According to Aristotelian ideas of causation, a “substantial form” is a holistic entity with its own nature and “ends”, and that distinguishes it from an artifact, which is only a collection of parts given some “end” by an outside manufacturer.
And so if a child assembles parts – an orange box, four pram wheels, an old floorboard and a piece of washing line – he has a go-kart. And if his parents angrily disassemble it, they can restore the oranges, the pram , the floor and the washing line, provided the kid didn’t smash them up on a hill. The parts have no intrinsic relationship to one another.
However, a living being, A-T thinking says, is an entirety in which every part is intrinsic to the whole. I may be able to localise thought to my brain and walking to my limbs, but it is I who think and I who walk. Remove my arm, and I will be a defective person, not just a different design. It’s a commonsense approach that answers a lot of questions about individual identity, homeostasis, volition and the like, as well as why creatures come in species (sharing the same essential nature) and have both specific and individual ways of being that may be understood in increasing depth as we get to know them. And so rabbits and hares, though fairly closely related, each have their unique way of doing life, at every level, and that goes far beyond their physical morphology.
The importance of this to biology is that, if the A-T approach has any truth, it sits uneasily with our current views on biology, especially evolution, which are profoundly atomistic. In biological evolution, whichever theory or mechanism is considered, individual parts of an organism (or rather, a population of organisms) change piecemeal, and the resulting new creature is merely the sum of the changes. Genotype gives rise to phenotype as a recipe gives rise to a cake.
In practice, for the most part, genotype does not give rise to phenotype in any simple way, and that has to do with the fact that whichever part of the genome one looks at, it affects and is affected by many other parts in ways we cannot predict, perhaps because they are too complex to analyse, or perhaps for more fundamental reasons of form. See here for some recent work on this.
Does the complexity of life prevent us assessing whether the “atomistic approach” or the “formal causation” approach has better explanatory power? The organism appears to be more than the sum of its parts, and closely related species appear to display very different characteristics, and that might put Aristotelian thinking on the table. But the whole thing might simply be a function of the compexity of genetic interactions, and still living things might be, essentially, molecular artifacts slung together like the child’s go-kart. On this latter understanding Evolutionary Creation is very much analogous to the go-kart model, only with rather less evidence of efficient purpose.
There is, however, a simpler model for the A-T scheme, and that is simple chemical compounds, which are held to be every bit as much substantial forms in their own right as living things. Now the point about forms is, as has been said, that they are holistic. And so the Thomist would say that, when elements combine to form molecular compounds, such as water, the forms of hydrogen and oxygen are actually lost, and an entirely new form, that of water, replaces them. In other words, there is a sense in which, unlike the conventional view that water “contains” both hydrogen and oxygen, A-T thinking says that hydrogen and oxygen are no longer truly present at all (or only virtually), but only this new, unique form, water, with its own quite separate properties. If that’s counterintuitive, it’s because our intuitions are guided by customary atomistic models. After all, we don’t balk at the idea that Uranium 235 disappears completely in a fission reaction.
One line of evidence regarding this is whether it is possible, on first principles, to predict the properties of a compound from those of its constitutive chemical elements. Perhaps surprisingly the answer is a qualified “No.” True, certain basic features can be predicted, such as mass, obviously, which is conserved, and also a few other things from the “valence shell electron pair repulsion (VSEPR)” of the elements in question – which can determine such things as the angles between bonds and so on.
But with even simple molecules – and water is a prime example as it has so many unique, and unpredictable, properties – to a large extent one has to actually make the stuff, and see what it does, as if its form has truly replaced that of the elements whose properties it no longer displays.
One of the recent wonders of science is that it’s possible to produce true visual representations of molecules which, to our amazement, closely resemble the diagrams in our chemistry textbooks. I guess the amazement comes from the fact that atoms, being at the quantum scale, really have no definite appearance, so we half expect molecules to look like bleary spheres too. But note that what you see in the linked images are mainly the bonds, not the atoms. The “reality” lies in the interactions constituting the “form”, not in the constituents. But that’s only to consider the structure – the other properties of the molecule are in most cases not obvious from the structure, and even less from the properties of the elemnts.
In more complex organic compounds, such as proteins, the sheer complexity of the molecules complicates this. Prediction is hampered here partly by the number of components within the molecule (as in, for example, proteins which can fold in unpredictable ways), and conversely it is helped by the fact that, to an extent, parts of large molecules act as units. Even so, the same problem of needing to make the thing to see what is does applies. Even sophisticated computer simulations can provide only pointers to which molecules are worth exploring for some desired property; drug companies still have to succeed in the synthesis of thousands of likely compounds to produce one beneficial drug.
The issue is exemplified by this forum post, in which a student expresses admiration that:
One of my professors does computational chemistry and he is actually able to determine certain physical properties (e.g. pka, boiling point, melting point, etc.) based on certain structures (e.g. hydrocarbon chain lengths and other features) of the molecule, without having to synthesize the molecule itself. Pretty neat stuff.
A bright guy can predict a few properties of complex organic molecules based on their stuctures. Yes indeed – but that does not weaken the basic position of the A-T supporter, because some elements of animal form can also be predicted, such as that a close descendant of a herbivore is not likely to be a meat-eater, and something can be known of a creature’s lifestyle from its fossilized bones, just as molecular structure will give some information on molecular chemistry.
If formal causes like this are a better explanation of the transformation of properties of molecular compounds or living creatures than the reductionist model that they are the sum of their molecules and no more, then how these formal causes work remains mysterious. But that’s not really different from the mystery of how hydrogen and oxygen lose their native properties and gain those of water in combination, under atomism, and there ia difference in that no real work has been done on formal causation for centuries.
These causes would be, though, in some sense immaterial (the material of water, after all, is just hydrogen and oxygen), and they would be universals in the sense of being real natures, shared by all molecules or organisms of the same type. Without them, such universals really have little explanation under atomism – that is, a concept like “human nature” is somewhat illusory, and the “normal values” of homeostasis beg the question of the meaning of “normal” – to what standard do they conform?
And the Son of God just took on a particular configuration of molecules with some resemblance to the configuration we happen to have too.