How do we understand consciences today? Oftentimes they are used as an excuse for moral relativism or subjectivism. This is not so. Consciences, and our duty to form them in truth for the sake of our freedom, are an integral task of the moral life. For St. Thomas, conscience is never detached from the larger moral life. A good moral act involves object, circumstances, and end (ST I-II, q. 18, a.
4), and it also requires that reason rightly order the will toward the good (ST I-II, q. 18, aa. 5–6). That is why conscience must be joined to virtue, especially prudence. Prudence is not timidity or caution, but the virtue that judges rightly about what must be done in concrete cases. More deeply, prudence depends upon love of the good and the just.
One of the great truths modern man resists is precisely this: we do not only sin because we think badly; we also think badly because we sin. A person who is morally compromised will not see clearly. Vice clouds judgment, bends perception, and makes reality harder to recognize. A man ruled by lust will not see chastity clearly. A man ruled by ambition will not judge justice clearly.
A man who lies habitually will lose the ability to recognize truthfulness with ease. The problem, then, is not merely intellectual but moral and spiritual. St. Thomas says as much when he asks whether blindness of mind is a sin (ST II-II, q. 15, a. 3) and whether folly is caused by lust (ST II-II, q. 46, a. 8).
In the treatise on temperance, he goes further, listing among the “daughters of lust” blindness of mind, thoughtlessness, and rashness (ST II-II, q. 153, a. 5). Vice, then, does not merely lead us to choose badly; it can make