We might easily imagine, for example, that thinking is a purely spiritual act, and that in the way it comes about on earth in the human soul it has nothing to do with existence in a body. In one sense this is so. But spiritually independent as human thinking is, it could not take its course here in earth existence were it unable to have the support of the body and its processes. I may avail myself of a comparison which I have often used here on similar occasions. When a man is walking, the ground he walks on is certainly not the essential part of his activity; the essential part is inside his skin; but without the support of the ground he could not get along.
It is the same with thinking. In essence, thinking is certainly not a brain-process, but without the support of the brain it could not take its earthly course. In the light of this comparison one gets a right conception of the spirituality as well as of the physical limitations of human thinking In short, my dear friends, here in earthly life there is nothing in man that does not depend on the body for support.
Source: Rudolf Steiner – GA 218 – MEMORY AND LOVE – Stuttgart, 4th December, 1922