The physical body is made up of countless numbers of tiny material particles called molecules and atoms. These material particles are constantly being replaced by material particles from the outside environment (in the form of food, water, air, and so on). Over a period of five to seven years, this process of metabolism brings about a complete change of the matter that makes up your body.
Many people believe that a person is the brain or some part of the brain. You may be one of them. If so, the following should boggle your mind:
Recent studies on the turnover of the molecular population within a given nerve cell have indicated that ... their macromolecular contingent is renewed about ten thousand times in a lifetime.*
In other words, the matter making up each brain cell is completely renewed every three days.
Your brain—that mass of matter which is contained in your skull today—is not the same brain that was in your skull last week.
* Paul Weiss, “The Living System: Determinism Stratified,” in Arthur Koestler and J.R. Smythies, eds., Beyond Reductionism (London: Hutchinson, 1969), p. 13.
You can eat so much food that your belly aches—yet you still want more! Even though your belly is filled to the point of physical pain, you, the self, are not full; you still desire to consume more. The fact that the body can be full or satisfied while you still feel empty is evidence that the body is not you.