I may add that Mr. Ellis agrees with me in noting the invention of the conception by which the laws of phenomena are interpreted as something additional to _Induction_, both in the common and in the Baconian sense of the word. He says (General Preface, Art. 9), "In all cases this process [scientific discovery] involves an element to which nothing corresponds in the Tables of Comparence and Exclusion; namely the application to the facts of a _principle_ of arrangement, an _idea_, existing in the mind of the discoverer antecedently to the act of induction." It may be said that this principle or idea is aimed at in the Baconian analysis. "And this is in one sense true: but it must be added, that this _analysis_, if it be thought right to call it so, is of the essence of the discovery which results from it. To take for granted that it has been already effected is simply a _petitio principii_. In most cases the mere act of induction follows as a matter of course as soon as the _appropriate idea_ has been introduced." And as an example he takes Kepler's invention of the ellipse, as the idea by which Mars's motions could be reduced to law; making the same use of this example which we have repeatedly made of it. Mr. Ellis may at first sight appear to express himself more favourably than I have done, with regard to the value of Bacon's _Inquisitio in Naturam Calidi_ in the Second Book of the _Novum Organon_. He says of one part of it[198]: "Bacon here anticipates not merely the essential character of the most recent theory of heat, but also the kind of evidence by which it has been established.... The merit of having perceived the true significance of the production of heat by friction belongs of right to Bacon." But notwithstanding this, Mr. Ellis's general judgment on this specimen of Bacon's application of his own method does not differ essentially from mine. He examines the _Inquisitio_ at some length, and finally says: "If it were affirmed that Bacon, after having had a glimpse of the truth suggested by some obvious phenomena, had then recourse, as he himself expresses it, to certain 'differentiæ inanes' in order to save the phenomena, I think it would be hard to dispute the truth of the censure."