What makes money make money?

A sentence from Henry Miller's Tropic of Capricorn in the comments of Stephanie Kelton's New York Times op-ed Learn to Love Trillion-Dollar Deficits:

"To walk in money through the night crowd, protected by money, lulled by money, dulled by money, the crowd itself a money, the breath money, no least single object anywhere that is not money, money, money everywhere and still not enough, and then no money or a little money or less money or more money, but money, always money, and if you have money or you don't have money it is the money that counts and money makes money, but what makes money make money?"