  Besides rejecting universal hylomorphism, William also rejects the doctrine of a plurality of souls in a human being. This doctrine, which was popular among Franciscan thinkers, holds that the body of a human being is informed not just by a single soul, the rational soul, but also by a vegetative and a sensitive soul. This multiplicity of souls, it is held, serves to explain how human beings have the vital operations characteristic of plants and animals in addition to peculiarly human ones, and also explains the development of the fetus's vital functions. Against this doctrine, William claims to have shown earlier in On the Soul that not only the operation of understanding, but also the sensitive operations of seeing and hearing “are essential and proper to the soul itself” (OO II suppl., 108b; Teske 2000, 164) and “hence, it cannot be denied except through insanity that one and the same soul carries out such operations.” Rather than posit a multiplicity of souls, William holds that higher souls incorporate the kinds of operations attributed to lower ones. The rational soul is infused into the body directly by God when the fetus reaches a suitable stage of organization, and the prior animal or sensitive soul in the fetus ceases to be. In this case too a view along similar lines was to be developed by Aquinas.