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
- This implied that there must be one, and can be only one, sample of every possible kind or degree of entity. To consider Leibniz's attitude toward this form of the principle would involve too much technical metaphysics.
- "Contemplation de la Nature" (1764), 2d ed., 1769, I., 26-27.