30. This sentence is a direct quotation from d’Alembert’s “Introduction au Traité de dynamique ” (1743), Oeuvres, I, 392. Aram Vartanian, in Diderot and Descartes (Princeton, 1953), p. 157, sees this part of the Discourse as a continuation of Descartes’ rationalistic ideal of relating the totality of physical data to a unique cause. Condillac seems to have incorporated ideas from this same passage of d’Alembert’s almost word for word into the first paragraph of his Traité des systèmes (1749), Oeuvres, I, 121:
A system is nothing more than the arrangement of different parts of an art or a science in an order in which they all support one another, and in which the last are explained by the first. Those which explain the others are called principles; and the system is so much the more perfect as the principles are fewer in number: it is even to be hoped that one could reduce them to a single one.
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