37. In these paragraphs, as d’Alembert moves from a description of the unity of geometry to the statement of the assumption of unity in all phenomena, we have a most perfect exposition of the marriage of the “geometric spirit” with the empiricism for which Condillac gave the theoretical definition in his Traité des systèmes (1749). There Condillac asserted that the only legitimate system was one that explained facts by facts, and the best system was one in which one fact would explain all the others.

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