9. The “substantial forms” and “occult qualities” to which d’Alembert refers are technical concepts which he alleges are derived from ancient philosophy and were incorporated into the medieval scholastic philosophical-theological system. The philosophes disagreed contemptuously with the scholastics about what was an adequate explanation of any given phenomenon. One of the ways the scholastics “explained” physical phenomena, for instance, was by postulating the existence of “occult qualities” (unknown causes of manifest effects) hidden within bodies. These scholastic inventions are summarily dealt with as being utterly useless explanations of anything in the article “Quality (Metaphysics),” Encyclopédie, XIII, 651, which was probably written by d’Alembert. The term “substantial form” is defined in d’Alembert’s article “Substantial Form,” Encyclopédie, VII, 176, as a “barbarous term of old scholastic philosophy which was principally used to designate alleged material beings which were nevertheless not matter.”


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