REPLIES TO THE QUARTERLY REVIEWERS. 549 



answering to the axiom that action and reaction are equal and oppo- 

 site. In the last case, as in the first, ideas of the terms and their rela- 

 tions require to be made, by practice in thinking, so vivid that the 

 involved truths may be- mentally seen. But when the individual ex- 

 periences have been multiplied enough to produce distinctness in the 

 representations of the elements dealt with, then, in the one case, as in 

 the other, those mental forms, generated by ancestral experiences, 

 cannot be occupied by the elements of one of these ultimate truths 

 without perception of its necessity. If Prof. Tait does not admit this, 

 what does he mean by speaking of " physical axioms" and by saying 

 that the cultured are enabled " to see at once their necessary truth ? " 



Again, if there are no physical truths which must be classed as a 

 priori, I ask why Prof. Tait joins Sir W. Thomson in accepting, as bases 

 for Physics, Newton's Laws of Motion ? Though Newton gives illus- 

 trations of prolonged motion in bodies that are little resisted, he gives 

 no proof that a body in motion will continue moving, if uninterfered 

 with, in the same direction at the same velocity ; nor, on turning to 

 the enunciation of this law, quoted in the above-named work, do I find 

 that Prof. Tait does more than exemplify it by facts which can them- 

 selves be asserted only by taking the law for granted. Does Prof. 

 Tait deny that the first law of motion is a physical truth ? If so, what 

 does he call it ? Does he admit it to be a physical truth, and, denying 

 that it is a priori, assert that it is established a posteriori — that is, by 

 conscious induction from observation and experiment ? If so, what is 

 the inductive reasoning which can establish it ? Let us glance at the 

 several conceivable arguments which we must suppose him to rely on. 



A body set in motion soon ceases to move if it encounters much 

 friction, or much resistance from other bodies struck. If less of its 

 energy is expended in moving, or otherwise affecting, other bodies, or 

 in overcoming friction, its motion continues longer. And it continues 

 longest when, as over smooth ice, it meets with the smallest amount 

 of obstruction from other matter. May we then, proceeding by the 

 method of concomitant variations, infer that were it wholly unobstruct- 

 ed its motion would continue undiminished ? If so, we assume that the 

 diminution of its motion observed in experience is proportionate to 

 the amount of energy abstracted from it in producing other motion, 

 either molar or molecular. We assume that no variation has taken 

 place in its rate, save that caused by deductions in giving motion to 

 other matter ; for, if its motion be supposed to have otherwise varied, 

 the conclusion, that the differences in the distances traveled result from 

 differences in the obstructions met with, is vitiated. Thus the truth 

 to be established is already taken for granted in the premises. ,Nor is 

 the question begged in this way only. In every case where it is re- 

 marked that a body stops the sooner, the more it is obstructed by 

 other bodies or media, the law of inertia is assumed to hold in the ob- 

 structing bodies or media. The very conception of greater or less 



