Saint Thomas Aquinas on the Incompleteness of the Human Soul

SAINT THOMAS Aquinas's metaphysical account of the human soul and its hylomorphic union with the body has long been lauded as a way to avoid substance dualism while affirming the immortality of the soul. Aquinas maintains that the human soul is an immaterial subsistent part of the human being that survives the death of the person.¹ It owes its metaphysical status to its role as the intellective principle in the human being. During life, it serves as the seat of our intellective acts, and even in its postmortem, separated state it continues to carry out intellective operations.² Yet despite this existential and operational independence from the body, Aquinas argues that soul and body are not two substances accidentally united. The human being is not an aggregate or accidental unity.³ Instead, he maintains that the soul is the substantial form of the body and argues that the hylomorphic unity this provides renders the human being a single, unqualifiedly unified substance, that is, a substantial unity.⁴

For as long as it has been lauded, however, Aquinas's view has also been plagued with charges of incoherence. A thing that can exist and operate apart from a larger whole—a thing like the human soul—seems paradigmatic of a complete substance, and in Aquinas's metaphysics no complete substance can be united to anything further to form a substantial unity. Thus, the soul's existential and operational independence from the body generates, at the very least, a prima facie tension with the human being's substantial unity.⁵ But if Aquinas is not philosophically entitled to the substantial unity of the human being, his view inevitably collapses into the dualism he seeks to avoid.⁶

Aquinas recognizes the tension between the soul's independence and its union with the body but attempts to dissolve that tension by blocking any inference from the soul's capacity for separate existence and operation to its completeness as a substance. He argues that the soul ultimately fails to be a complete substance because it is not complete in a specific nature.⁷ Human nature is only complete when the soul is united to matter to constitute the human body. But to a critic this may appear simply to beg the question. Presumably a thing that exists and operates on its own ought to count as specifically complete—perhaps not in human nature, but in some intellective nature of its own. In that case, the soul ought to count as a complete substance. Aquinas's claim seems ad hoc. Worse still, it seems to conflict with his broader metaphysical framework in which a thing's existence and its species membership derive from the same source.⁸

Aquinas has more to say in his defence. In particular, he provides two accounts meant to motivate the claim that the soul is specifically incomplete. In the first and more prominent of these, found in the disputed question De anima, he appeals to the soul's dependence on the body for its intellective operation as a marker of its specific incompleteness. During life, the soul relies on images stored in the brain to produce the intelligible forms through which it understands. Aquinas argues that a thing cannot be complete in a specific nature if it lacks within itself those things required for its essential and natural operations. Because the soul depends on the body for these images, on its own it lacks the means of producing the intelligible forms required for its natural and optimal mode of intellective understanding. It is, therefore, specifically incomplete.⁹

In a second account, from the disputed question De spiritualibus creaturis, Aquinas argues that the soul is incomplete because, on its own, it cannot fully express what it contains virtually, qua form. There are things of which it is the principle that cannot be realized apart from its union with matter to constitute the body—for instance, the sensitive and nutritive powers, which can only be realized in the body. Aquinas maintains that a thing cannot be complete in a...