Aquinas wonders whether intellectual substances (human souls and angelic spirits) could possibly be united as a form to any other body besides the human (SCG 2.90). He answers in the negative. Why? Bodies are either “mixed” (a complex of parts of heterogeneous nature) or “simple” (a partless conglomeration of a single element). Intellectual substances couldn’t be united as a form to simple bodies, since their very simplicity would preclude the possibility of the conjoined intellectual substance being able to do anything by means of such a body (e.g., what would a human soul or angelic spirit joined to, say, a rock as its form do with such a rock?). What about mixed bodies, then? Intellectual substances, by their very superiority to corporeal reality, could only be joined to the best or “noblest” of such bodies. Such a body would have to “possess that harmonious quality in the highest degree.” But the “most evenly tempered body,” Aquinas avers, “is the human,” a fact confirmed by its exceeding “fine texture and keen sense of touch,” qualities which Aquinas takes as a sure, corporeal “sign” of an otherwise “mental acuteness.” Human bodies, in short, are smart bodies, indeed, the smartest bodies, and therefore no intellectual (“smart”) substance may be joined to any body other than the human.
This raises an intriguing possibility that, to my knowledge, Aquinas himself doesn’t anywhere explore, and that is the role that the human body (and not just the human powers of intellect and reason) may play in man’s being made in the image of God.