  Among William's many arguments for the existence of souls is the argument that bodies are instruments of souls and the power to carry out operations through an instrument cannot belong to the instrument itself. Thus, there must be something other than the bodies of living things that operates by means of bodies, and this is their soul. In the case of the human or rational soul, he argues that its existence follows in particular from the fact that understanding and knowledge are found in a human being but not in the whole body or any part of it. Since these are operations of a living substance, there must be a living incorporeal substance in which they are, and this is what people mean when they speak of their soul.