Fire and Knowledge

I do not believe one can settle how much we ought to give [to others and to charity]. I am afraid the only safe rule is to give more than we can spare. In other words, if our expenditures on comforts, luxuries, amusements, etc., is up to the standard common among those with the same income as our own, we are probably giving away too little. If our charities do not at all pinch or hamper us, I should say they are too small. There ought to be things we should like to do and cannot do because our charities expenditure excludes them.

—C.S. Lewis. Mere Christianity, p. 86