"The psychologist Dedre Gentner and her collaborators have shown that a focus on relationships is the key to the power of analogy as a tool of reasoning. They note that many scientific theories were first stated as analogies, and often are still best explained that way: gravity is like light, heat is like a fluid, evolution is like selective breeding, the atom is like a solar system, genes are like coded messages. For an analogy to be scientifically useful, though, the correspondences can’t apply to a part of one thing that merely resembles a part of the other. They have to apply to the relationships between the parts, and even better, to the relationships between the relationships, and to the relationships between the relationships between the relationships."