Changes

Hathayoga aims at the conquest of the life and the body whose combination in the food sheath and the vital vehicle constitutes, as we have seen, the gross body and whose equilibrium is the foundation of all Nature’s workings in the human being. The equilibrium established by Nature is sufficient for the normal egoistic life; it is insufficient for the purpose of the Hathayogin. For it is calculated on the amount of vital or dynamic force necessary to drive the physical engine during the normal span of human life and to perform more or less adequately the various workings demanded of it by the individual life inhabiting this frame and the world-environment by which it is conditioned. Hathayoga therefore seeks to rectify Nature and establish another equilibrium by which the physical frame will be able to sustain the inrush of an increasing vital or dynamic force of Prana indefinite, almost infinite in its quantity or intensity. In Nature the equilibrium is based upon the individualisation of a limited quantity and force of the Prana; more than that the individual is by personal and hereditary habit unable to bear, use or control. In Hathayoga, the equilibrium opens a door to the universalisation of the individual vitality by admitting into the body, containing, using and controlling a much less fixed and limited action of the universal energy. <ref>
http://incarnateword.in/cwsa/23/the-systems-of-yoga#p8
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===Limitations of Hathayoga===