The first in this series of modern rationalists, Descartes, is credited with defining a "mind-body problem" which is a major subject of discussion for university philosophy courses. According to the presentation his , the human mind and body are different in kind, and while Descartes agrees with Hobbes for example that the human body works like a clockwork mechanism, and its workings include memory and imagination, the real human is the thinking being, a soul, which is not part of that mechanism. Descartes explicitly refused to divide this soul into its traditional parts such as intellect and reason, saying that these things were indivisible aspects of the soul. Descartes was therefore a dualist, but very much in opposition to traditional Aristotelian dualism. In his he deliberately uses traditional terms and states that his active faculty of giving ideas to his thought must be corporeal, because the things perceived are clearly external to his own thinking and corporeal, while his passive faculty must be incorporeal (unless God is deliberately deceiving us, and then in this case the active faculty would be from God). This is the opposite of the traditional explanation found for example in Alexander of Aphrodisias and discussed above, for whom the passive intellect is material, while the active intellect is not. One result is that in many Aristotelian conceptions of the "nous", for example that of Thomas Aquinas, the senses are still a source of all the intellect's conceptions. However, with the strict separation of mind and body proposed by Descartes, it becomes possible to propose that there can be thought about objects never perceived with the body's senses, such as a thousand sided geometrical figure. Gassendi objected to this distinction between the imagination and the intellect in Descartes. Hobbes also objected, and according to his own philosophical approach asserted that the "triangle in the mind comes from the triangle we have seen" and "essence in so far as it is distinguished from existence is nothing else than a union of names by means of the verb is". Descartes, in his reply to this objection insisted that this traditional distinction between essence and existence is "known to all".