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THE POPULAR SCIENCE MONTHLY. 



^ tainly the form in which parts of the book 

 are cast savors of the past epoch. It may 

 bo said that the object of the book is to pre- 

 sent the ' facts ' of zoology in a logical 

 order; and that this sufficiently explains 

 the arrangements to which objection might 

 be taken as above, viz., the commencing 

 with the higher instead of the lower groups, 

 the prominent position assigned to external 

 and little-significant characters, the absence 

 of any recognition of the leading doctrine 

 of modern zoology, the doctrine of filiation. 

 To this there is nothing to say excepting 

 that of the very many logical methods of 

 treatment possible in a hand-book of zoology, 

 many are easy to follow out, and that one, 

 which aims at presenting a logical classifica- 

 tion of the kind spoken of by Mill, in which 

 objects ' are arranged in such groups, and 

 those groups in such an order as shall best 

 conduce to the ascertainment and remem- 

 brance of their laws,' is a very difficult one 

 to follow out. This kind of classification 

 involves nothing less than an attempt (how- 

 ever inadequate) to trace the animal pedi- 

 gree ; for the laws to be ascertained and 

 remembered are the laws of heredity and 

 adaptation. We may regret, then, that so 

 able a zoologist as Prof. Carus has remained 

 in the old grooves and not ventured on to 

 the inevitable track where Gegenbaur and 

 Haeckel have preceded him." 



LITERARY NOTICES. 



Problems of Life and Mind. By George 

 Henry Lewes. First Series, Vol. II. 

 Boston : J. R. Osgood & Co., pp. 487. 

 Price, $3. 



The " problems " discussed by Mr. 

 Lewes in the two volumes which constitute 

 his first series are six in number. Of these, 

 the first, " Limitations of Knowledge," was 

 considered in the preceding volume ; the 

 rest are considered in the volume before us. 

 The author's purpose in this series is " to 

 lay down the foundations of a creed, by 

 exhibiting the method which determines 

 all successful inquiry, and by specifying 

 certain general results reached on that 

 method." Thoroughly imbued with the sci- 

 entific spirit, Mr. Lewes applies to meta- 

 physical discussions the same methods 

 which have in modern times given such 

 brilliant results in the field of natural sci- 

 ence. The problems which he discusses 

 arc among the most intricate which have 



worried the mind of man from the dawn 

 of reason. They have been the object of 

 profound study ever since philosophy first 

 had a place in human thought ; men have 

 viewed them from every side, and attempt- 

 ed their solution in every conceivable way, 

 but still they remain in all their pristine 

 obscurity. The scientific method of inves- 

 tigation has at last been brought to bear 

 upon these problems, and already we seem 

 to be gaining some headway, under the 

 guidance of Spencer, Bain, Lewes, and 

 their fellows. 



The first problem discussed in the vol- 

 ume before us is that of Certitude. The 

 author's one test of truth, or of certitude, 

 is the principle of equivalence. When two 

 terms have the same import, i. e., are equiv- 

 alent^ they may be predicated of one an- 

 other ; and all errors, both in reasoning and 

 conduct, arise from assuming equivalence in 

 terms where it does not exist. Mathemati- 

 cal truths are exact, necessary, only when 

 the terms in which they are stated are 

 equivalent ; mathematical propositions be- 

 come inexact, or contingent, whenever 

 they are applied to cases involving condi- 

 tions not included in the terms. The ob- 

 jection might be made that this reduces 

 truth to an identical proposition " a thing 

 is itself." Yet, in propounding any truth, 

 what more does one intend than to express 

 what the facts are ; and what is a state- 

 ment of facts more than the assertion that 

 they are what they are ? When the two 

 terms of a proposition can be thus shown 

 to be equivalent, the proposition is a truth, 

 and we possess certitude of it. Mr. Lewes 

 shows that this principle of equivalence is 

 the same as the Universal Postulate of 

 Herbert Spencer, of which it is merely the 

 positive statement. Our author's principle 

 is, " Truth is the equivalence of its terms." 

 He states Spencer's principle as follows : 

 " Whenever a subject and predicate are so 

 united that the one term is incapable of 

 appearing to thought without the other, 

 the proposition is necessary ; and its nega- 

 tive being unthinkable, the proposition it- 

 self must be true." Our author further 

 shows that the three scholastic principles, 

 Identity, Contradiction, and Ratio Sufficicns, 

 are all reducible to the principle of equiva- 

 lence. 



