548 THE POPULAR SCIENCE MONTHLY. 



• 

 necessary datum by the theory of Energy, is not otherwise considered 

 in that theory. I will confine myself to the proposition, disputed at 

 great length by the reviewer, that our cognition of the Persistence of 

 Force is a priori. He relies much on the authority of Prof. Tait, 

 whom he twice quotes to the effect that — • 



" Natural philosophy is an experimental, and not an intuitive science. No 

 a 'priori reasoning can conduct us demonstratively to a single physical truth. ,? 



Were I to take an hypercritical attitude, I might dwell on the fact 

 that Prof. Tait leaves the extent of his proposition somewhat doubt- 

 ful, by speaking of " Natural philosophy " as one science. Were I to 

 follow further the reviewer's example, I might point out that " Natural 

 philosophy," in that Newtonian acceptation adopted by Prof. Tait, in- 

 cludes Astronomy ; and, going on to ask what astronomical " experi- 

 ments " those are which conduct us to astronomical truths, I might 

 then " counsel " the reviewer not to depend on the authority of one 

 who (to use the reviewer's polite language) " blunders " by confound- 

 ing experiment and observation. I will not, however, thus infer from 

 Prof. Tait's imperfection of statement that he is unaware of the differ- 

 ence between the two ; and shall rate his authority as of no less value 

 than I should had he been more accurate in his expression. Respect- 

 ing that authority I shall simply remark that, if the question had to 

 be settled by the authority of any physicist, the authority of one who 

 is diametrically opposed to Prof. Tait on this point, and who has been 

 specially honored, both by the Royal Society and by the French In- 

 stitute, might well counterweigh his, if not outweigh it. I am not 

 aware, however, that the question is one in Physics. It seems to me a 

 question respecting the nature of proof. And, without doubting Prof. 

 Tait's competence in Logic and Psychology, I should decline to abide 

 by his judgment on such a question, even were there no opposite judg- 

 ment given by a physicist, certainly of not less eminence. 



Authority aside, however, let us discuss the matter on its merits. 

 In the " Treatise on Natural Philosophy," by Profs. Thomson and 

 Tait, § 243, I read that, " as we shall show in our chapter on ' Expe- 

 rience,' physical axioms are axiomatic to those only who have sufficient 

 knowledge of the action of physical causes to enable them to see at 

 once their necessary truth." In this I agree entirely. It is in Physics, 

 as it is in Mathematics, that, before necessary truths can be grasped, 

 there must be gained, by individual experience, such familiarity with 

 the elements of the thoughts to be framed that propositions about 

 those elements may be mentally represented with distinctness. Tell a 

 child that things which are equal to the same thing are equal to 

 one another, and the child, lacking a sufficiently abstract notion of 

 equality, and lacking, too, the needful practice in comparing relations, 

 will fail to grasp the axiom. Similarly, a rustic, never having thought 

 much about forces and their results, cannot form a definite conception 



