=B. Mental.=--The law of perfect adaptation also explains why ideal feelings may at one time result in a pleasant, and at another time in a painful, feeling tone. According to the principle of apperception, the new experience must organize itself with whatever thoughts and feelings are now occupying consciousness. It necessarily happens that a given experience does not always equally harmonize with our present thoughts and feelings. The recognition of a friend under ordinary circumstances is agreeable, but amid certain associations or in a certain environment, such recognition would be disagreeable. So, too, while an original experience may have been agreeable, the memory of it may now be disagreeable; and vice versa. For instance, the memory of a former success or prosperity may, in the midst of present failure and poverty, be disagreeable; while the recollection of former failure and defeat may now, in the midst of success and prosperity, be agreeable. What is it that makes a sensation, a perception, a memory, or an apprehended relation pleasant under some circumstances and unpleasant under others? The rule appears to be that when the experience harmonizes with our present train of thought, when it promotes our present interests and intentions, it is pleasant; but when, on the other hand, it does not harmonize with our train of thought or thwarts or impedes our interests and purposes, it is unpleasant. =Function of Pleasure and Pain.=--From what has been noted concerning co-ordination between the adaptation of the organism to impression and the quality of the accompanying feeling, it is evident that pleasure and pain each have their part to play in promoting the ultimate good of the individual. Pain is beneficial, because it lets us know that there is some misadjustment to our environment, and thereby warns us to remove or cease doing what is proving injurious. In this connection, it may be noted that no disease is so dangerous as one that fails to make its presence known through pain. Pleasure also is valuable in so far as it results from perfect adaptation to a perfect environment, since it induces the individual to continue beneficial acts. It must be remembered, however, that so far as heredity or education has adapted our organism to improper stimuli, pleasure is no proof that the good of the organism is being advanced. In such cases, redemption can come to the fallen world only through suffering.