But why ought we to obey instincts? Is there another instinct of a higher order directing us to do so, and a third of a still higher order directing us to obey it? - an infinite regress of instincts? This is presumably impossible, but nothing else will serve. From the statement of psychological fact "I have an impulse to do so and so" we cannot by any ingenuity derive the practical principle "I ought to obey this impulse." . . . Telling us to obey instinct is like telling us to obey "people." People say different things: so do instincts. Our instincts are at war.1Those who follow the teachings and example of Jesus know that many of our "normal" and "natural" instincts are opposed to the life of a disciple of Jesus. It is not normal or natural to "love our enemies" (Luke 6:27-36). Our instincts do not tell us to "love our neighbour as ourselves" (Matthew 22:34-40). Our instincts tend toward self-preservation, self-enjoyment, getting ahead of others, and revenge for wrongs done against us. The Bible clearly teaches, and the actions of humans show, that we are capable of rising above the impulse of instincts. It is not natural or easy to do so; but we have the ability to do so. The Bible also teaches that humans are not simply intelligent animals. We have been created in the image of God (Genesis 1:27); a concept that has much to do with our ability to be creative and to be able to over-ride our natural instincts.
I agree with Lewis that our instincts are at war. They are at war with the teachings of justice and fairness. They are at war with the image of God. They are at war with our higher calling.
1. A Severe Mercy [reprint: HarperOne, 2009, pp. 146-148]). The letter is dated 14 May 1954. I have only quoted a small portion of Lewis' letter. The whole letter is specific to sexuality and sexual sin and is a helpful comment on wider issues.