From: The Collected Letters of C. S. Lewis, Vol. III: Narnia, Cambridge, and Joy 1950-1963, edited by Walter Hooper, HarperSanFrancisco, 2007; 3 June 1956; letter to Keith Masson, pp. 758-759:
You rather take the line that a traditional moral principle must produce a proof of its validity before it is accepted: I rather, that it must be accepted until someone produces a conclusive refutation of it.
But apart from that: — I agree that the stuff about ‘wastage of vital fluids’ is rubbish. For me the real evil of masturbation would be that it takes an appetite which, in lawful use, leads the individual out of himself to complete (and correct) his own personality in that of another (and finally in children and even grandchildren) and turns it back: sends the man back into the prison of himself, there to keep a harem of imaginary brides. And this harem, once admitted, works against his ever getting out and really uniting with a real woman.
For the harem is always accessible, always subservient, calls for no sacrifices or adjustments, and can be endowed with erotic and psychological attractions which no real woman can rival. Among those shadowy brides he is always adored, always the perfect lover: no demand is made on his unselfishness, no mortification ever imposed on his vanity. In the end, they become merely the medium through which he increasingly adores himself. . . . And it is not only the faculty of love which is thus sterilized, forced back on itself, but also the faculty of imagination. . . .
Masturbation involves this abuse of imagination in erotic matters (which I think bad in itself) and thereby encourages a similar abuse of it in all spheres. After all, almost the main work of life is to come out of our selves, out of the little, dark prison we are all born in. Masturbation is to be avoided as all things are to be avoided which retard this process. The danger is that of coming to love the prison.
Masturbation Remains a Grave Sin (Debate w Steve Hays) [1-6-07; links added on 8-13-19]
Photo credit: “Our contributor was tagging for this image: Sculpture, Monument, Vain, Self-love, Narcissistic.” [Max Pixel / public domain]