Aquinas on conscience (ST I, 79)

 Synderesis is a characteristic disposition rather than a power….as nature needs to implant in us the first principles concerning theoretical matters, so also does nature needs to implant in us the first principles concerning practical matters.

 But the first principles about theoretical matters, principles implanted in us by nature, do not belong to any special power but to a special characteristic disposition, which we call “the understanding of principles”….and so the principles of practical matters, principles implanted in us by nature, likewise do not belong to a special power but to characteristic disposition from nature, and we call this disposition synderesis.  And so we also say synderesis  incites to good and complains of evil, since we progress from first principles to discovery  and judge about the things we have discovered…so we judge by both reason and synderesis.

 Thirteenth article

 Conscience is an act, not a power.  And this is evident both from the maning of the word and from the things that we in our ordinary way of speaking attribute to conscience.  It signifies the relation of knowledge to something else, since we define con-science as knowledge with something else. But acts connect knowledge with things.  And so it is clear from the meaning of the word that conscience is an act.

 We say that conscience bears witness, morally obliges or stirs to action, and accuses or disquiets or reproves.  And all of these things result from connecting some knowledge of ours to what we do.

 This connection arises in three ways.  It arises in one way as we recognize that we have or have not done something…and we accordingly say that conscience bears witness.

 In a second way, we connect our knowledge to something as we by our conscience judge that we should or should not do something.  And we accordingly say that conscience incites to action or morally obliges.

 In a third way, we connect our knowledge to something as we by our conscience judge that we have or have not done something worthily.  And we accordingly say that conscience excuses or accuses and disquiets.

 But all three of these ways clearly result from the actual connection of knowledge to what we do.  And so, properly speaking, conscience designates an act.

Source: Aquinas On Law, Morality and Politics 2nd ed translated by Richard J. Regan

Hackett 2002 (p2-3)