… Aristotle develops Plato’s account of perseity as “communion” into an account of the various relationships of universality between the subject and the predicate. We can first note that some predicates are said of all instances of some subject. Such predication seems to always involve perseity, though it need not be made explicit. In looking for an explanation of malaria, for example, we might notice that all cases of malaria arise from being near swamps, but this does not mean that it getting malaria and being near swamps considered precisely as such have a per se relation to each other. We can observe, for example, that the two are connected always or as a rule without being convinced that there is any intrinsic feature of a swamp that is the direct cause of malaria.
A more fundamental relation between subject and predicate is one that exists between them in virtue of something intrinsic or per se to them. For example, we don’t just believe that fire and its heat are simply together as a rule, we are also convinced that there is something intrinsic to fire that makes this so. Again, we do not just observe cones being a third of the area of the cylinders that contain them, we also can see that it is an intrinsic feature of the cone that makes this the case. We are (perhaps) not entitled to hold that these particular convictions are indefeasible, and perhaps either one is open to being overturned by the discovery of a cold fire or a new and more complete theory of geometry, but to be mistaken about which features are intrinsic to things is not the same thing as to deny that we can come to know any intrinsic features of some subject. At the bare minimum an intrinsic feature is a heuristic that guides our explanations of things from what merely happens to be so towards what must be so.
Aristotle’s account of the per se is based on the more fundamental axiom that our explanations of what things are must start off confused and imprecise and gradually be made more complete. Malaria does not come to us with a label or tag that tells us exactly what place to look for it or how we are to consider any of the objects in that place. It might first be related to being in hot climates; and then not to a hot climate as such but to hot areas with swamps; and then not to the swampiness of the place as such but to an extrinsic feature of swamps (sc. that mosquitoes adapted to use them as ecosystems); and then not to mosquitoes as such but to the fact that mosquitoes both carry and transmit a certain parasite. Notice that, with each progressive development in our understanding, we might be relatively convinced that we have found malaria as such. We might be convinced that malaria is just a peculiar way of being weakened or broken down by heat, just as some people still believe that colds are a peculiar way of being weakened or broken down by the cold. Again, we might be convinced that malaria is just a swamp fever, or that it is caused by something either intrinsic to swamps (e.g. their brackish) or something else. Every stage in this process might be supported by evidence to the point of convincing a rational observer; but for all that, we can recognize from our comfortable perch as outside observers that we have not found malaria itself until we have found the parasite. This terminal point of the explanation is simply when we reach to what the thing is in itself, that is, when we come to get a distinct look at the thing which we first understood only nominally and in a confused way.
Notice that on this account of explanation it consists in moving from some X to an account of what X is intrinsically or in itself. Explanation does not terminate in some brute fact in the face of which we can say nothing more than “it just is that way”, but rather in moving from something that is merely named to what the thing named is in itself. The explanation does not end with a shrug that can do no more than accept that “all explanations have to stop somewhere” but with the conviction that we have actually found the thing which we had initially done no more than name. The simplest account we can give of this sense of explanation is that it consists simply in discovering what we mean. We meant something by malaria, but this initial meaning occurred in a jumble of confused facts that required a difficult process of discovery and many sophisticated theories and tools to discover that malaria itself is the name for a mosquito-borne parasite.
Because this account of explanation is based on the general fact that explanations move from the confused to the distinct, every sort of cause will admit will move from some X that is merely named to the X itself. For example, the if we witnessed the first atomic attack on Hiroshima, we would want to know who was responsible for it and so we would be looking for an explanation in the order of agent causes. Now notice that the first person we could actually see dropping the bomb would be the bombardier on the plane, though it would be pretty easy to establish that he only dropped it at the behest of the flight commander. But neither of these persons is who we are looking for when we ask who is responsible for the event, but only the one who was responsible for all of them doing what they did, sc. Harry Truman. Likewise, if we caught the Watergate spies, they would be the first persons we knew were responsible for the break in, but they would not be the ones we are looking for when we ask who is responsible for the break in.
Just as there is a long series of diverse subordinate agents there is a corresponding series of distinct goals or final causes: the bombardier was only intending to pull a lever at the proper time while Truman had the much broader motive of terrifying his enemies into unconditional surrender. Here again, explaining a fact consists in discovering who we mean when we consider “the one responsible for this” or what motive we are looking for when we ask “why did this happen?” Truman is not a brute fact explaining why the bomb dropped, he is the one we meant to talk about from the beginning when we spoke of the one responsible for the action. Again, it is ridiculous to say that the explanation of motives “breaks down” after we discover the motive to terrify the Japanese into submission, since this would be like saying that our ability to look for something “breaks down” after we find it.
On this account of explanation, the explanans is simply “the thing itself”: e.g. malaria itself is the parasite as opposed to any other environmental feature, the one who dropped the bomb was Truman himself as opposed to any of his subordinates. Though speaking of a “thing itself” certainly suggests the familiar Platonic theory, Aristotle’s account of explanation is a complete redefinition and repudiation of it. The things themselves are not separated entities, or even forms inhabiting matter, but just the precise realities that we first target in our merely nominal and confused accounts. The trajectory of explanation is not from the facts to a form that is outside of them but from a name which we impose in the midst of a confused awareness of facts to a thing among those facts that actually deserves the name. The thing itself – or the thing per se – is not given separately from what we start with, but within what we start with, though indistinctly.
Aristotle distinguished two senses of perseity. In the first, any intrinsic connection between a subject and predicate will be a per se connection. In this sense, when we say an exothermic reaction is hot we say something per se, since it falls in the very definition of exothermic reaction that they give off heat. There is, however, a stricter sense in which it is not precisely the exothermic reaction that is hot, but only the mean molecular motion, since it is only this latter that is precisely what heat is, and it is in virtue of exothermic reactions giving rise to such motion that they are hot. Aristotle called the looser sense of the per se kath’ auto, which can be unproblematically translated as “per se”, but he called the stricter sense katholou (or “universal” in the genitive case) which we will here call primo since this was the Medieval usage. It is this strict sense of the per se – the per se and primo – that we target when we seek to explain something, even if, for practical reasons, we are often content with explanations that fall short of this level of rigor.
The Per se and Primo in The Fourth Way
Any causal explanation targets the primo and per se, and all cosmological arguments are a causal explanations of various things manifest to sensation. We can see that the Fourth Way is appealing to this principle because it twice appeals to the Medieval theory of fire as the cause of what was hot, which the Medievals saw as the per se and primo cause of heat, in exactly the same way that we now see mean molecular motion and the per se and primo cause of heat. It is crucial that we describe the Fourth Way as reaching, for example, what is per se and primo good, true, etc. because there are all sorts of things that are per se good – virtue, charity, food, and even everything that exists – which are nevertheless not good such first. This is why it is not enough to appeal to a principle like “things that are participated reduce to things that are essential”. Just as a thing can be essentially hot without being what is hot first, so too a thing can be essentially good without being what is good first of all.
St. Thomas places the principle that the causal explanations reduce to some first in at the logical beginning of his cosmological arguments, though he first applies it to the special case of a first cause of motion, saying that a series of causes cannot be infinite because: “then there would not be a first mover, and it would follow that there would be no other movers, since a second mover does not move except by the motion of the first mover, as a stick does not move unless it is moved by the hand.”  St. Thomas’s example is clearly from the order of efficient causes, and we saw above that the a first cause is necessary in this order since such a cause arises simply from giving a distinct account of what one means in speaking about “what is responsible for the motion”. One simply can’t mean to speak about an instrument or something with a derivative responsibility for an action when he asks about what is responsible for an action. Doing so would be like answering a child who asked “why are we driving to Church?” by saying “because I’m pointing the car towards it”. To ask the question about my what intention is responsible for the action means asking about what is responsible first of all. Any other explanation explains only in a qualified sense, and is in some way an indistinct grasp of the facts. While it is true that there are all sorts of reasons why we do not press our questions to completely distinct answers – reasons ranging from practical concerns to the dimness of our intellect to the desire to restrict ourselves to a limited domain of explanation – nevertheless St. Thomas is claiming that the completely distinct answer to “what is responsible for the stick moving” is “God”.
The Fourth Way starts from no specific class of facts, but from any fact about the world which, when understood per se and primo, deserves to be called God. St. Thomas need not be seen as restricting himself only to “transcendental perfections”, as the manual Thomists have read him. The Fourth Way can, in fact, start with things given in all the other four proofs, and can reach all the conclusions they reach by its own proper way of proceeding and without appeal to any extra premises. For example, we see movers that are more and less immobile, and so there must be some mover that is immobile per se and first; we see causes that are more and less causal, so some cause must be per se and first; and we see necessary things that are more and less necessary, and thus there is something whose necessity is per se and first. If such a being is “what all call God” in the preceeding proofs, it is also in this case.
Reading the Fourth Way in this way leads to several good results, which I will here only sketch in outline:
It gives the proof scientific value, even in the contemporary sense of science. The Fourth Way, as we have continually stressed, appeals to a principle that is common in all causal explanations, not just those that are supposedly metaphysical, but also those that are scientific, or medical. We are looking for the per se and primo just as much when we are looking for the cause of malaria or diabetes as we are when we are looking to establish the existence of God. This explains why this proof is uniquely concerned with proving its relation to the empirical and scientific, not only by twice appealing to the Medival account of the per se and primo cause of heat, but also by referencing De Caelo et mundo, an essentially Astronomical book.
It gives a simple and elegant account of the analogy of names from creatures to God. On our account of the per se and primo, it is the end point of an explanation that starts from something that is merely named and moves to something most deserving of the name. But this how St. Thomas divides analogous names from univocal and metaphorical names in the Summa theologiae. If, St. Thomas says, we consider the thing we impose the name on first then we will call the creature “good” or “existent”, but if we consider what the name signifies, then we see goodness and existence as belonging not to the creature but to God. Words like “good” or “existent” can be understood as naming either the things we first grasp in an indistinct and simply nominal manner, and in this sense we “mean” to indicate creatures when we speak of them. But we can never mean to indicate the indistinct and secondary as such in a causal explanation, and so in the proper sense of meaning or signification we do not signify the creature. It is precisely this double sense of meaning, arising with respect to a single word, that gives rise to the diverse proportional or analogous names that are said of God and creatures.
It can contribute to a larger dialogue about the explanatory necessity of form. Though Thomists frequently lament that the modern sciences have apparently cast out all formal and final causes, they have not yet come up with a principle actually useful to modern science that points to the explanatory power of form. But an understanding of perseity seems to be just such a principle, for it reduces causal explanations to something “in itself” or “of itself”, which have been, since Plato, unmistakable references to formal causality; and such causes become final causes as soon as we recognize the role that they play in a causal process that brings them about.
 This premise seems to be particularly important to preserve the real integrity and perseity of creatures, and to keep a proof for the existence of God from negating their proper existence. This problem becomes particularly acute when we consider the question of human freedom in the face of the causal power of God; though, as we have just seen, there is no problem with saying that the creature is essentially free and even autonomous, we only deny that this essential freedom is the first such essential freedom.
 ST 1.2.3.co. Note that the principle that causal explanations reduce to some first, though it occurs temporally at the end of the argument, is logically the first premise of the argument. No other principle or truth in the argument is more general. This general conclusion about causes is presupposed in the subsequent proofs.
 It’s interesting to note that some explanations are explicitly targeting means, and so are targeting things that are in themselves secondary. But even when we ask these “how” questions in the order of means, we are still we are asking for something that is first. A catapult, for example, in an instrument that uses a rock as an instrument to destroy things, but if someone asks “how did we destroy the city wall” the answer is not “with rocks” but “with catapults”. If we said only the first, we would mean that the rock was our primary instrument, i.e. that we held it in hand and hammered the wall, or hurled it at the wall with our bare hands.
 The recent finding of the ATP/P2X7R pathway by researchers at Boston’s Children’s Hospital is simply a finding of the per se and primo cause of diabetes