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 cerebro-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 "Principles 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 testimony 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, particularly 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