The Fly raises at least two philosophical questions. The first concerns what kind of thing it is Seth Brundle (the Jeff Goldblum character) has become by the end of the movie. Having had his genetic material accidentally spliced together with that of a housefly, he gradually transforms into something more and more fly-like, albeit at a (repulsively) human size. So is he still a human being by the end? Or a fly? Or of some new, hybrid species?
Part of the problem in answering questions about even the most carefully thought-out scenarios of this sort is that we simply have no actual facts to go on. The scenario might be inspired by reflection on certain bits of factual knowledge, but it is not itself factual. Moreover, such scenarios are typically under-described in crucial respects. What does Brundle’s interior anatomy look like at different stages in the transformation? How many human-like organs remain by the end? Is a basically vertebrate skeletal plan preserved throughout? (Presumably so or he’d have collapsed into a puddle of goo.) And what does the “Brundlefly” creature’s genetic make-up actually look like? There is no fact of the matter, and even if the movie had given an answer it would have been sheer speculation based on a “splicing” set up (the teleportation device) that is itself at best only tenuously related to any actual scientific knowledge. But, to go along with the gag, let’s speculate based on what we do “know” from the movie. (Spoiler alert: If you haven’t seen the movie, be aware that certain key plot points will be revealed below.)
The classical definition of a human being, one central to A-T traditionally, is, of course, that man is a rational animal. That does not mean that every human being actually reasons or is even capable of exercising his power of reason. Injury or genetic defect might impede the exercise of this power. Still, the power is always there to be impeded as long as we have a human being. A severely brain damaged human being is a brain damaged human being, and not (literally) a “vegetable,” even if he is reduced to his vegetative functions. He is still the sort of thing which when in “good working order” will reason. Contrast this with a dog, which never can reason even if it is healthy and whole.
So, even if Brundle at the end of his transformation had had no exercise of reason, that would not by itself show that he was no longer a rational animal. But as it happens, he does exercise reason to the bitter end. To be sure, after Veronica (the Geena Davis character) accidentally tears off his jaw and thereby (apparently) triggers the sloughing off of his other remaining human facial features to reveal a gigantic fly-like head (see above), he loses the power of speech. But he continues his attempt to carry out his scheme of splicing himself with Veronica and their unborn child to create a bizarre new three person/one fly hybrid. (More on that below.) He is, of course, completely mad by this point, but to be mad is to be irrational, not non-rational. Moreover, after his failure to accomplish the splice and the associated accidental merging of himself with part of the telepod, he is clearly “begging” Veronica to put him out of his misery when he grasps the end of the shotgun she is holding and raises it to his head. This is obviously meant to indicate that Brundle is still “in there.” So, he is still a rational animal, even if an absolutely horrific one.
But how could he thereby still be human given how radical his transformation has been? Wouldn’t he be some other, non-human sort of rational animal? There are two reasons to answer in the negative, one biological, the other metaphysical. To take them in order, first, as we have said, Brundle retains reason throughout; and while from an A-T point of view, having something like a functioning human neurophysiology is not a sufficient condition for being a rational animal (since the intellect is immaterial), it is a necessary condition. Since flies have nothing remotely like that, it is evident that “Brundlefly” has a more or less human nervous system. Add to that the fact that he must surely also have a human-like skeletal and muscular structure in order to do the things he does, and no doubt other human-like anatomical features as well, and it is plausible that what he is is essentially a severely damaged human being who has acquired certain fly-like features as a result of the genetic alteration he has undergone, rather than something essentially non-human.
But second, even if one insisted on judging him to be something other than a damaged Homo sapiens sapiens – though again, there is little in the way of “hard evidence” from the movie to go on – it wouldn’t necessarily follow that he isn’t human in a deeper, metaphysical sense. For I’ll see your David Cronenberg and raise you a David Oderberg: As the latter David argues in the section of Real Essentialism on the Porphyrian Tree, human is best understood as a metaphysical category under which any rational animal would fall even if it did not have a body plan or genetic code like ours. Though the point is, I think, moot. For “Brundlefly” is not merely a rational animal; given the evident psychological and bodily continuity he manifests throughout his radical transformation, there is no reason to doubt that he is the same rational animal as the rational animal who existed pre-transformation. And since that rational animal was a Homo sapiens sapiens, so too is the post-transformation Brundle.
So that’s one metaphysical question the movie raises, and there’s my answer to it. The other question it raises concerns the issue of personal identity, though only by implication. Again, there isn’t really any question that the creature as he actually exists onscreen is one and the same person, Seth Brundle, all the way through the movie, certainly given what has been said (and even if one wanted to divorce – as one shouldn’t – Brundle “the person” from Brundle “the human being”). But suppose Brundle had been able to carry out his scheme of “splicing” himself via the telepods with Veronica and their unborn child? Who would the resulting person have been?
Since we’re not given even a hint of how this disturbing scenario would have played out, there’s even less to go on in answering these questions than there was vis-à-vis the question already dealt with. But we can certainly imagine various outcomes – one in which the grotesque amalgam that walks out of the pod has a mixture of man-like, woman-like, and fly-like characteristics, one in which it is considerably more human-looking than “Brundlefly” was before but more androgynous and still somewhat fly-like, one in which the resulting creature appears to have competing personalities vying for control, one in which it appears to have a single personality with both Brundle-like and Veronica-like aspects, and so forth. As is typical with weirdo thought experiments of the kind personal identity theorists delight in, though, the “correct” interpretation of such fantasies cannot be determined from imagined behavioral and physiological phenomena alone. A metaphysics established on independent grounds – and in light of actual, normal cases – must be brought to bear. And from an A-T point of view, what we can know independently of all such thought experiments is that the human soul is the substantial form of the living human body, and (unlike other forms) an immaterial, subsistent form. (See Aquinas ch. 4, or Oderberg again, ch. 10.)
Hence, given the worked-out A-T hylemorphic conception of the soul, if we had evidence of both Brundle’s and Veronica’s psychological traits in whatever amalgam walked out of the machine, the correct interpretation would seem to be something like this: Both Brundle and Veronica will have survived as separate individuals in the amalgam, since their souls have each evidently survived and (being immaterial) cannot themselves intelligibly have gotten jumbled together. (The same thing will be true of Veronica’s unborn baby, even if it is not yet capable of manifesting any psychological characteristics.) But their bodies have gotten entangled, in a way that is more radical than, but still properly interpreted as an extension of, the sort of entanglement we see in the case of Siamese twins. In the Siamese case, there are two persons and two bodies, even if the bodies have been intimately linked and/or one or both have not been completely formed. And the same thing is true of the imagined Brundlefly/Veronica/baby splicing, even if the entanglement is more thorough and the disfigurement of the respective strands (the Brundle strand, Veronica strand, and baby strand) more radical.
So, that’s my take on the metaphysics of The Fly. Maybe I’ll get to The Thing some time. (It’s spaghetti you don’t want to eat while watching that one. And no orange crispy beef while watching Aliens. I’ve got a list…)