Page:Popular Science Monthly Volume 79.djvu/471

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to link them together by the force of analogy, and to endeavor to attain that high degree of knowledge in which particular effects are recognized as dependent upon more general effects, nature is compared with herself in her larger processes, and thus ways are opened before us by which the different parts of physical science may be perfected. For success in the former sort of study there are needful only a good memory, assiduity and careful attention; but for the sort of which we are here speaking other qualities are requisite: breadth of view, steadiness of vision, a power of reasoning formed by the practise of reflection even more than by learning. For such study, in short, a man must have that quality of mind which enables him grasp remote relations between things, to bring them together, and thereby to form a body of reasoned conclusions, after having duly estimated similarities and weighed probabilities.

But these judicious and stimulating, if slightly vague, appeals for the conversion of natural history into a science of causal relations and generalized laws, were not the principal purpose of the preliminary discourse. The thought of Buffon at the time when he wrote that essay seems to have been dominated above all by a single idea, which was also one of the two or three ruling ideas of the whole of the first half of the eighteenth century—namely, the Leibnitian "principle of continuity'* (lex continui). In the intellectual fashions of this period, next to the blessed word "Nature" the most sacred phrase was "the Great Chain of Beings"; indeed, one of the truths that man was supposed to know most surely about nature was that she "makes no leaps." In the form, especially, of the neo-Platonic and Spinozistic metaphysical assumption that all possible forms must exist, the principle was much older than the philosophy of Leibniz; but it owed to him and his disciples a more definite formulation and a greatly increased popular currency. It declared that all entities are arranged in a graded scale of similarity, so that for every being that exists there also exists some other (in the strict version of the principle, one and only one other) from which its difference is infinitesimal, i. e., less than any assignable difference. A typical statement of the doctrine is Bonnet's:

All this (as Bonnet's language intimates) was held by the Leibnitian philosophy to be logically implied by the still more fundamental "principle of sufficient reason." For if the gradations found in nature