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 (Q. 76, A. 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 (A. 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 Obj. 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.