As the pioneer of modern neuroscience, Santiago Ramón y Cajal may be the most influential figure in the history of biology you’ve never heard of. Along with Charles Darwin and Louis Pasteur, Cajal ranks among the most brilliant and original scientists of the nineteenth century, and his discoveries have done for our understanding of the human brain what the work of Galileo and Sir Isaac Newton did for our conception of the physical universe. Cajal was awarded the Nobel Prize in 1906 for his lifelong investigation of the structure of neurons—“the mysterious butterflies of the soul,” he called them, “whose beating of wings may one day reveal to us the secrets of the mind.” And he produced a dazzling oeuvre of anatomical drawings, whose otherworldly beauty conjured a vivid image of our mental life. Benjamin Ehrlich’s is the first major biography in English of this rare genius, who embarked on a scientific odyssey that mirrored the rocky journey of his beloved homeland of Spain into the twentieth century. Born into poverty in a mountaintop hamlet, Cajal was a willful and unruly child who at first struggled to live up to the expectations of his imperious father, a country doctor. A portrait of a nation as well as a biography, follows Cajal from the hinterlands to Barcelona and Madrid, where he became an internationally celebrated figure, single-handedly raising the scientific reputation of Spain in the process. To momentous effect, Cajal demonstrated a truth that was as controversial in his own time as it is universal in ours: that the nervous system is composed of individual cells with distinctive roles, just like any other organ in the body. is at once the story of how we arrived at our modern understanding of the brain and a finely wrought portrait of an individual as remarkable and complex as the subject to which he devoted his life.