The point is that [our excuses] 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? The truth is, we believe in decency so much—we feel the Rule of Law pressing on us so—that we cannot bear to face the fact that we are breaking it, and consequently we try to shift the responsibility. For you notice that it is only for our bad behaviour that we find all these explanations. It is only our bad temper that we put down to being tired or worried or hungry; we put our good temper down to ourselves.
C.S. Lewis, Mere Christianity, p. 8