"Prana is the name by which we designate a universal principle, which principle is the essence of all motion, force or energy, whether manifested in gravitation, electricity, the revolution of the planets, and all forms of life, from the highest to the lowest. It may be called the soul of Force and Energy in all their forms, and that principle which, operating in a certain way, causes that form of activity which accompanies life."
- Ramacharaka, Yogi, The Hindu-Yogi Science of Breath, pp. 16, 17.