3. Further, I asked how it happened that Professor Tait accepted as bases for Physics, Newton’s Laws of Motion; which were illustrated but not _proved_ by Newton, and of which no _proofs_ are supplied by Professor Tait, in the _Treatise on Natural Philosophy_. I went on to examine what conceivable _a posteriori_ warrant there can be if there is no warrant _a priori_; and I pointed out that neither from terrestrial nor from celestial phenomena can the First Law of Motion be deduced without a _petitio principii_ . . . No reply: the Reviewer characterized my reasoning as “utterly erroneous” (therein differing entirely from two {317} eminent authorities who read it in proof); but beyond so characterizing it he said nothing. 4. To my assertion that Newton gave no proof of the Laws of Motion, the Reviewer rejoined that “the whole of the _Principia_ was the proof.” On which my comment was that Newton called them “axioms,” and that axioms are not commonly supposed to be proved by deductions from them . . . The Reviewer quotes from one of Newton’s letters a passage showing that though he called the Laws of Motion “axioms,” he regarded them as principles “made general by induction;” and that therefore he could not have regarded them as _a priori_. 5. In rejoinder, I pointed out that whatever conception Newton may have had of these “axioms,” he explicitly and distinctly excluded them from the class of “hypotheses.” Hence I inferred that he did not regard the whole of the {318} _Principia_ as the proof, which the Reviewer says it is; since an assumption made at the outset, to be afterwards justified by the results of assuming it, is an “hypothesis” . . . No reply. 6. Authority aside, I examined on its merits the assertion that the Laws of Motion are, or can be, proved true by the ascertained truth of astronomical predictions; and showed that the process of verification itself assumed those Laws. No reply.