Thomas Aquinas and the complex simplicity of the rational soul

Abstract

AbstractThomas Aquinas holds that the rational soul is not only, like other created immaterial substances, mereologically simple, in that it is completely lacking in any kind of material parts, but also mereologically complex, in that it includes within its composition its own essence, an act of existence, and various powers. Aquinas's account of the mereological complexity of the rational soul introduces several tensions with his understanding of the soul as the substantial form of the body and his larger ontology of the human person. After providing an overview of several key mereological notions operative in Aquinas's thought, and an overview of the mereological simplicity and mereological complexity of immaterial substances in his ontology, I introduce three potential concerns for the Thomistic account, all of which might have been avoided had Aquinas instead understood the rational soul to be entirely mereologically simple—serving as the source and subject of, but bearing some other nonmereological relation to, the aforementioned parts. Though Aquinas does not pursue this simpler solution, I argue that he already has built into his ontology the resources to make such a solution consistent with the rest of his thought.