  These arguments serve not only to show the existence of souls, but also that they are substances. In the case of the human soul, William argues, the operations of understanding and knowing must be treated as proper operations of the soul, and this requires that the soul itself be a substance. To the objection that the soul itself does not literally understand or know but is instead that thanks to which a human being can understand and know, William will argue that the soul and human being, which he equates with the referent of the pronoun “I,” are one and the same thing. More generally, the substantiality of souls—and not just of human souls—is indicated by the fact that bodies are their instruments.