  Second, he recognized that the senses have an essential role to play in natural philosophy. The older interpretive literature sometimes had Descartes claiming that he could derive all natural philosophical or scientific knowledge from the pure intellect, independent of the senses. But Descartes knew full well that he could not do that. He distinguished between the general principles of his physics and the more particular mechanisms that he posited to explain natural phenomena, such as magnetism or the properties of oil and water. He claimed to derive the general principles “from certain seeds of truth” that are innate in the mind (6:64). These include the fundamental doctrine that the essence of matter is extension (Princ. II.3–4, IV.203). As to particular phenomena, in general he had to rely on observations to determine their properties (such as the properties of the magnet), and he acknowledged that multiple hypotheses about subvisible mechanisms could be constructed to account for those phenomena. The natural philosopher must, therefore, test the various hypotheses by their consequences, and consider empirical virtues such as simplicity and scope (Disc. VI; Princ. IV.201–6). Further, Descartes knew that some problems rely on measurements that can only be made with the senses, including determining the size of the sun (7:80) or the refractive indexes of various materials (Met. VIII).