For the father of modern neuroscience, cellular anatomy was like the most exciting fiction.

Fiction is, by definition, a world away from fact—but Santiago Ramón y Cajal, often heralded as “the father of modern neuroscience,” used it to find objective truth. Cajal spent his days at the microscope, gazing down at faint, entangled fibers that appeared to his fellow anatomists as inscrutable labyrinths. Contrary to prevailing theory, the Spaniard discerned that…