How do we decide what is good or evil? The usual answer is that we decide by conscience. But probably no one thinks now of conscience as a separate faculty, like one of the senses. Indeed, it cannot be so thought of. For an autonomous faculty like a sense cannot be argued with; you cannot argue a man into seeing green if he sees blue. But the conscience can be altered by argument; and if you did not think so, you would not have asked me to come and argue with you about the morality of obeying the civil law when it tells us to serve in the wars. Conscience, then, means the whole man engaged in a particular subject matter.
But even in this sense conscience still has two meanings. It can mean (a) the pressure a man feels upon his will to do what he Thinks is right; (b) his judgment as to what the content of right and wrong are. In sense (a) conscience is always to be followed. It is the sovereign of the universe, which “if it had power as it has right, would absolutely rule the world.” It is not to be argued with, but obeyed, and even to question it is to incur guilt. But in sense (b) it is a very different matter. People may be mistaken about wrong and right; most people in some degree are mistaken. By what means are mistakes in this field to be corrected?
From The Weight of Glory
Compiled in Words to Live By