"We know nothing of the principle of health, the positive of which pathology is the negative, except from observation and experience. Nothing but observation and experience will teach us the ways to maintain or to bring back the state of health. It is often thought that medicine is the curative process. It is no such thing; medicine is the surgery of functions as surgery proper is that of limbs and organs."

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Desired substance, things, patterns, or sequences of experience that are in some sense "good" for the organism - items of diet, conditions of life, temperature, entertainment, sex, and so forth - are never such that more of the something is always better than less of the something. Rather, for all objects and experiences, there is a quantity that has optimum value. Above that quantity, the variable becomes toxic. To fall below that value is to be deprived.

— Gregory Bateson