Some people prefer to talk about moral ‘ideals’ rather than moral rules and about moral ‘idealism’ rather than moral obedience. Now it is, of course, quite true that moral perfection is an ‘ideal’ in the sense that we cannot achieve it. In that sense every kind of perfection is, for us humans, an ideal; we cannot succeed in being perfect car drivers or perfect tennis players or in drawing perfectly straight lines. But there is another sense in which it is very misleading to call moral perfection an ideal. When a man says that a certain woman, or house, or ship, or garden is ‘his ideal’ he does not mean (unless he is rather a fool) that everyone else ought to have the same ideal. In such matters we are entitled to have different tastes and, therefore, different ideals. But it is dangerous to describe a man who tries very hard to keep the moral law as a ‘man of high ideals’, because this might lead you to think that moral perfection was a private taste of his own and that the rest of us were not called on to share it. This would be a disastrous mistake. Perfect behaviour may be as unattainable as perfect gear-changing when we drive; but it is a necessary ideal prescribed for all men by the very nature of the human machine just as perfect gear-changing is an ideal prescribed for all drivers by the very nature of cars. And it would be even more dangerous to think of oneself as a person ‘of high ideals’ because one is trying to tell no lies at all (instead of only a few lies) or never to commit adultery (instead of committing it only seldom) or not to be a bully (instead of being only a moderate bully). It might lead you to become a prig and to think you were rather a special person who deserved to be congratulated on his ‘idealism’. In reality you might just as well expect to be congratulated because, whenever you do a sum, you try to get it quite right. To be sure, perfect arithmetic is ‘an ideal’; you will certainly make some mistakes in some calculations. But there is nothing very fine about trying to be quite accurate at each step in each sum. It would be idiotic not to try; for every mistake is going to cause you trouble later on. In the same way every moral failure is going to cause trouble, probably to others and certainly to yourself. By talking about rules and obedience instead of ‘ideals’ and ‘idealism’ we help to remind ourselves of these facts.
C.S. Lewis, "Christian Behaviour," Part 3 of Mere Christianity (1952; Harper Collins edition 2001) 69-71