Though the soul animates the body and not the other way around, being embodied is essential to being a human being. A human is body and soul. The soul is not spatially constrained but nevertheless is properly located in the body, though not in any one part. Rather, the soul pervades (or “infuses) the body, in union but not mixed therewith.
Hence, there is interdependence between body and soul, even though only one is immaterial and immortal. For instance, the soul senses in harmony with the body when it experiences pain. But it does so without the body when it senses emotion, although data gathered by bodily organs may give rise to the emotion sensed. Further, the soul functions apart from the body in abstract thought, though it has ideas (or “phantasms”) in its consciousness which are connected to the operation of bodily senses. The point is that, for Aquinas, thinking, choosing, and the like are not simply functions of the brain. The brain may be necessary for thinking as part of the sensory apparatus of the body, but it is not a sufficient condition of thought. Intellect and will are powers of the soul, not any organ of the body. As Aquinas says,
the soul, by its essence and not through the medium of certain other powers, is the origin of those powers which are the acts of organs, even as any form, from the very fact that by its essence it informs its matter, is the origin of the properties which result naturally in the composite.
This is why persons with mental and otherwise physical disabilities are no less a rational creature. The deficiency lies in the organ through which the rational powers of the soul act, not in the rational soul itself. The full potentiality remains even if the bodily means for actuality is diminished.