746 THE POPULAR SCIENCE MONTHLY. 



vious lie had read to the British Association a paper containing the 

 fundamental positions of his later work, " Theory of Political Econ- 

 omy," In 1865 appeared the treatise " On the Coal Question," deal- 

 ing with the problem of the exhaustion of tlie English coal-mines, the 

 calculations of which were adopted by Mr. Gladstone and Mr. Mill in 

 their treatment of the subject. In 1869 appeared the small work, 

 " Substitution of Similars the True Principle of Reasoning ; " in 

 1870 he read a paper before the Royal Society " On the Mechanical 

 Performance of Logical Inference ; " and about this time he com- 

 pleted his well-known Logical Machine. In 1870 appeared the first 

 edition of the " Elementary Lessons in Logic." The " Theory of Po- 

 litical Economy " was published in 1871 ; and the author's great work, 

 " The Principles of Science," was issued in two volumes in 1 874. 



" The Principles of Science " is a comprehensive treatise on pure 

 and applied logic, or on the formal theory of inference and the 

 methods of scientific investigation. The first book resumes the au- 

 thor's previous researches in pure logic, and carries them a step 

 further. All inference is regarded as essentially reasoning from simi- 

 lars to similars, afiirming that what is true of one thing is true of 

 its like. The rules of inference flowing from this general principle, 

 and the symbolical notation employed to express all the forms of 

 thought, are stated and exemplified with great fullness. The par- 

 ticular novelty introduced is the view of induction, which Prof. 

 Jevons regards as merely the inverse process of deduction. Thus, 

 in deduction, we have given to us certain relations among terms or 

 notions, and by the application of the formal laws of thought we 

 develop all the possible combinations which are consistent with 

 given relations. In induction, on the other hand, the combinations 

 of terms are given, and we require to reason backward to the pos- 

 sible relations from whicli they may result. Insisting strongly upon 

 his view of inductive inference. Prof. Jevons is led to criticise and 

 reject the ordinary accounts of the process. He declines to admit 

 that inductive research necessarily involves the idea of causation, 

 and assimilates it more nearly to the mathematical doctrine of proba- 

 bility. The chapter in which he expounds the philosophy of induc- 

 tive inference is peculiarly valuable, and deserves more careful criti- 

 cism than it has yet received. As final result we have the complete 

 subordination of induction to deduction; all inductive research, ac- 

 cording to Prof. Jevons, consisting of three steps framing an hy- 

 pothesis as to the general law, deductively inferring results from it, 

 and comparing the inferred conclusions with real fact. 



The subordinate points involved in this theory of induction, such 

 as the principles of combination and the general method of calculat- 

 ing probabilities, are treated very elaborately. The problem of in- 

 verse probability, which is, in Prof. Jevons's view, identical wifh the 

 problem of induction, receives most careful attention. Some attempts 

 have recently been made to carry out one or two of the elaborate 



