Objection 1. It would seem that the reason cannot be overcome by a passion, against its knowledge. For the stronger is not overcome by the weaker. Now knowledge, on account of its certitude, is the strongest thing in us. Therefore it cannot be overcome by a passion, which is weak and soon passes away.
Objection 2. Further, the will is not directed save to the good or the apparent good. Now when a passion draws the will to that which is really good, it does not influence the reason against its knowledge; and when it draws it to that which is good apparently, but not really, it draws it to that which appears good to the reason. But what appears to the reason is in the knowledge of the reason. Therefore a passion never influences the reason against its knowledge.
Objection 3. Further, if it be said that it draws the reason from its knowledge of something in general, to form a contrary judgment about a particular matter--on the contrary, if a universal and a particular proposition be opposed, they are opposed by contradiction, e.g. "Every man," and "Not every man." Now if two opinions contradict one another, they are contrary to one another, as stated in Peri Herm. ii. If therefore anyone, while knowing something in general, were to pronounce an opposite judgment in a particular case, he would have two contrary opinions at the same time, which is impossible.
Objection 4. Further, whoever knows the universal, knows also the particular which he knows to be contained in the universal: thus who knows that every mule is sterile, knows that this particular animal is sterile, provided he knows it to be a mule, as is clear from Poster. i, text. 2. Now he who knows something in general, e.g. that "no fornication is lawful," knows this general proposition to contain, for example, the particular proposition, "This is an act of fornication." Therefore it seems that his knowledge extends to the particular.
Objection 5. Further, according to the Philosopher (Peri Herm. i), "words express the thoughts of the mind." Now it often happens that man, while in a state of passion, confesses that what he has chosen is an evil, even in that particular case. Therefore he has knowledge, even in particular.
Therefore it seems that the passions cannot draw the reason against its universal knowledge; because it is impossible for it to have universal knowledge together with an opposite particular judgment.
On the contrary, The Apostle says (Romans 7:23): "I see another law in my members, fighting against the law of my mind, and captivating me in the law of sin." Now the law that is in the members is concupiscence, of which he had been speaking previously. Since then concupiscence is a passion, it seems that a passion draws the reason counter to its knowledge.
I answer that, As the Philosopher states (Ethic. vii, 2), the opinion of Socrates was that knowledge can never be overcome by passion; wherefore he held every virtue to be a kind of knowledge, and every sin a kind of ignorance. In this he was somewhat right, because, since the object of the will is a good or an apparent good, it is never moved to an evil, unless that which is not good appear good in some respect to the reason; so that the will would never tend to evil, unless there were ignorance or error in the reason. Hence it is written (Proverbs 14:22): "They err that work evil."
Experience, however, shows that many act contrary to the knowledge that they have, and this is confirmed by Divine authority, according to the words of Luke 12:47: "The servant who knew that the will of his lord... and did not... shall be beaten with many stripes," and of James 4:17: "To him... who knoweth to do good, and doth it not, to him it is a sin." Consequently he was not altogether right, and it is necessary, with the Philosopher (Ethic. vii, 3) to make a distinction. Because, since man is directed to right action by a twofold knowledge, viz. universal and particular, a defect in either of them suffices to hinder the rectitude of the will and of the deed, as stated above (Question 76, Article 1). It may happen, then, that a man has some knowledge in general, e.g. that no fornication is lawful, and yet he does not know in particular that this act, which is fornication, must not be done; and this suffices for the will not to follow the universal knowledge of the reason. Again, it must be observed that nothing prevents a thing which is known habitually from not being considered actually: so that it is possible for a man to have correct knowledge not only in general but also in particular, and yet not to consider his knowledge actually: and in such a case it does not seem difficult for a man to act counter to what he does not actually consider. Now, that a man sometimes fails to consider in particular what he knows habitually, may happen through mere lack of attention: for instance, a man who knows geometry, may not attend to the consideration of geometrical conclusions, which he is ready to consider at any moment. Sometimes man fails to consider actually what he knows habitually, on account of some hindrance supervening, e.g. some external occupation, or some bodily infirmity; and, in this way, a man who is in a state of passion, fails to consider in particular what he knows in general, in so far as the passions hinder him from considering it. Now it hinders him in three ways. First, by way of distraction, as explained above (Article 1). Secondly, by way of opposition, because a passion often inclines to something contrary to what man knows in general. Thirdly, by way of bodily transmutation, the result of which is that the reason is somehow fettered so as not to exercise its act freely; even as sleep or drunkenness, on account of some change wrought on the body, fetters the use of reason. That this takes place in the passions is evident from the fact that sometimes, when the passions are very intense, man loses the use of reason altogether: for many have gone out of their minds through excess of love or anger. It is in this way that passion draws the reason to judge in particular, against the knowledge which it has in general.
Reply to Objection 1. Universal knowledge, which is most certain, does not hold the foremost place in action, but rather particular knowledge, since actions are about singulars: wherefore it is not astonishing that, in matters of action, passion acts counter to universal knowledge, if the consideration of particular knowledge be lacking.
Reply to Objection 2. The fact that something appears good in particular to the reason, whereas it is not good, is due to a passion: and yet this particular judgment is contrary to the universal knowledge of the reason.
Reply to Objection 3. It is impossible for anyone to have an actual knowledge or true opinion about a universal affirmative proposition, and at the same time a false opinion about a particular negative proposition, or vice versa: but it may well happen that a man has true habitual knowledge about a universal affirmative proposition, and actually a false opinion about a particular negative: because an act is directly opposed, not to a habit, but to an act.
Reply to Objection 4. He that has knowledge in a universal, is hindered, on account of a passion, from reasoning about that universal, so as to draw the conclusion: but he reasons about another universal proposition suggested by the inclination of the passion, and draws his conclusion accordingly. Hence the Philosopher says (Ethic. vii, 3) that the syllogism of an incontinent man has four propositions, two particular and two universal, of which one is of the reason, e.g. No fornication is lawful, and the other, of passion, e.g. Pleasure is to be pursued. Hence passion fetters the reason, and hinders it from arguing and concluding under the first proposition; so that while the passions lasts, the reason argues and concludes under the second.
Reply to Objection 5. Even as a drunken man sometimes gives utterance to words of deep signification, of which, however, he is incompetent to judge, his drunkenness hindering him; so that a man who is in a state of passion, may indeed say in words that he ought not to do so and so, yet his inner thought is that he must do it, as stated in Ethic. vii, 3.
Source: Summa Theologica (New Advent)