The English word life does duty for many very different shades of meaning; but the word Prana familiar in the Upanishad and in the language of Yoga is restricted to the life-force whether viewed in itself or in its functionings. The popular sense of Prana was indeed the breath drawn into and thrown out from the lungs and so, in its most material and common sense, the life or the life-breath ; but this is not the philosophic significance of the word as it is used in the Upanishads. The Prana of the Upanishads is the life-energy itself which was supposed to occupy and act in the body with a fivefold movement, each with its characteristic name and each quite as necessary to the functioning of the life of the body as the act of respiration. Respiration in fact is only one action of the chief movement of the life-energy, the first of the five, — the action which is most normally necessary and vital to the maintenance and distribution of the energy in the physical frame, but which can yet be suspended without the life being necessarily destroyed.
The existence of a vital force or life-energy has been doubted by western Science, because that Science concerns itself only with the most external operations of Nature and has as yet no true knowledge of anything except the physical and outward. This Prana, this life-force is not physical in itself; it is not material energy, but rather a different principle supporting Matter and involved in it. It supports and occupies all forms and without it no physical form could have come into being or could remain in being. It acts in all material forces such as electricity and is nearest to self-manifestation in those that are nearest to pure force; no material force could exist or act without it, for from it they derive their energy and movement and they are its vehicles. But all material aspects are only field and form of the Prana which is in itself a pure energy, their cause and not their result. It cannot therefore be detected by any physical analysis; physical analysis can only resolve for us the combinations of those material happenings which are its results and the external signs and symbols of its presence and operation.
How then do we become aware of its existence ? By that purification of our mind and body and that subtilisation of our means of sensation and knowledge which become possible through Yoga. We become capable of analysis other than the resolution of forms into their gross physical elements and are able to distinguish the operations of the pure mental principle from those of the material and both of these from the vital or dynamic which forms a link between them and supports them both. We are then able to distinguish the movements of the Pranic currents not only in the physical body which is all that we are normally aware of, but in that subtle frame of our being which Yoga detects underlying and sustaining the physical. This is ordinarily done by the process of Pranayama, the government and control of the respiration. By Pranayama the Hathayogin is able to control, suspend and transcend the ordinary fixed operation of the Pranic energy which is all that Nature needs for the normal functioning of the body and of the physical life and mind, and he becomes aware of the channels in which that energy distributes itself in all its workings and is therefore able to do things with his body which seem miraculous to the ignorant, just as the physical scientist by his knowledge of the workings of material forces is able to do things with them which would seem to us magic if their law and process were not divulged. For all the workings of life in the physical form are governed by the Prana and not only those which are normal and constant and those which, being always potential, can be easily brought forward and set in action, but those which are of a more remote potentiality and seem to our average experience difficult or impossible.
But the Frame energy supports not only the operations of our physical life, but also those of the mind in the living body. Therefore by the control of the Pranic energy it is not only possible to control our physical and vital functionings and to transcend their ordinary operation, but to control also the workings of the mind and to transcend its ordinary operations. The human mind in fact depends always on the Pranic force which links it with the body through which it manifests itself, and it is able to deploy its own force only in proportion as it can make that energy available for its own uses and subservient to its own purposes. In proportion, therefore, as the Yogin gets back to the control of the Prana, and by the direction of its batteries opens up those nervous centres (Chakras) in which it is now sluggish or only partially operative, he is able to manifest powers of mind, sense and consciousness which transcend our ordinary experience. The so-called occult powers of Yoga are such faculties which thus open up of themselves as the Yogin advances in the control of the Pranic force and, purifying the channels of its movement, opens up communication between the consciousness of his subtle, subliminal being and the consciousness of his gross, physical and superficial existence.
Thus the Prana is vital or nervous force which bears the operations of mind and body, is yoked by them as it were like a horse to a chariot and driven by the mind along the paths on which it wishes to travel to the goal of its desire. Therefore it is described in this Upanishad as yoked and moving forward and again as being led forward, the images recalling the Vedic symbol of the Horse by which the Pranic force is constantly designated in the Rig-veda. It is in fact that which does all the action of the world in obedience to conscious or subconscious mind and in the conditions of material force and material form. While the mind is that movement of Nature in us which represents in the mould of our material and phenomenal existence and within the triple term of the Ignorance the knowledge aspect of the Brahman, the consciousness of the knower, and body is that which similarly represents the being of the existent in the mask of phenomenally divisible substance, so Prana or life-energy represents in the flux of phenomenal things the force, the active dynamis of the Lord who controls and enjoys the manifestation of His own being. It is a universal energy present in every atom and particle of the universe, and active in every stirring and current of the constant flux and interchange which constitutes the world.