, 

 taking experience for guide, that variations of its pressure are 

 exactly proportional to variations of the density, there being an 

 actual instance of this law in air of given temperature. More- 

 over this is the simplest relation conceivable ; and in an inves- 

 tigation of the present kind a law less simple than the simplest 

 is inadmissible. The rule of simplicity is recognized by Newton, 

 where he says (Reg. I.), " Natura simplex est et rerum causis 

 superfluis non luxuriat;" and again (Reg. III.), " Natura sim- 

 plex esse solet et sibi semper consona." On the same ground 

 of simplicity the aether must also be conceived to be always and 

 everywhere of the same density, excepting so far as variations 

 of density are superinduced by a state of motion. 



The fundamental idea relative to force, to which these con- 

 siderations conduct, may be thus expressed : — All force is action 

 by pressure of a uniform fluid, the pressure of which is pro- 

 portional to its density. 



I have now to meet some objections which I anticipate will be 

 raised against these views. It will probably be urged that since 

 the progress of science has shown that bodies consist of discrete 

 parts, there can be no continuity of surface, and the forms of 

 solids are not actually what they appear to be ; that absolute 

 constancy of form is not actual, all known bodies being capable 

 of yielding to force ; and that as two solids cannot be made to 

 come into actual contact, mutual pressure of their parts is not 

 actual. After all this has been fully admitted, there remains 

 the fact that we do acquire, by our senses and the power of abs- 

 traction, distinct perceptions of extension, of continuity of sur- 

 face, of form, of constancy of form, of contact, and of pressure 

 by contact. If this be denied, the common sense of mankind 

 would reclaim. It is the very essence of philosophy to rest 

 upon the foundation of common perceptions, and by reasoning 

 from these to account for phenomena. If the reasoning be 

 good, and an explanation of the phenomena be shown to result 

 from the antecedent fundamental conceptions of matter and 

 force, we may be said to know their causes in the sense in which 

 we know anything else. Also, that the fundamental ideas, in 

 their exact form, are not mere abstractions, but conceptions of 



