Thomas Aquinas’s attempted solution to the problem of substance dualism (i.e., the mind/body connection) involved his use of Aristotelian language to define the intellectual soul as the “form” of the body. Yet, in order to account for the separated state of the soul after death Thomas was forced to stretch Aristotle’s concept of “formal substance” to explain the Christian teaching on the soul’s disembodied afterlife. He maintains that the soul is the intellectual form of the body but is also per se subsistens (existing on its own) and that the agent intellect does not make use of any bodily organ in the act of intellection. Krista Hyde, in a master’s thesis submitted to the University of Missouri at Saint Louis, makes note of Aquinas’s attempt to connect the mind and body in one substantial form but also points out the metaphysical problem that this causes:
In this way, Aquinas reframes the mind-body connection and escapes the interaction problem, but this opens him up to another, potentially fatal, flaw. The weakness of his argument is that it is not clear how such a soul could separate from the body and survive its death, despite his protestations that this act of the body is also substantial. Aquinas maintains that the human soul is an abstract form and a concrete particular thing. He has accounted for the soul as the form of the body. It is intuitive, however, that such forms would have to be Platonic (that is, an incorporeal, abstract idea, having the highest level of reality and to which humans have access only through the world of perception and sensation) if they can be said to depart from the body and remain in existence with some function until the time of the resurrection and judgment, as required by Christian doctrine. Aquinas, of course, would never accept his philosophy as Platonic, especially on this account, because he insists that matter is for the edification of the soul. Plato maintains that the soul simply “uses” matter, but that matter is unnecessary.
If the soul is a state, it must be one that can exist apart from that which bears it. Though the concept of the free radical is helpful as an illustration of the soul as form, it fails to exemplify this aspect; the free radical is not immaterial. The soul as organizing force of the body may be fatal to Aquinas’s psychology. If the soul is seen as a sort of genetic code or as software to the body’s hardware, however, it may be possible to imagine its survival in an immaterial state, to be employed (later, during the afterlife) in organizing prime matter once more.
A counter-objection might suggest that the soul survives between death and resurrection, but is “activated” again upon the resurrection. It does not seem likely that Aquinas would be willing to accept that the soul is totally incapacitated until Judgment, since he insists that the soul retains its proper operations: will, intellectual memory, and understanding.
~ Hyde, Thomas Aquinas: Soul-Body Connection and the Afterlife, pp. 29-30.)
An interesting note to add to this is that many of Aquinas’s followers did move in a more Platonic direction after his death. Fast-forward to the 15th century and one can see with Marsilio Ficino a student of Aquinas’s thought who turns to the original sources of the Platonic tradition to explain this very problem. He argues that the soul uses various vehicles (pace Proclus) and thus maintains its formal functionality even when separated from the body. Ficino’s solution is no less problematic, however, (it does not fully explain the necessity of the soul’s embodiment) and begs the question of whether the immortality of the soul can be proven philosophically without at the same time directly appealing to such doctrines as the hypostatic union of Christ’s natures as found in the Chalcedonean definition. Whether Hyde’s proposed solution with regard to the soul as “software” or “genetic code” of the body is helpful is, of course, up to the reader to decide.