[This is, as I say, an attempt. Criticisms are welcome.]
1. Perhaps now is the proper time to discuss the principle “forma dat esse” in depth. In what sense does a thing’s form give it being? Recall that form is simply what makes a thing what it is. A thing’s form is that characteristic act or manner of being which makes it this sort of thing as opposed to any other. We can talk about form in various ways, depending on which aspect of what a thing is we want to get at. For example, “substantial form” designates the act of a thing that makes it a substance, i.e., a unified thing, over and above the mere agglomeration of its parts. “Accidental form” by contrast designates those features of a thing that are added to its substantial form, which can be changed or lost without destroying the unifying act that makes this thing as a whole the sort of thing it is. (Thus a man may cut his hair and remain human.)
2. Being is always being something. Not to have a form is not to have any sort of characteristic act, unity, identity, feature, etc. The answer to “What is a formless being?” is by definition “Nothing.” And note that this nothingness is a real, absolute nothingness. Ideas in the mind (for example, “unicorn”) have a certain existence, as ideas, as accidental features of the mind of the one thinking of them, and are thus not genuinely nothing, even if they lack existence outside the mind. But something that has no form is not just an unrealized possible kind, like a unicorn, but has no kind, no act, nothing at all either unique to itself or in common with other things. And thus it does not exist at all.
3. Philosophies of the post-Scholastic variety tend to have a conception of “being” as synonymous with “instantiation in the world”, which makes existence totally horizontal. Nothing “is” any more than anything else, and the supposition that some things could “be” more than others even seems bizarre. But once we grant that to be is to have form, and that nonexistence is utterly unformed, we begin to see the emergence a hierarchy of being. Formlessness does not exist, has no act. But if we give something act, then it is. Now suppose we add something to that act, in such a way that it does not lose what it was before, but its former act is subsumed under something more: its capacity to do things is increased, etc. In this case it is clear that we are adding “to be” (i.e. existence) to the thing, not just according to our ideas, but really in itself. And the more the thing’s act is compounded and enriched, the more it has form, and the more it exists. This scale of possible possession of forms, of acts, of existence, is perfection. To have more act is to be the sort of thing the form of which is further removed from nothingness by the power, the self-diffusive virtue, of its act. In this way, forma dat esse.
4. Consider a maple tree. Altogether it is one, living thing, and its act as a whole (its substantial form) subsumes all the acts of its parts into a unity which none of them could be or produce separately, which is distinct from the parts in their partition. This act is the form of the tree, its life or soul. Now, suppose we remove the form of the whole from the tree, so that what it is is no longer distinct from the form of its parts in their partition. In this case we have a dead tree. To clean things up, let’s rearrange the parts (since their arrangement is no longer particularly important). Now we have a heap of timber. Now, the timber remains a substance, albeit a different one than the tree. It has its own characteristic activity and features, its own form, though the pile has no special unity. One piece of timber is just as much timber as the whole stack (though one branch is not just as much “a tree” as the whole tree… in fact it is not a tree at all).
5. Suppose we remove the substantial form of this piece of timber, so that its parts (the lifeless cellulose strands making it up) lose their unity and the whole loses its distinction from the parts in their partition. We do this by sticking the log in an incinerator. Now we have a heap of ash and some fumes. There is some distinction between the different kinds of parts, but let’s focus in again on one of these parts, say a carbon dioxide molecule. Here again the whole is distinct in its unity from its parts in their partition. We remove the form of the whole, so that the parts no longer act as a whole, but merely as parts. In this case we have some Carbon and Oxygen ions. We proceed to the atomic scale, and the subatomic scale, and after removing the substantial form at each level, we arrive at some elementary particle, which is (the physicists tell us) some complex wave-function with a particular spin, velocity, wavelength, etc. And when we remove these forms, we have only the vague residue of “mass-energy” latent in a force field, and when we remove that, there is (so far as I know) nothing left at all.
6. Now, it is clear that the form of mass-energy participates in the act of a living maple tree, as well as the form of atoms, of cellulose strands, and woody tissue (i.e. timber). And yet at each level it is not difficult at all to say that mass-energy is not in itself the same as an oxygen atom, because evidently there is something different about the act of mass-energy in an oxygen atom than in other kinds of atom. Something has been added to this atom to make it what it is, and to increase the form of the mass energy (though, n.b., it is not simply an increase in the amount of mass-energy). Granted, many different kinds of “something” can be added to that mass-energy, to make it chromium or carbon or calcium or iron, but clearly the act of an iron atom is not simply the same as the act of mere mass-energy, or we could not claim that the two are really distinct. And the same is true at every other level. As we ascend from elementary particles toward living trees, something real is being contributed to the being of the stuff in question, so that without losing anything, the end product is simply speaking more, without being quantitatively, materially more. The form of the whole adds existence to the form of the parts, so that what was not a tree, did not have the activity of the tree, becomes a tree, and is everything a tree is. The tree-ness of the tree gives it its perfection, its reality, its being, such that when that form is obliterated, what remains is less than it was before, without losing an iota of its material quantity.