 Still, using the notion of form in this way to characterize the substantiality of the ultimate constituents of reality still leaves much to be explicated with respect to the origin of living beings. Taurellus considers a creation theory of the origin of human souls (PT: 144), but he also voices doubt concerning such a theory. In particular, he cautions that an act of divine creation would render the imperfection of human souls inexplicable (PT: 166). Therefore, he offers an integrated naturalistic account of the origin of the souls of non-human animals and humans.