Everyone goes out conquering the rest of the world. The greatest conquest is self conquest and for an average person, that begins with conquest of the body. There are very few people who first conquer the mind and thereby conquer the body. The body should not be your enemy, it should not be the one who dictates to you; you are the one who dictates to your body, breaking some of its habits. All of this comes under the category of tapas, heating up, exertion, changing the habit by doing something over and over again.
In ancient and medieval times there were certain undesirable schools of teaching among whom tapas or asceticism was so extreme that it was unpleasant both to the doer and the viewer. On the other hand, in the 19th and 20th centuries, extreme asceticism has given way to extreme comfort. Hatha yoga is comfortable asceticism. Do not make yourself too uncomfortable, and yet exert a little. If you do it harmoniously and gently, your purpose will be accomplished.
Why this conquest of the body? We are not talking merely of mastering one desire as against another but of mastering all the functions of the entire body as a whole. While you are practicing postures there is no other desire. Why? Because the mind can desire only one thing at a time. For the duration of time you have set your mind to watch the body developing, becoming relaxed, releasing tensions, removing its pain and so on that is its focus until the body is so trained that it learns to obey the mind. A person who practices hatha yoga regularly, daily, will have an easier time mastering desires of the senses than a person who does not. We are not speaking of a puritanical ideal. As another example, take the desire to overeat: when you are sensitive to the body, it is much easier to conquer such a desire. You learn how to absorb your energies into your system and reuse them.
This conquest by the mind of the body then is the first philosophical goal of hatha yoga: that the body should be under the direction of the mind. The mind says, move, the body moves. The body does not say ‘oh, I feel so lazy, I think I’ll just lie here in this comfortable bed surrounded with these comfortable cushions. I am so comfortable’. That kind of powerless power that the body has over the mind has to be broken. Otherwise you can neither sit in meditation for a long time, nor can you stay healthy, nor can your run your digestive system in any kind of proper order.
If the body is not purified, it will have adverse effects on the processes of mental purification. Sooner or later, one place or another, the body will become a hindrance to the mind in its own practice or worship, prayer or meditation. This tapas, this gentle exertion, is where the entire practice of hatha yoga fits in with the teaching of the great master, Patanjali.