How do I know what desires fall into the various categories of natural and necessary desires, and are there hard and fast rules about how to apply the resulting categorization of a particular desire I am considering?"
- natural and necessary - desire for something for survival, for health of the body, for happiness of the soul
- natural yet unnecessary - desire for something which is difficult to get or causes much more pain
- unnatural and unnecessary - desire for something which is an "empty" concept which has no limit, and does not contribute to a happy life - such as power, great wealth, or immortality.
This works as a type of remedy, by reasoning through one's desires. It is common sense to know what is necessary for survival and health of the body. For happiness we must rely on the knowledge and experience of wise people (elders) and likely Epicurus gave clear guidance in his writing (which we no longer have). We do see in the Principal Doctrines that friendship is necessary for happiness.