Objection 1. It would seem that pleasure does not hinder the use of reason. Because repose facilitates very much the due use of reason: wherefore the Philosopher says (Phys. vii, 3) that "while we sit and rest, the soul is inclined to knowledge and prudence"; and it is written (Wisdom 8:16): "When I go into my house, I shall repose myself with her," i.e. wisdom. But pleasure is a kind of repose. Therefore it helps rather than hinders the use of reason.
Objection 2. Further, things which are not in the same subject though they be contraries, do not hinder one another. But pleasure is in the appetitive faculty, while the use of reason is in the apprehensive power. Therefore pleasure does not hinder the use of reason.
Objection 3. Further, that which is hindered by another, seems to be moved, as it were, thereby. But the use of an apprehensive power moves pleasure rather than is moved by it: because it is the cause of pleasure. Therefore pleasure does not hinder the use of reason.
On the contrary, The Philosopher says (Ethic. vi, 5), that "pleasure destroys the estimate of prudence."
I answer that, As is stated in Ethic. x, 5, "appropriate pleasures increase activity... whereas pleasures arising from other sources are impediments to activity." Accordingly there is a certain pleasure that is taken in the very act of reason, as when one takes pleasure in contemplating or in reasoning: and such pleasure does not hinder the act of reason, but helps it; because we are more attentive in doing that which gives us pleasure, and attention fosters activity.
On the other hand bodily pleasures hinder the use of reason in three ways. First, by distracting the reason. Because, as we have just observed, we attend much to that which pleases us. Now when the attention is firmly fixed on one thing, it is either weakened in respect of other things, or it is entirely withdrawn from them; and thus if the bodily pleasure be great, either it entirely hinders the use of reason, by concentrating the mind's attention on itself; or else it hinders it considerably. Secondly, by being contrary to reason. Because some pleasures, especially those that are in excess, are contrary to the order of reason: and in this sense the Philosopher says that "bodily pleasures destroy the estimate of prudence, but not the speculative estimate," to which they are not opposed, "for instance that the three angles of a triangle are together equal to two right angles." In the first sense, however, they hinder both estimates. Thirdly, by fettering the reason: in so far as bodily pleasure is followed by a certain alteration in the body, greater even than in the other passions, in proportion as the appetite is more vehemently affected towards a present than towards an absent thing. Now such bodily disturbances hinder the use of reason; as may be seen in the case of drunkards, in whom the use of reason is fettered or hindered.
Reply to Objection 1. Bodily pleasure implies indeed repose of the appetite in the object of pleasure; which repose is sometimes contrary to reason; but on the part of the body it always implies alteration. And in respect of both points, it hinders the use of reason.
Reply to Objection 2. The powers of the appetite and of apprehension are indeed distinct parts, but belonging to the one soul. Consequently when the soul is very intent on the action of one part, it is hindered from attending to a contrary act of the other part.
Reply to Objection 3. The use of reason requires the due use of the imagination and of the other sensitive powers, which are exercised through a bodily organ. Consequently alteration in the body hinders the use of reason, because it hinders the act of the imagination and of the other sensitive powers.
Source: Summa Theologica (New Advent)