Mere Christianity, in The Complete C. S. Lewis Signature Classics

p. 47

Lewis compares individual morality to what a person does to his own ship (his own life) and group morality to the intermeshing of all the ships in the convoy and its destination.  Then this: “Let’s go back to the person who says that a thing cannot be wrong unless it hurts some other human being.  He quite understands that he must not damage the other ships in the convoy, but he honestly thinks that what he does to his own ship is his own business.  But does it not make a great difference whether his ship is his own property or not?  Does it not make a great difference whether I am, so to speak, the landlord of my own mind and body, or only a tenant, responsible to the real landlord?  If somebody else made me for his own purposes, then I shall have a lot of duties which I should not have if I simply belonged to myself.”