Thomas Aquinas

“ I answer that, A twofold movement of the appetite ensues from a present evil. One is that whereby the appetite is opposed to the present evil; and, in this respect, sorrow is of no use; because that which is present, cannot be not present. The other movement arises in the appetite to the effect of avoiding or expelling the saddening evil: and, in this respect, sorrow is of use, if it be for something which ought to be avoided. Because there are two reasons for which it may be right to avoid a thing. First, because it should be avoided in itself, on account of its being contrary to good ”

“ Just as sorrow for an evil arises from a right will and reason, which detest the evil, so sorrow for a good is due to a perverse reason and will, which detest the good. Consequently such sorrow is an obstacle to the praise and merit of the virtuous good; for instance, when a man gives an alms sorrowfully. ”

“ Because, however, in the science of Morals, we consider things individually---for actions are concerned about individuals---that which is good on some supposition, should be considered as good: just as that which is voluntary on some supposition, is judged to be voluntary, as stated in Ethic. ”

“ Therefore man's greatest good consists in the fulfilment of his will. Now pain consists in something happening contrary to the will, as Augustine declares (De Civ. Dei xiv, 6,15) . Therefore pain is man's greatest evil. ”