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—
Were I to take an hypercritical attitude, I might dwell on the fact that Prof. Tait leaves the extent of his proposition somewhat doubtful, 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, includes Astronomy; and, going on to ask what astronomical "experiments" 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 confounding experiment and observation. I will not, however, thus infer from Prof. Tait's imperfection of statement that he is unaware of the difference between the two; and shall rate his authority as of no less value than I should had he been more accurate in his expression. Respecting 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 Institute, 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 judgment 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 'Experience,' 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