The concepts of pleasure and good, both separately and in their relation to one another, have for centuries been a favourite and fruitful subject of philosophical discussion. The contrasting concepts of pain and evil, however, though by no means entirely neglected, have been, and still are, less popular among philosophers. The reason for this disparity is not altogether clear. The title of a recent autobiography, “Philosophers lead sheltered lives,” might support the explanation that philosophers are (no doubt very properly) reluctant to write on matters of which they have little or no first-hand knowledge. Pleasure and good, it may be said, form part of their own personal experience: but their academic seclusion has little place for either pain or evil. A more serious reason is the assumption, often made but rarely defended, that, since pain and pleasure, evil and good, are in some sense pairs of “opposites,” a full discussion of all four concepts is unnecessary. The nature of pain, it is thought, can easily be inferred from the nature of pleasure, and that of evil from that of good. But even if this were true, it ought not to be taken for granted; the belief (it is more often implicit than explicit) that these concepts are symmetrically disposed opposites ought to be held (if at all) only after an investigation into all of them. And in fact the belief is not true. It does not need profound insight or observation to see that we cannot (as some of the utilitarians once thought) construct a simple hedonic scale on which pleasure appears as a plus, and pain as a minus quantity. Contemporary hedonists have often paid more attention to the psychological complexities involved in this problem than did their classical predecessors. A. L. Hilliard, for example, distinguishes carefully between pain and unpleasantness. Pain he describes as a sensitivity, “correlated with the excitation of specific receptors in the nervous system”: unpleasantness is an affectivity (i.e. roughly, an emotion) to which physiological correlates, though they must be assumed to exist, have not yet been discovered.