  William argues at length that a soul cannot be a body, a conclusion he applies not just to human or rational souls, but also to those of plants and animals. In the case of human souls, he uses, among others, Avicenna's “flying-man” argument to establish this point, arguing that a human being flying in the air who lacks and never had the use of his senses would know that he exists but deny that he has a body. Therefore “it is necessary that he have being that does not belong to the body and, for this reason, it is necessary that the soul not be a body” (OO II suppl., 83a; Teske 2000, 91).