The relations between a person's awareness and his body have been problems for modern thought ever since Descartes pointed out that each can completely be conceived without essential reference to the other. Whatever thought may be and however wide its range, no thoughts are spatially extended; even the thoughts of spatially extended things are not spatially extended. Similarly the realm of extended things may be of whatever character physics discovers it to have, yet none of those extended things need be conceived to think. Descartes' own resolution of the problem broke down in two related ways. He suggested that mind and body are two distinct substances that interact through conjunction in the pineal gland. Yet if the mind were casually related only to mental elements, it did not seem possible for it to move the pineal gland; or if it could, then why not other bodily parts also? Furthermore, most people have come to believe that whatever a person is, he is one thing, not two essentially different things; therefore the mind—body problem now is one of conceiving of two somehow different aspects of a person. Put in more contemporary language, the problem is to understand the relation between physical and mental functions.