May 7, 1874 1 



NATURE 



the earth. The coloured illustrations showing the charac- 

 teristic appearances of the various zones are as success- 

 ful as anything of the kind we have seen, although, what 

 perhaps cannot be avoided in coloured illustrations of 

 this kind, there is a little too much of " the light that 

 never was on sea or land " upon them. 



LETTERS TO THE EDITOR 



The Editor does not hold himself responsible for opittions expressed 

 by his correspondents. No notice is taken of anonymous 

 communications .] 



Necessary Truths — Physical and other 



I AM not about to continue a controversy which I regret hav- 

 ing been provoked into by the misrepresentations of one who 

 ignored the contents of works he professed to review. Reply 

 and rejoinder may go on endlessly. I could not, to much pur- 

 pose, argue with Kir. Hayward, who, instead of takmg such 

 unconsciously-formed preconceptions as those resuhing from the 

 infinite experiences of muscular tensions and their efl'ects, pro- 

 poses to exemplify unconsciously-formed preconceptions by a 

 consciously-formed hypothesis concerning the relation be- 

 tween weight and motion. Nor sliould I care to discuss any 

 question with my new anonymous assailant ; who, when certain 

 examples given show the '* exact quantitative relations'' spoken 

 of to be those of direct proportion, describes me as "intensely 

 unmathematical " because I subsequently use the more general 

 expression as equivalent to the more special — which, in the case 

 in question, it is. 



The fiist of my objects in now writing is to remind "some 

 bystanders, who may from their antecedents be presumed com- 

 petent to judge," that the essential question is not a matliematical 

 one, but a logical and psychological one, in respect of which I 

 am not aware that senior wranglers, as such, can claim any 

 special competence. Further, even admitting the assumption 

 that the question is mathematical, I have to warn the reader that 

 he will be much misled if he infers that there are not "some 

 bystanders who may from their antecedents be presumed " niort 

 " competent to judge," who concur in the opinion that the laws 

 of motion cannot be demonstrated experimentally. 



My second object is to inclose, for publication in Nature, a 

 passage novv standing in type to be added to future impressions of 

 " First Principles " in further elucidation of necessary truths, 

 and our apprehensions of them. 



Herbert Spencer 



"The consciousness of logical necessity, is the consciousness 

 that a certain conclusion is implicitly contained in certain 

 premisses explicitly stated. If, contrasting a young child and an 

 adult, we see that this consciousness of logical necessity, absent 

 from the one, is present in the other, we are taught that there is 

 & growing iipto the recognition of necessary truth, merely by the 

 unfolding ol the inherited intellectual forms and faculties. 



" To state the case more specifically : — Before a neceesary 

 truth can be known as such, two conditions must be fulfilled. 

 Theie must be a mental structure capable of grasping the terms 

 of the proposition and the relation alleged between them ; and 

 there must be such definite and deliberate mental representation 

 oi these terms as makes possible a clear consciousness of this 

 relation. Non-fulfilment of either condition may cause non- 

 recognition of the necessity of the truth ; and may even lead to 

 acceptance of its contrary as true. Let us take cases. 



" The savage who cannot count the fingers on one hand, can 

 frame no definite thought answering to the statement that 7 and 

 5 make 12 ; still less can he frame the consciousness that no 

 other total is possible. 



"The boy adding up figures inattentively, says to himself 

 that 7 and 5 make 1 1 ; and may repeatedly bring out a wrong 

 result by repeatedly making this error. 



" Neither the non-recognition of the truth that 7 and 5 make 

 12, wliich in the savage results from undeveloped mental 

 structure, nor the assertion, due to the boy's careless mental 

 action, that they make 1 1, leads us to doubt the necessity of the 

 relation between these two separately-existing numbers, and the 

 sum they make when existing together. Nor does failure from 

 either cause to apprehend the necessity of this relation make us 

 hesitate to say, that when its terms are distinctly represented in 

 thought, its necessity will be seen ; and that apart from any 

 multiplied experiences, this necessity becomes cognisable when 



structures and functions are so far developed that groups of 7 and 

 5 and 12 can be intellectually grasped. 



" Manilesfly, then, there is a recognition of necessary truths, 

 as such, which accompanies mental evolution. Along with 

 acquirement of more complex faculty and more vivid imagina- 

 tion, there comes a power of perceiving to be necessary truths 

 what were before not recognised as truths at all. And there 

 are ascending gradations in these recognitions. Thus a boy 

 who has intelligence enough to see that things which are equal 

 to the same thing are equal to one another, may be unable to see 

 that ratios which are severally equal to certain other ratios, that 

 are unequal to each other, are themselves unequal ; though to a 

 more developed mind this last axiom is no less obviously neces- 

 sary than the first. 



" All this, which holds of logical and mathematical truths, 

 holds, with change of terms, of physical truths. There are 

 necessary truths in Physics, for the apprehension of which, also, 

 a developed and disciplined intelligence is required ; and before 

 such intelligence arises, not only may there be failure to appre- 

 hend the necessity of them, but there may be vague beliefs in 

 their contraries. Up to comparatively recent times, all mankind 

 were in this state of incapacity with respect to physical axioms ; 

 and the mass of mankind are so still. Various popular notions 

 betray inability to form clear ideas of forces and their relations, 

 or carelessness in thinking, or both. Effects are expected with- 

 out causes of fit kinds ; or effects extremely disproportionate to 

 causes are looked for ; or causes are supposed to end without 

 effects. But though many are thus incapable of grasping physi- 

 cal axioms, it no more follows that physical axioms are not 

 knowable a priori by a developed intelligence, than it follows 

 that there is no necessity in logical relations because many have 

 intellects not developed enough to perceive the necessity. 



" The ultimate physical truth of which clear apprehension is 

 eventually reached, is that force can neither arise without an 

 equivalent antecedent, nor disappear without an equivalent con- 

 sequent. Along with power ol introspection there comes recog- 

 nition of the fact that existence cannot be conceived as beginning 

 or ending : the Laws of Thought themselves negative any such 

 mental representation. And if it be asked why this intuition, 

 which all physical axioms indirectly imply, and which is postu- 

 late in every physical experiment, is to be taken as authoritative 

 because its negation is inconceivable, the answer is that no argu- 

 ment which sets out to discredit it can do this without logical 

 suicide ; since there is no other warrant for asserting the depen- 

 dence of any conclusion on its premisses than the inconceivability 

 of its negation." 



This passage forms part of a revised version of the chapters 

 on Matter, Motion, and Force, which I have contemplated 

 making for this year past. When those chapters were written 

 and stereotyped in April 1861 (see Preface), the modem doc- 

 trines concerning Force and its transformation were so imperfectly 

 developed, that some of the leading technical words now currently 

 used were not introduced. The reorganisation of "First Prin- 

 ciples," which I made in 1867, for the purpose of more truly pre- 

 senting the general Theory of Evolution, did not implicate these 

 chapters, and I believe I did not even re-read them : the stereo- 

 type plates, in common with those of many other chapters, 

 with the numberings of pages and sections altered, were used 

 afresh, and continue still to st.ind as they originally did. But 

 while now rectifying defects of statement which it was scarcely 

 possible to avoid thirteen years ago, I find no reason for changing 

 the essential conception set forth in those chapters ; nor is the 

 need for changing it suggested to me by those on whose judg- 

 ments I have the best reasons for relying. — H.S. 



Royal Society Soiree 



With reference to your account of the Royal Society's soiree 

 (Nature, vol. ix. p. 502), will you allow me to explain that all 

 I "promised" concerning the missing pair of Paradise-birds was 

 to deliver them when sent for. 



They were not sent for, owing to some mistake, and conse- 

 quently not exhibited. 



May 5 1'. L. Sclatek 



Father Secchi's Work on the Sun 

 W'lTH great surprise I read in Nature, vol. ix. p. 390, the 

 following note : — 



" Father Secchi is preparing at Gauthier Villars a second 



