54 THE POPULAR SCIENCE MONTHLY. 



Quite recently, while conversing with a scholar and logician of far 

 more than usual powers, we chanced to talk of the alleged feats of 

 levitation, and he asked me how they were to be explained. I told him 

 that there was no evidence that they had ever occurred ; and that it 

 was known deductively, by the established laws of physiology, that they 

 had not and could not occur. I furthermore stated that claims of this 

 sort could be and should be only studied by experts ; that experiments 

 with living human beings could only be conducted by experts in cere- 

 bro-physiology, and that probably there were not half a dozen persons 

 in the world capable of making experiments of that kind. My friend 

 failed to see the justness of this view, and confessed himself unable to 

 understand how so simple a matter as the rising of a body in a room 

 could not be settled by the eyes of any honest, well-balanced man. 

 " Why," said he, " if a dozen George Washingtons should testify that 

 they had all seen a man rise in the air, I should be compelled, by the 

 rules of evidence, to believe them. What is the need of an expert in 

 a matter of simple eyesight and common honesty ? " 



I refer to these conversational experiences, because they represent, 

 in a concrete form, the present attitude of scholars and logicians toward 

 the principles of evidence. 



That these instances are not exceptional is proved by the literature 

 of science, of religion, of logic, and of law, in all of which departments 

 the subject of human testimony is more or less discussed. Neither in 

 Whewell's " History of the Inductive Sciences " nor in Jevons's " Prin- 

 ciples of Science " do we find a correct or thorough analysis of human 

 testimony, on which all science depends ; by these authors, as much as 

 by religious, apologetic writers, it is assumed that the senses are to 

 be trusted. In the department of logic we do not find, either in Mill 

 or Hamilton, any attempt even to build up a science of human testi- 

 mony which must everywhere constitute the premises of reasoning, and 

 by which the results of reasoning are to be determined. Constantly 

 Sir William Hamilton reiterates that logic deals only with the forms of 

 reasoning, and is not at all responsible for the premises ; but nowhere 

 does he point out, in a satisfactory manner, the principles on which 

 premises are to be obtained. It is true that Bacon, under the fantastic 

 titles, " Idols of the Tribe," " Idols of the Den," " Idols of the Forum," 

 and " Idols of the Theatre," first pointed out some of the more obvious 

 sources of error, and writers on logic repeat his views ; but other sources 

 of error, equally important but far more subtile, are not referred to even 

 in the most recent treatises on reasoning. Students of science, par- 

 ticularly of physiological science, and, above all, experimenters with 

 living human beings, must either trust to their instincts, as many do, or 

 find out for themselves, by study and experience, the special sources of 

 error in researches of this character, and guard against them. 



Coming to law, we find that Prof. Greenleaf, one of the most valued 

 writers on the principles of evidence, says that " the credit due to the 



