Sri Aurobindo translates Taittiriya Upanishad, Brahmanandavalli, Chapter One (third part): “and from the earth, herbs and plants; and from the herbs and plants, food; and from food man was born. Verily man, this human being, is made of the essential substance of food. And this that we see is the head of him, and this is his right side and this is his left; and this is his spirit and the self of him; and this is his lower member whereon he resteth abidingly. Whereof this is the Scripture.”
Food, in this context, represents the world of Matter. The human being is constituted out of Matter, and the forces of the life-energy, as well as higher evolved forces such as mental energy, etc. all find their foundation for action within the human being on the basis of Matter, and conditioned by the limits and mode of action of material substance.
Sri M. P. Pandit notes: “Man comes into being with the birth of his body which is constituted of the essence and the modifications of Matter and he dies with the disintegration of that body of Matter. Whatever the other principles active in him like mind, life, etc. they depend for their existence and functioning on the physical body and are, normally, wholly conditioned by it. Whichever way one looks at this human being in creation, horizontally or vertically, he is an evolute of Matter — Food.”
Sri Aurobindo writes in The Life Divine (Chapter XXVI): “Here in the material world everything is founded upon the formula of material substance. Sense, Life, Thought found themselves upon what the ancients called the Earth-Power, start from it, obey its laws, accommodate their workings to this fundamental principle, limit themselves by its possibilities and, if they would develop others, have even in that development to take account of the original formula, its purpose and its demand upon the divine evolution. The sense works through physical instruments, the life through a physical nervous-system and vital organs, the mind has to build its operations upon a corporeal basis and use a material instrumentation, even its pure mental workings have to take the data so derived as a field and as the stuff upon which it works.”
The Upanishad here is not simply describing the basis of Matter in human life and activity, but setting up a paradigm which it will subsequently utilize to describe the more subtle and powerful sheathes that, together with the material sheath, form the completeness of the human being.