CSL really is master of the metaphor.
I am talking of Affection as it is when it exists apart from the other loves. It often does so exist; often not. As gin is not only a drink in itself but also a base for many mixed drinks, so Affection, besides being a love itself, can enter into the other loves and colour them all through and become the very medium in which from day to day they operate.
C. S. Lewis, The Four Loves