March i6, 191 i] 



NATURE 



97 



not exist. To deprive mankind of microscopes or tele- 

 scopes would be hardly a more serious blow to science. 

 We do not, of course, depend on our eyes for the notion 

 of time — for the congenitally blind perceive time — but so 

 soon as we wish to know accurately the relation of chang- 

 ing events to time intervals, we depend upon having them 

 recorded in a visible form. It is the practical acknowledg- 

 ment of the superiority of the eye as an agent to make 

 clear the correlation of data. 



Scientific men base their work upon a series of assump- 

 tions : first, that there is absolute truth, which includes 

 everything we know or shall know ; secondly, that we 

 ourselves are included in this absolute truth ; thirdly, that 

 objective existence is real ; fourthly, that our sensory per- 

 ception of the objective is different from the reality. 

 These conceptions constitute our fundamental maxims, and 

 even when not definitely put in words they guide all sound 

 scientific research. Metaphysicians find such maxims 

 interestingly debatable ; but science applies them un- 

 hesitatingly, and is satisfied because their application 

 succeeds. Philosophy, ever a laggard and a follower after 

 her swifter sister, has lately, and somewhat suddenly, 

 termed the scientific habit of work pragmatism, and has 

 taken up the discussion of it with delightful liveliness. 

 Let us acknowledge the belated compliment and continue 

 on our way. 



The practical result of the four maxims has been that 

 we further assume that all errors are of individual human 

 origin, and that there are no objective errors. We make 

 all the mistakes, nature makes none. To render the pur- 

 suit of new knowledge successful, our basic task is to 

 eliminate error, or in other words to decide when we 

 have sufficient proof. The elimination of error depends 

 primarily upon insight into the sources of error, which, 

 since methods of all sorts are employed, involves an 

 intimate technical acquaintance with the methods, with 

 just what they can show, with what they cannot show, 

 and with the misleading results they may produce. In 

 the laboratory training of a young scientific man, one 

 chief endeavour must always be to familiarise him with 

 the good and the bad of the special methods of his branch 

 of science. Not until he thoroughly understands the 

 character and extent of both the probable and the possible 

 errors is he qualified to begin independent work. His 

 understanding must comprise the three sources of observa- 

 tional error, namely, the variation of the phenomena, the 

 imperfections of the methods, and the inaccuracy of the 

 observer. The personal equation always exists, although 

 it can be quantitatively stated only in a small minority of 

 cases. 



The history of science at large, the history of each 

 branch of science, and the personal experience of every 

 active investigator, all equally demonstrate that the. 

 greatest source of error is in our interpretations of the 

 observations, and this difficulty depends, it seems to me, 

 more than upon any other one factor ; upon our uncon- 

 querable tendency to let our conclusions exceed the sup- 

 porting power of the evidence. Since generalisation is the 

 ultimate goal, we are too easily inveigled into assuming 

 probabilities to be certainties, and into treating theories, 

 and even hypotheses, as definite conclusions. Each 

 generation of investigators in its turn spends much time 

 killing off and burying older erroneous interpretations. 

 The business is seldom accomplished by direct attack, for 

 error perishes only in the light of truth, as micro- 

 organisms are said to perish suddenly when struck by 

 ultra-violet rays. Owing to the load of false theories, we 

 work like a mental chain-gang, and are never unfettered. , 

 The handicap imposed by wrong hypotheses has always | 

 imneded thp growth of science. 



The multitude of such experiences, great and small, has 

 gradually created among scientific men a special highly 

 characteristic mental attitude. They regard the majority 

 of the accumulated data and many of the inductions of 

 science as correct. This is their 'estimate of the great 

 body of information which, though personal in its origin, 

 has been in the course of time so tested and verified that 

 it is looked upon as established and secure. When Asellus 

 in 1622 discovered the lymphatics, or so-called lacteals, of 

 the mesentery, and demonstrated that thoy convev pro- 

 ducts of digestion from the intestine, his knowledge was 

 his own, and at first his onlv. Since then the observa- 

 NO. 2159, VOL. 86] 



tions have been repeatedly verified, and of course ex- 

 tended, and all uncertainty has vanished from our 

 minds. Similarly, in innumerable other cases reasonable 

 impersonal certainty has been attained. Yet the investi- 

 gator lives in an atmosphere of concentrated uncertainty, 

 for he is convinced that at any time new data may turn 

 up, and that all generalisations are likely to require 

 modification. We might well adopt as our cry : 

 Incredulity towards the known ; open credulity towards the 

 unknown. 



We think of science as a vast series of approximations, 

 and our task is constantly to render our approximations 

 closer to absolute truth, the existence of which we take 

 for granted. We use our approximations as best we may, 

 treating them in large part and, at least for the time 

 being, as if they were accurately true, yet meanwhile we 

 remain alert to better them. This has long been the 

 standard of scientific thought. It is the pragmatic atti- 

 tude of mind, but its new name has not rendered it a 

 novelty. 



The pivot of all research is adequate proof. It would 

 certainly aid science if some competent philosopher should 

 make a study of the practice of investigators in the various 

 branches of science sufficient to render clear the general 

 principles by which investigators decide when a new 

 observation or a new induction is sufficiently proven. If 

 we follow the advance of research in any particular direc- 

 tion, we soon realise that there is a more or less definite 

 standard of proof, which, though never clearly formulated, 

 is none the less insisted upon, so that any paper which 

 does not come up to this standard is subject to unfavour- 

 able criticism. Two elements of this standard we know, 

 the first the elimination of the recognised sources of error, 

 secondly, the repetition of the observations so that the 

 constancy of the phenomenon is assured. We cannot do 

 more than allude to this theme, which I must leave to the 

 future and to a more competent mind to analyse and 

 develop. 



To sum up. The method of science is not special or 

 peculiar to it, but only a perfected application of our 

 human resources of observation and reflection — to use the 

 words of von Baer, the greatest embryologist. To secure 

 trustworthiness, the method of science is, first, to record 

 everything with which it deals, the phenomena themselves 

 and the inferences of the individual investigators, and to 

 record both truly ; secondly, to verify and correlate the 

 personal knowledges until they acquire impersonal validity, 

 which means, in other words, that the conclusion approxi- 

 mate so closely to the absolute truth that we can be safely 

 and profitably guided by them. The method of science is 

 no mystic process. On the contrary, it is as easily com- 

 prehended as it is infinitely difficult to use perfectly, and 

 at its best the method supplies merely available approxi- 

 mations to the absolute. 



W^e set science u[x>n the throne of imagination, but we 

 have crowned her with modesty, for she is at once the 

 reality of human power and the personification of human 

 fallibilitv. 



THE CALORIC THEORY OF HEAT, AND 

 CARNOT'S PRINCIPLE.' 



'T'HE caloric theory of heat as developed by Carnot in 

 his famous " Reflexions on the Motive Power of 

 Heat " (Paris, 1824) leads immediately to the correct solu- 

 tion of the relations between heat and motive power 

 (energy or work) in all reversible processes, and appears 

 to be in some respects preferable to the mechanical theory 

 as a method of expression, because it emphasises more 

 clearly the distinction, first clearly stated by Carnot, 

 between reversible and irreversible transformations, and 

 because it directly provides the natural measure of a 

 quantity of heat as distinct from a quantity of thermal 

 energy. 



Carnot first introduced the method of the cyclical pro- 

 cess in discussing the action of a heat engine, and showed 

 that, in the ideal case, if there were no direct transference 

 of heat between bodies at different temperatures, the 

 transformations of heat and motive power in such a cycle 

 were reversible. Assuming that it was impossible to 



1 Abstract of the pfe«idential address delivered before the Physical Society 

 on February 10 by Prof. H. L. Callendar, F.R.S. 



