 What follows from this is that conscience, whether right or erroneous, always binds. As to the question of whether an erroneous conscience excuses, on the other hand, that depends on the type of ignorance that causes it. This is because some types of ignorance render an act involuntary and therefore excusable, and some do not. Ignorance that excuses is called invincible, and concerns ignorance about the facts of the situation (assuming, of course, there is no negligence involved). Such cases, that is, where an agent could not have known better under the circumstances and would not have thus acted had he known better, are excusable. Aquinas uses the example of fornication—voluntary, illicit sex—to illustrate his point. Although it is a sin to commit adultery, a man is excused if he has sexual relations with a woman who is not his wife, so long as he is genuinely, if improbably, mistakes the women for his wife. This is