The Yogis of ancient India took a somewhat different approach toward understanding the physical world than we have seen in the West. Western scientists have generally started from the most external physical forms and from there, systematically moved inward toward more and more subtle understanding. The Yogis quickly recognized that the physical form is the outer, grossest expression and could not possibly be the ultimate cause, and that therefore, something else must be building, creating and operating the outer world and all its forms. The Taittiriya Upanishad, indeed, started from the outer physical and moved toward the vital, mental, knowledge and ultimately the spiritual levels of existence, with each one more subtle than the prior, and having a causative action on the former level.
These Yogis, when they looked at the interaction between the subtle inner levels and the gross physical forms, developed a detailed understanding of the principles of action of the vital force, which they called Prana. Sri Aurobindo describes briefly this view: “This mental or psychical body, which the soul keeps even after death, has also a subtle pranic force in it corresponding to its own subtle nature and substance,–for wherever there is life of any kind, there must be the pranic energy and a substance in which it can work,–and this force is directed through a system of numerous channels, called nadi,–the subtle nervous organisation of the psychic body,–which are gathered up into six (or really seven) centres called technically lotuses or circles, cakra, and which rise in an ascending scale to the summit where there is the thousand-petalled lotus from which all the mental and vital energy flows. Each of these lotuses is the centre and the storing-house of its own particular system of psychological powers, energies and operations,–each system corresponding to a plane of our psychological existence,–and these flow out and return in the stream of the pranic energies as they course through the Nadis.”
With this understanding, the practitioners of Yoga are able to understand how they can open to and utilize the energies flowing through or focused within any of these chakras and apply that energy to spiritual growth and realization, inner mental, emotional and vital development, interactions and relations with the outer world, and physical strength and well-being of the physical body.
Sri Aurobindo, The Synthesis of Yoga, Part Two: The Yoga of Integral Knowledge, Chapter 28, Rajayoga, pp. 514-515