Why we make excuses for our bad behaviour

It seems, then, we are forced to believe in a real Right and Wrong. People may be sometimes mistaken about them, just as people sometimes get their sums wrong; but they are not a matter of mere taste and opinion any more than the multiplication table. Now if we are agreed about that, I go on to my next point, which is this. None of us are really keeping the Law of Nature. If there are any exceptions among you, I apologise to them. They had much better read some other book, for nothing I am going to say concerns them. And now, turning to the ordinary human beings who are left:
    I hope you will not misunderstand what I am going to say. I am not preaching, and Heaven knows I do not pretend to be better than anyone else. I am only trying to call attention to a fact; the fact that this year, or this month, or, more likely, this very day, we have failed to practise ourselves the kind of behaviour we expect from other people. There may be all sorts of excuses for us. That time you were so unfair to the children was when you were very tired. That slightly shady business about the money—the one you have almost forgotten—came when you were very hard-up. And what you promised to do for old So-and-so and have never done—well, you never would have promised if you had known how frightfully busy you were going to be. And as for your behaviour to your wife (or husband) or sister (or brother) if I knew how irritating they could be, I would not wonder at it—and who the dickens am I, anyway? I am just the same. That is to say, I do not succeed in keeping the Law of Nature very well, and the moment anyone tells me I am not keeping it, there starts up in my mind a string of excuses as long as your arm. The question at the moment is not whether they are good excuses. The point is that they are one more proof of how deeply, whether we like it or not, we believe in the Law of Nature. If we do not believe in decent behaviour, why should we be so anxious to make excuses for not having behaved decently?

Quotes from Mere Christianity, part 4
C.S. Lewis, Mere Christianity (1952, this edition: 2001) 7-8.

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