In his seventh chapter, “Human Pain, Continued,” from The Problem of Pain, C.S. Lewis writes:
Whatever its merits, self-torture is quite a different thing from tribulation sent by God. Everyone knows that fasting is a different experience from missing your dinner by accident or through poverty. Fasting asserts the will against the appetite – the reward being self-mastery and the danger pride: involuntary hunger subjects appetites and will together to the Divine will, furnishing an occasion for submission and exposing us to the danger of rebellion. But the redemptive effect of suffering lies chiefly in its tendency to reduce the rebel will. Ascetic practices, which in themselves strengthen the will, are only useful in so far as they enable the will to put its own house (the passions) in order, as a preparation for offering the whole man to God. They are necessary as a means; as an end, they would be abominable, for in substituting will for appetite and there stopping, they would merely exchange the animal self for the diabolical self. It was, therefore, truly said that ‘only God can mortify.’ Tribulation does its work in a world where human beings are ordinarily seeking, by lawful means, to avoid their own natural evil and to attain their natural good, and presupposes such a world. In order to submit the will to God, we must have a will and that will must have objects. Christian renunciation does not mean stoic ‘Apathy’, but a readiness to prefer God to inferior ends which are in themselves lawful. Hence the Perfect Man brought to Gethsemane a will, and a strong will, to escape suffering and death if such escape were compatible with the Father’s will, combined with a perfect readiness for obedience if it were not.