urity is achieved by freedom from desire, and desire should be distinguished from necessity. When one is hungry, one should eat, when one is thirsty one should drink, when one is fatigued, one should sleep. These absolute necessities cannot be called desires because without these necessities one cannot even survive.
Desire is a destructive form of longing which innervates the whole system, longing for things which are not necessary for the maintenance of the body. Every kind of luxury should be considered as desire. One has to judge for oneself whether this particular thing is absolutely necessary, or one can be comfortable even without that. Each one is one's own judge. If one can comfortably live happily without certain things, asking for those things once again is called desire. It will disturb the mind.
There are levels of desire, of a lower category and a higher category, etc. You may feel like having a cup of tea, it is a minor desire, have it. You may like to go for a walk - go, go for a walk. Any object that brings about tumult in the emotion, that is an object of desire by which you either want it intensely or hate it intensely. Intense wanting and intense hating will affect the emotions. The test of good health is freedom from emotion, correct understanding without any kind of ebullition, burning desire of any kind. This is briefly the definition of desire.
There are two kinds of desires, anabolic and catabolic. Anabolic desires are constructive, helpful and necessary for maintaining health and peace of mind. Catabolic desires are destructive in their nature. They throw the energy out. Any procedure by which we can conserve our energy is anabolic. Any process by which we deplete our energy and then become weak, that is catabolic. One must be able to very carefully distinguish between one and the other. Understanding is the judge, it is called Viveka-Shakti, capacity to distinguish between what is absolutely essential for living a normal life and what is irrelevant. This is a preliminary definition to the question "What is purity?".
So purity is not like an apple that falls from a tree, it has shades of definition. You will not find any such clear description of this subject in any book, each one has to use one's common sense.
It is necessary to save life. Suppose you find a snake is wriggling encircled by forest fire, you would like to save it but you do not go and catch hold of its neck. So even a good desire like wanting to save the snake should not be fulfilled in a reckless way. People generally take a long stick and throw it like this, etc. etc.
There are desires of a different type, like sexual desire. It is neither good nor bad, like fire. Do you consider fire as a good thing or a bad thing? You cannot say anything about it. It can destroy or it can cook your food. So, likewise, sexual desire is a conservative process of maintaining a balance in the system, it is not capable of definition in a cut-and-dried manner. It is, as any desire is, relative to circumstance. But if it is a passion, you may distinguish between desire and passion. Passion is voluptuous, tumult-like, and makes one sick afterwards. Great discrimination has to be exercised here.
There is a famous passage called Kama Gita in the Mahabharata. The desire says, "People try to conquer me, but they do not know that even the desire to conquer me is a desire, they don't understand that, so I am behind all their attempts."
Desire for God sublimates all other mortal desires. The higher absorbs the lower, the lower should be transmuted to the higher by meditation.