Genius is akin both to madness and inspiration and, as every one is both more or less inspired and more or less mad, every one has more or less genius.
 Samuel Butler, The Note-Books of Samuel Butler (1912). copy citation

Context

“to my mind, those who hold it, and hold it firmly, are the only people worth thinking about. They will also hold that, important as the physical world obviously is, the spiritual world, of which we know little beyond its bare existence, is more important still. Genius i Genius is akin both to madness and inspiration and, as every one is both more or less inspired and more or less mad, every one has more or less genius. When, therefore, we speak of genius we do not mean an absolute thing which some men have and others have not, but a small scale-turning overweight of a something which we all have but which we cannot either define or apprehend—the quantum which we all have being allowed to go without saying.” source