Da. 2, 1880] 



NA TURE 



lOI 



unknown form of existence with which they are supposed 

 to be connected, readers Uttlc thought that Prof Tait 

 meant by this unknown form of existence his own mind. 

 Yet this is all that he now names as the missing term of 

 the relation between the seen universe and the unseen 

 imiverse. 



The second sample which I gave of Prof. Tait's views 

 on matters pertaining to his own subject, concerned the 

 nature of inertia, which he describes by implication as a 

 positive force. Here I quoted Prof. Clerk Maxwell. To 

 repeat his criticism in full would cause me to trespass on 

 the pages of N.\ture even more unduly than I must do. 

 If, however, any reader turns to Nature, July 3rd, 1879, 

 and reads the passage in question, he will be able to judge 

 ■whether it is, or is not, a joke, and if a joke, at whose 

 expense. Meanwhile, the essential question remains. 

 Prof. Tait says that matter has " an innate power of 

 resisting external influences.'' I, contrariwise, say that 

 the assertion of such a power is at variance with established 

 physical principles. 



One further illustration of Prof. Tait's way of thinking 

 was added. Quoting from a lecture given by him at 

 Glasgow, for the purpose of dispelling '' the widespread 

 ignorance as to some of the most important elementary 

 principles of physics,'' I compared two different definitions 

 of force it contained. In a passage from Newton, em- 

 phatically approved by Prof. Tait, force is implied to be 

 that which changes the state of a body, or, in modern 

 language, does w-ork upon it. Later on in the lecture, 

 Prof. Tait says — " force is the rate at which an agent does 

 work per unit of length.'' I contended that these defini- 

 tions are irreconcilable with one another ; and I do not 

 see that Prof. Tait has done anything to reconcile them. 

 True, he has given us some mathematics, by which he 

 considers the reconciliation to be effected ; and, possibly, 

 some readers, awed by his equations, and forgetting that 

 in symbolic operations, carried on no matter how rigour- 

 ously, the worth of what comes out depends wholly on 

 what is put in, will suppose that Prof Tait must be right. 

 If, however, his mathematics prove that while force is an 

 agent which does work, it is also the rate at which an 

 agent does work, then I say — so much the worse for his 

 mathematics. 



From these several tests of Prof Tait's judgment, in 

 respect to which I fail to see that he has disposed of my 

 allegations, I pass now to his implied judgment on the 

 formula, or definition, of Evolution. And here I have 

 first to ask him some questions. He says that because 

 he has used the word "definition " instead of " formula,'' 

 he has incurred my "sore displeasure and grave censure." 

 In what place have I expressed or implied displeasure or 

 censure in relation to this substitution of terms ? Alleging 

 that I have an obvious motive for calling it a "formula," 

 he says I am " indignant at its being called a definition." 

 I wish to see the words in which I have expressed my 

 indignation ; and shall be glad if Prof. Tait will quote 

 them. He says — "It seems I should have called him the 

 discoverer of tlie formula!'^ \\\i\.Q3.di of "the inventor of 

 the definition." 'Will he oblige me by pointing out where 

 I have used either the one phrase or the other .-' These 

 assertions of Prof Tait are to me utterly incomprehensible. 

 I have nowhere cither said or implied any of the things 

 which he here specifies. So far am I from consciously 

 preferring one of these words to the other, that, until I 

 read this passage in Prof. Tait's lecture, I did not even 

 know that I was in the habit of saying " formula " rather 

 than "definition." The whole of these statements are 

 fictions, pure and absolute. 



My intentional use of the one word rather than the 

 other, is alleged by him a propos of an incidental com- 

 parison I have made. To a critic who had said that the 

 formula or definition of Evolution "seems at best rather 

 the blank form for a universe than anything correspond- 

 ing to the actual world about us," 1 had replied that it 



might similarly be "remarked that the formula — 'bodies 

 attract one another directly as their masses and inversely 

 as the squares of their distances,' was at best but a 

 blank form for solar systems and sidereal clusters." 

 Whereupon Prof. Tait assumes that I put the " Formula 

 of Evolution alongside of the Law of Gravitation," in 

 respect to the definiteness of the previsions they severally 

 enable us to make ; and he proceeds to twit me with 

 inability to predict what will be the condition of Europe 

 four years hence, as astronomers "predict the positions 

 of known celestial bodies four years beforehand.'' Here 

 we have another example of Prof. Tait's peculiarity of 

 thought. Because two abstract generalisations are com- 

 pared as both being utterly unlike the groups of concrete 

 facts interpreted by them, therefore they are compared 

 in respect to their other characters. 



But now I am not unwilling to deal with the contrast 

 Prof. Tait draws ; and am prepared to show ^that when 

 the conditions are analogous, the contrast disappears. It 

 seems strange that I should have to point out to a scientific 

 man in his position, that an alleged law may be perfectly 

 true, and that yet, where the elements of a problem to be 

 dealt with under it are numerous, no specific deduction 

 can be drawn. Does not Prof. Tait from time to time 

 teach his students that in proportion as the number of 

 factors concerned in the production of any phenomenon 

 becomes great, and also in proportion as those factors 

 admit of less exact measurement, any prediction made 

 concerning the phenomenon becomes less definite ; and 

 that where the factors are multitudinous .and not measur- 

 able, nothing but some general result can be foreseen, and 

 often not even that ? Prof Tait ignores the fact that the 

 positions of planets and satellites admit of definite pre- 

 vision, only because the forces which .appreciably affect 

 them are few ; and he ignores the fact that where further 

 such forces, not easily measured, come into play, the pre- 

 visions are imperfect and often wholly wrong, as in the 

 case of comets ; and he ignores the fact that where the 

 number of bodies aftecting one another by mutual gravita- 

 tion is great, no definite prevision of their positions is 

 possible. If Prof Tait were living in one of the globular 

 star-clusters, does he think that after observations duly 

 taken, calculations based on the law of gravitation wculd 

 enable him to predict the positions of the component stars 

 four years hence ? By an intelligence immeasur.ably trans- 

 cending the human, with a mathematics to match, such 

 prevision would doubtless be possible ; but considered 

 from the human standpoint, the law of gravitation, even 

 when uncomplicated by other laws, can yield under such 

 conditions only general and not special results. And if 

 Prof Tait will deign to look into "First Principles," which 

 he apparently prides himself on not having done, he will 

 there find a sufficient number of illustr.ations showing that 

 not only other orders of changes, but even social changes, 

 are predictable in respect to their general, if not in respect 

 to their special, characters. 



There remains only to notice the opinion which Prof. 

 Tait seems still to hold, that the verbal transformation 

 which Mr. Kirkman has made in the formula or definition 

 of Evolution, suffices to show its hoUowness. Here I may 

 be excused for repeating what I have already said else- 

 where, namely, that " We may conveniently observe the 

 nature of Mr. Kirkman's belief, by listening to an imagi- 

 nary addition to that address before the Literary and 

 Philosophical Society of Liverpool, in which he first set 

 forth the leading ideas of his volume ; and we may fitly, 

 in this imaginary addition, adopt the manner in which he 

 delights. 



" Observe, gentlemen," we may suppose him saymg, 

 " I have here the yolk of an <t%z- The evolutionists, 

 using their jargon, say that one of its characters is 

 ' homogeneity ' ; and if you do not examine your thoughts, 

 perhaps you may think that the word conveys some idea. 

 But now if I translate it into plain English, and say that 



