In his later years, Descartes was well aware that he had not successfully finished the project that he had begun in the Treatise of man and had not been able to formulate one comprehensive mind-body theory. He sometimes expressed irritation when others reminded him of this. In reply to the questions “how can the soul move the body if it is in no way material, and how can it receive the forms of corporeal objects?” he said that “the most ignorant people could, in a quarter of an hour, raise more questions of this kind than the wisest men could deal with in a lifetime; and this is why I have not bothered to answer any of them. These questions presuppose amongst other things an explanation of the union between the soul and the body, which I have not yet dealt with at all” (12 January 1646, AT IX:213, CSM II:275). On other occasions, he came close to admitting defeat. “The soul is conceived only by the pure intellect; body (i.e. extension, shapes and motions) can likewise be known by the intellect alone, but much better by the intellect aided by the imagination; and finally what belongs to the union of the soul and the body is known only obscurely by the intellect alone or even by the intellect aided by the imagination, but it is known very clearly by the senses. […] It does not seem to me that the human mind is capable of forming a very distinct conception of both the distinction between the soul and the body and their union; for to do this it is necessary to conceive them as a single thing and at the same time to conceive them as two things; and this is absurd” (28 June 1643, AT III:693, CSMK 227). He admitted that the unsuccessfulness of his enterprise might have been his own fault because he had never spent “more than a few hours a day in the thoughts which occupy the imagination and a few hours a year on those which occupy the intellect alone” (AT III:692, CSMK 227). But he had done so for a good reason because he thought it “very harmful to occupy one’s intellect frequently upon meditating upon [the principles of metaphysics which give us or knowledge of God and our soul], since this would impede it from devoting itself to the functions of the imagination and the senses” (AT III:695, CSMK 228). He advised others to do likewise: “one should not devote so much effort to the Meditations and to metaphysical questions, or give them elaborate treatment in commentaries and the like. […] They draw the mind too far away from physical and observable things, and make it unfit to study them. Yet it is just these physical studies that are most desirable for people to pursue, since they would yield abundant benefits for life” (Conversation with Burman, 1648, AT V:165, CSMK 346–347). We will follow this wise advice.