duced; that matter is indestructible; that energy in its various transfor- 

 mations is constant in quantity ; that action and reaction are equal, but 

 in opposite directions ; and that a body subjected to no force will move 

 continuously with uniform velocity in a straight line. I shall take the 

 last mentioned theory as representative of the class. We shall con- 

 sider the question, then; can the theory that a body subjected to no 

 force will move continuously in a straight line, be verified? We can- 

 not withdraw a body from the action of every force, argues Poin- 

 care, at least we should not know that we had done so. An experi- 

 mental law is subject to revision, but this law may be extended to the 

 most general cases; for in these cases experiment is neither able to 

 confirm or contradict it. 



Now we first note that this theory deals with bodies, forces and 

 directions of motion; all of which are subjects capable of experi- 

 mental investigation. By experiment the conditions giving rise to 

 forces can be ascertained. A moving body is a thing of our expe- 

 rience, and its direction of motion can be investigated. Apparently 

 then, all the materials of the theory are capable of experimental in- 

 vestigation. As the conditions giving rise to forces are known, we can 

 ascertain what forces are acting upon a body. As its direction of 

 motion is a thing of experience, we can determine whether the motion 

 is straight or variable. The theory, however, makes an affirmation 

 concerning a body acted on by no force, and such a body we cannot 

 present in experience. We can, however, make inferences concerning 

 such a body, and if those inferences have an experimental foundation, 

 the theory is in so far verified, or may be. Although we cannot re- 

 move from the body all forces acting upon it, we can determine the 

 effect of those forces which continue to act upon it; and can infer in 

 what condition the body would be without the action of those forces. 

 However, it is urged, that we cannot know certainly but that some 

 force is acting which has escaped our notice, or which is of a mys- 

 terious nature. This is true, and is, I understand, the reason why 

 the law of inertia is a theory, is problematical, and not a fact. It 

 cannot be known positively to be true because we cannot give an ex- 

 hibition of the law in experience and be certain that ideal conditions 

 maintain. Neither can it ever be known absolutely to be false; for if 

 a body moves with unexplainable irregularity, we cannot be certain 

 but that some unknown force is acting upon it. However, we might 

 infer that the law is false, and verify our inference to a certain extent 

 by experiment; just as we infer it to be true. We infer, because facts 

 cannot be fully revealed in experience; and for the same reason, in- 

 ferences are always more or less uncertain. A verification is not an 

 actual display of the facts inferred by the theory, in experience; if it 



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