For Santiago Ramón y Cajal the brain was a beautiful, inconceivably complex, and self-regulating ecosystem, and he set out to write the field guide to its flora and fauna: “Like the entomologist in pursuit of brightly colored butterflies, my attention hunted, in the flower garden of the gray matter, cells with delicate and elegant forms, the mysterious butterflies of the soul, the beating of whose wings may someday—who knows?—clarify the secret of mental life.”