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 TESTIMONY AND AUTHORITY. 83 credentials ; in part the credentials of the Societies and other media of record and publication through which his work has in successive stages come to the individual reader ? If I submit myself to the knife of the Surgeon, how have I assured myself that he will do the right thing, unless by relying upon a complex tissue of testimony as to the pro- fessional ability of a large number of individuals ? The same thing is seen in the employment of mathematical results by non -mathematical persons ; and, as an attempt was made to show at the beginning of this paper, cannot by any process be avoided in even the best scientific work. The actual occurrence of these " bridges " of testimony, explicitly set out in the structure of each one's knowledge, is doubtless rare. The singling of them out must in general be a process of logical analysis. But a man can so little divest himself of his social nature that they exist impli- citly in almost every part of his knowledge. They are characteristic not merely of loosely and carelessly held floating opinion ; they chiefly rise into prominence in the more carefully and exactly ascertained portions of his know- ledge ; those, indeed, more than in any others, in which he is apt to take pride for having thoroughly worked out and sifted them for himself.