Objection 1. It would seem that insensibility is not a vice. For those are called insensible who are deficient with regard to pleasures of touch. Now seemingly it is praiseworthy and virtuous to be altogether deficient in such matters: for it is written (Daniel 10:2-3): "In those days Daniel mourned the days of three weeks, I ate no desirable bread, and neither flesh nor wine entered my mouth, neither was I anointed with ointment." Therefore insensibility is not a sin.
Objection 2. Further, "man's good is to be in accord with reason," according to Dionysius (Div. Nom. iv). Now abstinence from all pleasures of touch is most conducive to man's progress in the good of reason: for it is written (Daniel 1:17) that "to the children" who took pulse for their food (Daniel 1:12), "God gave knowledge, and understanding in every book and wisdom." Therefore insensibility, which rejects these pleasures altogether, is not sinful.
Objection 3. Further, that which is a very effective means of avoiding sin would seem not to be sinful. Now the most effective remedy in avoiding sin is to shun pleasures, and this pertains to insensibility. For the Philosopher says (Ethic. ii, 9) that "if we deny ourselves pleasures we are less liable to sin." Therefore there is nothing vicious in insensibility.
On the contrary, Nothing save vice is opposed to virtue. Now insensibility is opposed to the virtue of temperance according to the Philosopher (Ethic. ii, 7; iii, 11). Therefore insensibility is a vice.
I answer that, Whatever is contrary to the natural order is vicious. Now nature has introduced pleasure into the operations that are necessary for man's life. Wherefore the natural order requires that man should make use of these pleasures, in so far as they are necessary for man's well-being, as regards the preservation either of the individual or of the species. Accordingly, if anyone were to reject pleasure to the extent of omitting things that are necessary for nature's preservation, he would sin, as acting counter to the order of nature. And this pertains to the vice of insensibility.
It must, however, be observed that it is sometimes praiseworthy, and even necessary for the sake of an end, to abstain from such pleasures as result from these operations. Thus, for the sake of the body's health, certain persons refrain from pleasures of meat, drink, and sex; as also for the fulfilment of certain engagements: thus athletes and soldiers have to deny themselves many pleasures, in order to fulfil their respective duties. On like manner penitents, in order to recover health of soul, have recourse to abstinence from pleasures, as a kind of diet, and those who are desirous of giving themselves up to contemplation and Divine things need much to refrain from carnal things. Nor do any of these things pertain to the vice of insensibility, because they are in accord with right reason.
Reply to Objection 1. Daniel abstained thus from pleasures, not through any horror of pleasure as though it were evil in itself, but for some praiseworthy end, in order, namely, to adapt himself to the heights of contemplation by abstaining from pleasures of the body. Hence the text goes on to tell of the revelation that he received immediately afterwards.
Reply to Objection 2. Since man cannot use his reason without his sensitive powers. which need a bodily organ. as stated in I, 84, 7,8, man needs to sustain his body in order that he may use his reason. Now the body is sustained by means of operations that afford pleasure: wherefore the good of reason cannot be in a man if he abstain from all pleasures. Yet this need for using pleasures of the body will be greater or less, according as man needs more or less the powers of his body in accomplishing the act of reason. Wherefore it is commendable for those who undertake the duty of giving themselves to contemplation, and of imparting to others a spiritual good, by a kind of spiritual procreation, as it were, to abstain from many pleasures, but not for those who are in duty bound to bodily occupations and carnal procreation.
Reply to Objection 3. In order to avoid sin, pleasure must be shunned, not altogether, but so that it is not sought more than necessity requires.
Source: Summa Theologica (New Advent)