22i 



NATURE 



[April 21, 192 1 



loidal state. It should be such as to include at 

 one extreme bacteria growing in a medium. 

 It has been shown quantitatively that agglutina- 

 tion of bacteria occurs when the contact potential 

 difference at the surface of the bacteria is de- 

 stroyed. They, therefore, present a characteristic 

 feature. At the other extreme there would be such 

 a system as turned up accidentally during the war 

 at a certain factory. An oil was found to form 

 with water a stable emulsion remarkable for the 

 size of the drops, which averaged nearly 2 milli- 

 metres in diameter. When the drops were broken 

 up by violent shaking they slowly grew to the 

 characteristic large size, and at constant tem- 

 peratures persisted for months, forming a system 

 defined by a distinct curvature of the interfaces 

 fixed probably by frictional constraints. Clearly 

 delimitation can neither be simply dimensional nor 

 is it to be found in the chemical make-up ; it must 

 be sought and is to be found in the presence of 

 characteristic constraints. 



Both books present in a fair way the contem- 

 porary views of colloidal theory. It is to the 

 theory that criticism is directed, not to their pre- 

 sentation of it. Each book has its peculiar merits. 

 Prof. Zsigmondy, for instance, is particularly 

 good and complete in all that refers to the ultra- 

 microscopy of colloids. 



Dr. Ostwald's book gives the substance of 

 lectures delivered in America at the invita- 

 tion of certain universities. It is a good intro- 

 duction to the elements of the subject. One 

 special feature may be noticed. The book was 

 completed before the war, and the first preface 

 is dated 1914. Publication was deferred for 

 obvious reasons, and the second preface, dated 

 ^9i5» was written whilst the author was actually 

 at the front. From that agony of unrest the 

 author sends a message of peace, as dignified as 

 it is just, to his colleagues in what were then 

 enemy countries. For that message of goodwill 

 I for one thank him. W. B. Hardy. 



The Epistemological Problem. 



(i) 4 Study in Realism. By Prof. J. Laird. 

 Pp. xii-f228. (Cambridge: At the University 

 Press, 1920.) 145. net. 



(2) Studies in Contemporary Metaphysics. By 

 Prof. R. F. A. Hoernle. Pp. ix-t-314. (Lon- 

 don: Kegan Paul, Trench, Trubner, and Co., 

 Ltd., 1920.) 165. net. 



(0 "fL faut bien plus de principes que vous ne 

 1 pensez pour d^montrer ce dont personne 



ne doute," observes Malebranche in his " Entre- 

 NO. 2686, VOL. 107] 



tiens Metaphysiques." This came to mind 

 in reading the quotation from his adversary 

 Arnauld which Prof. Laird has placed at 

 the head of the introduction to his " Study 

 in Realism." There can be no knowledge 

 without object known, is the gist of Arnauld 's- 

 remark. How undeniable ! And yet Prof. Laird- 

 has to write a book and hint to us that he finds it 

 difficult to keep his study within reasonable 

 bounds. The realists are all alike; they disarm 

 their adversaries by the naivete of their defini- 

 tion, only to discover that there is no end to the 

 diversity of meanings their professedly obvious 

 affirmation may cover. " If the shade of Reid 

 could visit these regions to-day it would greet 

 Mr. Prichard, of Oxford ; but it would be startled 

 by Mr. Alexander, bewildered by Mr. Russell, and 

 distressed by Mr. Holt. Indeed, one is tempted" 

 to think that any realism defined to the quick 

 becomes nothing but the definer's private philo- 

 sophy." Such is one realist's confession. 



This troublesome problem of knowledge, how- 

 ever, is one to which searchers for truth, what- 

 ever be the scientific direction of their inquiry, 

 cannot be indifferent. It is impossible to avoid 

 its challenge, although it is not one of the great 

 problems of philosophy. It is riot, like the im- 

 mortality of the soul, the nature of the world, and 

 the existence of God, one of the problems which, 

 concern the whence, the why, and the whither of 

 human existence. The epistemological problem 

 is in effect the River Styx of the higher world of 

 philosophy, but there -is no Charon who can 

 be bribed with a fee to ferry us to the other 

 side. 



Why is realism called a theory? It is not a 

 theory in any proper meaning of the term. It is 

 simply an assumption concerning the reality of 

 things and the knowing relation, and the con- 

 tention that the assumption is consistent with the 

 facts. The assumption is that the object of know- 

 ledge is independent of the knowing, and that 

 knowledge is discovery, the independent things 

 or objects being directly revealed or given to the 

 mind. This is the ordinary assumption of common 

 sense, but neither the plain man nor the scien- 

 tific researcher calls it a theory or requires a 

 theory. It is the philosopher who wants a theory. 

 The argument of the realist seems to be that if 

 the assumption can be proved to be consistent 

 with the facts of perception, memory, imagination, 

 and such like processes, it will then become a 

 theory. To this the reply is, "Can the Ethiopian- 

 change his skin?" 



Prof. Laird is delightful to read. However diffi- 

 cult and abstruse the argument, it is bright with 



