If ‘being good’ meant simply joining the side you happened to fancy, for no real reason, then good would not deserve to be called good.
 C. S. Lewis, Mere Christianity (1952). copy citation

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Author C. S. Lewis
Source Mere Christianity
Topic reason meaning
Date 1952
Language English
Reference
Note Adapted from a series of BBC radio talks made between 1942 and 1944
Weblink https://www.dacc.edu/assets/pdfs/PCM/merechristianitylewis.pdf

Context

“Now if we mean merely that we happen to prefer the first, then we must give up talking about good and evil at all. For good means what you ought to prefer quite regardless of what you happen to like at any given moment. If ‘being good’ meant simply joining the side you happened to fancy, for no real reason, then good would not deserve to be called good. So we must mean that one of the two powers is actually wrong and the other actually right. But the moment you say that, you are putting into the universe a third thing in addition to the two Powers:” source