 For living things, their substantial form is their soul (anima). In saying this, Aquinas should not be understood to be ascribing some special sort of spirituality to plants and animals: he thinks they are material objects just as much as rocks and streams are. Rather, he is following the lead of Aristotle’s De anima in treating the soul as the first principle of life, whatever that may be. Since Aquinas thinks that the primary internal explanation for the existence of any substance is its substantial form (§4), it follows that every living substance has a soul that is its substantial form.