"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."