The form provides the organism with its actual existence and nature, and, in turn, the substantial form is individuated by the organism’s prime matter (together with its actual history). The form’s function is to animate and (we might say) rationalize matter, resulting in a sentient and rational organism (in the case of human beings). When everything goes well, what results is a determinate individual in an infima species. Consequently, the principle that no substance contains another substance as proper part is a corollary. The whole and its parts cannot have independent existence or independent natures. Ordinarily, one substantial form must be responsible for the existence of the whole individual and its parts, and for the nature of the whole individual and its parts.
No Substance in Substance (NSIS) No substance is a proper part of another substance.
However, all that we should conclude is that NSIS holds in the normal or typical case. There could be cases in which the action of a form does not produce a single, determinate individual. I want to consider four cases:
- Cases of fission: callosotomy, brain fission, twinning
- Manufacturing organisms “from scratch”
- Vague boundaries between colonies and multi-cellular organisms
- Transitional forms in evolution
Cases of Fission
Many lower animals and plants reproduce by budding or mitosis, in which a parent is divided into two or more daughters. It seems reasonable to think of this as involving substantial change, with the parent corrupted and new substances generated, although there is an alternative possibility, one of multi-location. On the multilocation model, the so-called “offspring” are all numerically identical to the parent, now wholly located in two or more places at once. The multilocation model would only be attractive if we discovered some kind of simultaneous coordination-at-a-distance (something analogous to quantum entanglement) linking the causal powers of the offspring. In the absence of such evidence, it seems clear that such budding generates entirely new, numerically distinct offspring.
Higher animals, including human beings, sometimes undergo identical (monozygotic) twinning, in which a single zygote (blastocyst) divides into two or more offspring. Late separation can result in conjoined twins or a parasitic twin. It seems reasonable to think of identical twins as a variation on the natural reproductive process, producing multiple substance, each numerically distinct from the others.
The case of conjoined twins, however, exemplifies a failure of complete individuation. Conjoined twins share organs and tissues. This can even involve sharing part of the brain, even the cerebral cortex or thalamus. In this case, it seems possible for the same parcels of prime matter to be actualized and informed by two different substantial forms. It might make sense in such a case to think of the forms themselves as overlapping mereologically, with a shared part (corresponding to the shared parcels of prime matter and shared developmental history) in common.
What about artificial cloning? Unlike twinning, this would be hard to classify as a case of natural reproduction. This might be a case of an exception to NSIS, with the clone a proper part of the donor organism, and the substantial form of one a proper part of the form of the other. Forms are ordinarily simple, wholly present in every part of the organism, but cases like conjoined twins and clones might provide exceptions.
In adult humans, the cerebral cortex is divided into two hemispheres, connected by a neural body, the corpus callosum. In an extreme treatment for epilepsy, the corpus callosum is transected (callosotomy), resulting in two somewhat separate “streams” of consciousness. The left brain sees through the left eye, and the right through the right eye, and each hemisphere controls the opposite side of the body below the neck. Language is typically controlled on the left side. In this case, we seem to have a partial failure of individuation, with the substantial form containing two distinct parts, one corresponding to each hemisphere. At the same time, there is strong evidence that the two hemispheres in these cases do manage to communicate and coordinate to a remarkable degree, which is what we would expect given the existence of a largely unified substantial form.
Let’s turn to fictional and speculative cases, like Parfitian brain fission. What would Hylomorphism predict if an animal’s brain were divided in half, and each half were provided enough new organs and tissues to survive? One possibility consistent with HM would be for at most one of the halves to be able to function as a normal, sentient animal. The other half would function only as a disunified heap of organs. A second possibility, still consistent with HM, would be for one of the two fission products to be dominant and fully normal, while the second is clearly deficient in its self-organization. In this second scenario, we should see some “biological entanglement,” some formal-action-at-a-distance, with the development and behavior of the secondary fission product influenced in some way so as to complement that of the dominant product. This would be comparable to the way in which divided hemispheres are able to coordinate in callosotomy patients.
The least likely outcome from the HM perspective would be for us to observe two completely normal organisms, without any unusual, ESP-like entanglement. If this were to happen, the best HM account might involve three substances (and three substantial forms): one for the original person and one for each of the fission products, with the two fission-products as proper parts of the original, surviving animal. Such a three-substance is also available in cases of budding/mitosis and monozygotic twinning.
The Analytic Thomist 