Like the Third Way, the Fourth Way consists of two principal parts. The first part establishes that there is something that is maximal in being, goodness, “truth”, and nobility. The second part reaches the conclusion that this maximal being is the cause of the being (and goodness, etc.) of all finite things. As in the case of the other Ways, St. Thomas does not suppose that he has yet proven the unity or uniqueness of God. So, we should really say that the first half involves the claim that there is some thing or things maximal in being, and the second half the claim that this thing or things is or are the cause(s) of all other things. For the sake of grammatical simplicity, I shall mostly ignore this important qualification.
 
The Big Question in interpreting the Fourth Way is this: just how Platonic is the argument? In his dialogue Phaedo (100a1-101a5), Plato has Socrates argue that he has discovered that the true cause of the beauty of things is something called Beauty Itself. Similarly, there is Justice Itself, the cause of the justice of all just things, Goodness Itself, Equality Itself, and so on. These are the so-called “Forms” or “Ideas”.
 
Anselm offered what seems to be a purely Platonic argument that parallels the Fourth Way in his Monologion. Anselm writes in Chapter 1:
 
“Necessarily, all good things are good through something, and this something is understood to be the same thing in each of the various good things…. And who would doubt that that through which all good things are good is a great good?.... That through which every good thing is good is good through itself…. The one thing that is good through itself is the one thing that is supremely good.”
 
Boethius makes a very similar argument in The Consolation of Philosophy (Book III, Chapter X). Is this exactly the argument Thomas Aquinas is making?
 
I think not, for two reasons. First, the steps of Aquinas’s argument are different. First, he establishes that there is a supreme being, and then he argues that this supreme being is the cause of other beings, which is the opposite of the Plato/Boethius/Anselm order. Second, he explicitly rejects in many places (following Aristotle) the argument from Many to One on which Plato/Boethius/Anselm rely. Human beings, for example, are each made human by his or her own human form, not by a Platonic Idea. It’s true that there is a kind of archetype of humanity (a divine idea) that was involved in our creation, but the divine idea is involved in God’s efficiently causing us to exist, which seems significantly different from the way that good things are supposed to be good “through” the Idea of the Good.
 
Kenny (in chapter 5 of The Five Ways) is pretty good on all this. He brings out the way in which a “common nature” for Aquinas is not some separate thing but rather a way of explaining the commonness of the members of a species in terms of some intimate relationship among their forms—namely, that the forms are not individual in themselves but only through their involvement with prime matter.
 
So, I think it is important to look closely at St. Thomas’s source: the end of chapter 1 of Book 2 (Alpha the Lesser) of the Metaphysics. I am also relying on some interpretive suggestions by Michael Augros in his paper, “Twelve Questions about the Fourth Way.” (The Aquinas Review, volume 12, 2005) In the Metaphysics, Aristotle argues that if something is the cause of all true things, then it must be supremely true. If we translate this into more familiar language about existence and nobility, we could say that if something of the existence of all existing things, then it must have supreme existence (likewise for nobility, goodness, and so on). This fits well with the place of the Fourth Way: we have already established that there is one existing thing that causes the existence of all other existing things (Second Way). Moreover, we know that this thing exists necessarily and causes the existence of everything else in every possible world (Third Way). The Fourth Way adds to this the conclusion that this necessary first cause must have supreme existence (nobility, etc.).
 
If this is right, then the second part of the Fourth Way is really the crucial part. The first part is simply designed to make the ultimate conclusion more plausible, by suggesting that there is something (at least, in the realm of possibility or potentiality) that has maximal existence. The second part assures us that this supreme thing actually exists, and, given the Third Way, actually exists and is supreme in existence in every possible world.
 
Alternatively, as Augros suggests, we could take the first part as simply arguing that there must be something that is, de facto, the highest thing on the scale of existence among the actually existing things. This need not, at this stage, be identified with the greatest possible being. Then the second part establishes that this de facto greatest thing must be the cause of the existence/greatness of all other things in all possible worlds, and so it must be the greatest possible being.
 
If this is right, then the standard translations of the crucial principle of the second part of the Fourth Way are misleading at best. Here’s the Latin original: “Quod autem dicitur maxima in aliquo genere est causa omnium quae sunt illius generis.” Kenny translates it as: "Now whatever is most F is the cause of whatever else is F.” The Leonine translation: “Now the maximum in any genus is the cause of all that is in that genus.” These make the principle a generalization of the form: all things that are maximal in a genus are the cause of the rest of the genus. But the Aristotelian principle in Metaphysics is the converse of this: all things that are the cause of the rest of the genus are maximal in that genus.
 
We should try to fit our translation to this Aristotelian principle, and I think it is possible, since the ‘quod’ is singular rather than general. Aquinas does not write “whatever” (omnia) is most F, as Kenny supposes, but rather “that which” (quod) is most F (or is supposed to be most F). The Leonine translation is better, if we take it to mean: the maximum in a genus and the cause of all that is in the genus (supposing that each of these exist) are one and the same thing. Ways 2 and 3 give us something that is the necessary cause of all other beings, and the first part gives us something that is a supreme in being, and the Aristotelian principle allows us to infer that the necessary cause of being is a supreme being, which is what we want. (Parenthetically, the presence of ‘dicitur’, ‘is said to be’, is some reason to prefer my reading of the first part over Augros’s: the supreme being of the first part is merely a supposed or hypothetical entity.)

​As in Maimonides’ case, Aquinas adds a second stage to the argument. He wants to establish not only the existence of a necessary being, but of a necessary being that has existence “in and of itself”, that doesn’t derive its necessary existence from something else.  Here again Aquinas has recourse to his no-infinite-regress assumption: the chain of causation explaining why derivatively necessary beings are necessary must terminate in a thing that is non-derivatively necessary, and this being will be God (a being whose essence is its existence).
 
Here’s a way of thinking about this second stage. Let’s suppose for contradiction that there is an infinite regress of necessary beings, each of which derives its necessity from its predecessor. So, N1 is caused to be necessary by N2, N2 is caused to be necessary by N3, and so on. And let’s assume that all necessary beings belong to such a regress: nothing is necessary in and of itself (unconditionally). Now, a world in which none of N1, N2, N3, etc. exist is an impossible world, since each of these beings exists necessarily and so exists in every possible world. So, the scenario in which none of the N’s exist is an “impossible world”, if you’ll allow me to talk of it that way. Let’s call this impossible world w!.
 
Let’s assume that if a scenario S is impossible, and this scenario S can be derived from some possible world w simply by deleting entities that exist in w, then there must be some ground or explanation of S's impossibility. Let's stipulate that the impossible world w! comes from the actual world (which is possible) by deleting all of the conditionally necessary beings in the actual world. Then the impossibility of w! must be explained in one of two ways: it fails to include something that is unconditionally necessary, or it violates some constraint of conditional necessity, i.e., it contains A but not B, even though A would (if it existed) necessitate B’s existence (which it could do by necessitating B’s necessary existence). But w! is not impossible in either of these ways. There is (by hypothesis) no unconditionally necessary being, so it isn’t impossible for that reason. And it satisfies all of the conditional constraints by never including any of the N’s. Its non-inclusion of Ni is permissible, because it also fails to include N(i+1), and Ni is necessary only conditional on N(i+1)’s existence. So, w! is possible, after all, which means that none of the N’s is necessary.
 
Therefore, it is impossible for anything to be necessary unless something is necessary unconditionally. And to be necessary unconditionally is to be necessary in and of oneself.

My colleague and friend Dan Bonevac has discovered a new interpretation the Third Way that resolves the problems that have puzzled readers from medieval times. The argument seems to involve two highly problematic claims:
 

1. If it is possible for a thing not to exist, then there must be some point of time at which the thing does not exist in reality.
2. If for each thing in some class C there is some time at which it does not exist, then there is a single time at which nothing in C exists.

 
Dan proposes that we interpret the temporal adverbs in the argument (quandoque, aliquando, modo) as modal rather than temporal modifiers. Such an interpretation is quite natural in many (if not all)  languages, including Latin and English. Dan notes that St. Thomas never uses the word 'time' ('tempus') or any other explicitly temporal term. In fact, if we look at the parallel argument in the Summa Contra Gentiles (I.13, paragraph 33), we see a complete absence there of temporality. Under this interpretation, the two problematic claims become:
 

1. If it is possible for a thing not to exist, then there is some possible situation in which it does not exist.
2. If for each thing in class C, there is some possible situation in which it does not exist, then there is a single possible situation in which nothing in C exists.

 
Now principle 1 is simply a tautology of modal logic. Principle 2 is still a substantive principle, but it is a quite plausible one, as we shall see.
 
Here is the Third Way under this modal interpretation:
 
Here’s the Third Way under this interpretation:
 

1. There are contingent (non-necessary) beings (because we see things coming into and going out of actual existence).
2. That which is contingent is non-existent in some possible world. (Modal logic)
3. Suppose for contradiction that all actual beings are contingent (non-necessary).
4. Given 2 and 3, there is a possible world in which nothing exists.
5. If it were possible for nothing to exist, then nothing would exist in the actual world.
6. Since it is impossible for something that doesn’t exist to exist as a possibility except by the causal agency of something actually existing. (Nihil ex nihilo)
7. But, obviously, something exists in the actual world (namely, the contingent things mentioned in premise 1).
8. Therefore, by contradiction (of the hypothesis in line 3), some actual being is a necessary being. (From 3-7)

Now everything is crystal clear, except for premises 4 and 5. Dan shows how we can construct, using resources available to Thomas Aquinas, plausible arguments for each. He calls premise 4 the “Annihilation Lemma” and 5 “the Dead End Lemma”.
 
Proof of premise 4: the Annihilation Lemma.
 

1. Suppose that everything is contingent in the actual world, w0.
2. Necessarily, all causal chains are finite (no regresses or cycles).
3. So, there are uncaused contingent things in w0. Let the xx’s be the plurality of those things.
4. Subtraction rinciple: if a world x contains one or more uncaused contingent things, then there is a world y in which those things do not exist, and the set of uncaused things in y is a subset of the set of uncaused things in x (i.e., no additional things exist uncaused in y).
5. So, there is a world w1 in which none of the xx’s exist, and such that the set of uncaused things in w1 is a subset of those in w0.
6. Since the xx’s are all the uncaused things in w0, this means that the set of uncaused things in w1 is empty.
7. However, if all causal chains are necessarily finite, then a world with no uncaused things must be an empty world, a world in which nothing exists.
8. So, w1 is an empty world.
9. So, it is possible for nothing to exist.

 
Proof of Premise 5: The Dead End Lemma
 

1. Suppose that w1 is an empty world.
2. Something can possibly exist relative to world w only if there is something in w that can cause it to exist. (A causal principle: Nihil ex Nihilo)
3. So, nothing can possibly exist relative to world w1.
4. The actual world is necessarily possible. (Axiom B of standard modal logic)
5. So, the actual world is possible relative to world w1. (4, definition of necessity)
6. So, nothing exists in the actual world. (From 3, 5)

 
Why think the Subtraction Principle is true? Suppose that there is an uncaused thing x which, if deleted from the world, necessitated the introduction of a new uncaused thing y in its place. In that case, it seems that the existence of y in the new world would be caused by the absence of x (together with the other conditions that, jointly with the non-existence of x, necessitated the existence of y). This is doubly problematic. First, and most importantly, because we seem to have a contradiction: the existence of y would be both caused and uncaused. And, second, because it doesn’t seem that the existence of anything could be wholly caused (or explained) by the non-existence of something else.
 
This version of the argument requires two causal principles: (i) necessarily, every causal chain is finite, and (ii) necessarily, it is impossible for something to exist unless (a) it actually exists, or (b) it could be caused to exist by something that actually exists. The second principle (Nihil ex Nihilo) is pretty strong. It would imply (given S5 modal logic) that every contingent thing in the actual world has a cause in the actual world. Here’s the proof. Suppose for contradiction that x is contingent and uncaused in w0 (the actual world). Consider any possible world w1 in which x does not exist. The existence of x is possible but not actual in w1 (by axiom B), so by Nihil ex Nihilo there must be some y that exists in w1 and is capable of causing x to exist. This plausibly entails that, in any world w in which x does exist, x is caused to exist by some y that also exists in w. Hence, since x exists in the actual world w0, x must be caused to exist in this world, contrary to our original assumption.
 
An interesting question: could we do without the first causal principle (namely, no infinite regresses or cycles)? Here’s a possible way of doing so. Suppose that there are infinite series or cycles of contingent things. We could plausibly strengthen our subtraction lemma, so that it allows for the simultaneous subtraction of all uncaused contingent things and all infinite contingent series and cycles, without requiring the addition of any new uncaused things or any new infinite series. If so, we could run the original argument without the first principle

The Third Way is clearly building on the Second Way. It adds an important component to the conclusion: we reach the conclusion that the uncaused cause of things must be a necessary being, in contrast to the contingent beings that are familiar to us. This immediately raises the question of what Aquinas means by 'necessary'. There are a range of possible interpretations:
 

1. Logically necessary. Its non-existence “entails a contradiction.” Or, perhaps, its non-existence is not ideally conceivable. Or, perhaps, its existence is an “analytic” truth (true by virtue of the meanings of the terms involved).
2. Metaphysically necessary. The narrowest form of real necessity. Not relative to a system of logic, and not epistemically or semantically defined.
3. Naturally necessary. Necessary given the metaphysically contingent order of nature within creation. Its existence is necessitated by the existence of anything or any kind of thing found in the actual creation.
4. Actually existing at every time.
5. Naturally incorruptible.
   

         a. Actually exists at some time and cannot naturally be corrupted.
         b. Actually exists at all times and cannot naturally be corrupted.
 
We can set aside meaning 1, since the argument has nothing to do with logical deducibility, analyticity, or conceivability.
 
Anthony Kenny argues (in his book, The Five Ways) that the Third Way must intend ‘necessary being’ in sense 5a or 5b. This is based on Aquinas’s assertion in the Summa Contra Gentiles (Book II, chapter 30) that the heavenly spheres are “necessary beings,” despite being created by God. However, this evidence does not rule out 3 or 4 either, although it does rule out 2.
 
​The next point of complexity concerns the distinction between things that are necessary per se (in and through themselves) and those whose necessity is caused by another. Are each of the four relevant categories of necessity (2, 3, 5a, and 5b) divided into two sub-categories by this distinction, or can some forms of necessity exist only per se or only when caused by another?
 
Now, suppose that the distinction cuts across all four meanings. Suppose, further, that Aquinas intended ‘necessary’ to correspond to meaning 3, 5a, or 5b. If so, the only result that Aquinas could reach would be that God is an incorruptible being that exists at all times or with merely natural (created) necessity. God is certainly incorruptible, but it wouldn’t make sense to attribute to Him existence in and through time, or merely natural necessity.
 
So, we must assume either that Aquinas intended meaning 2 throughout, or that he intended ‘necessary’ to include things that are necessary in senses 2, 3, 5a, or 5b, but he believed that only things with necessity in sense 2 (metaphysical necessity) could be necessary per se. Given his affirmation of created, metaphysically contingent beings (like the heavenly spheres) as ‘necessary’ in SCG II.30, the second assumption seems most likely. So, we can assume that necessary beings include all beings that are either metaphysically or naturally necessary, and that only metaphysically necessary beings are necessary per se. This leaves open the question of whether there could be metaphysically necessary beings (meaning 2) whose necessity is caused by something else. The most plausible candidates would be the divine ideas, although Aquinas never says that the necessary being of the divine ideas is caused (Summa Th. I q15, a1 and a2). He suggests instead that each divine idea is in some sense “identical” to the divine essence, despite the existence of many ideas.
 
The safest interpretation might be that only God is metaphysically necessary and only God is necessary per se (so those two categories coincide). Consequently, all of the merely naturally necessary beings have their necessity caused by God.
 
We still have to consider meanings 5a and 5b, which were favored by Kenny. I would rule out 5a immediately, since Aquinas never, as far as I know, labels the human soul as ‘necessary’, even though it is certainly incorruptible. It is true that Aquinas states (in Summa Theologica I q75 a6 ad 2) that the human soul has no ‘potentiality for non-existence’ but it isn’t clear that that is the same as necessary being.
 
The heavenly spheres are necessary beings in both sense 3 and 5b. So, the acid test will be to see which meaning makes most sense of the premises of the first part of Thomas’s argument. I will turn to this problem next week.

 All of Aquinas's Five Ways depend, in one way or another, on ruling out the possibility of infinite causal regresses. In the version of the First Way (the argument from motion) in the Summa Contra Gentiles (I.13), Aquinas follows Aristotle in offering two separate arguments against the causal regress. In the first three ways in the Summa Theologiae, he offers just one of the two Aristotelian arguments: an argument that depends on what I call the No-Intermediate-Real-Cause thesis. This is a thesis that states that if x is a cause of y, and y is a cause of z, then y is not really a cause in the strict sense but only secundum quid (only in a manner of speaking). An intermediate link in a causal chain is not in any sense the source of the reality of the ultimate effect--it is merely a conduit through which the first cause acts. Therefore, an infinite regress is impossible, because (as Aristotle and Aquinas note) every link in the regress would be only an intermediate cause. Hence, such a regress cannot contain any real causation.
 
This is a plausible argument, despite the fact that it is often dismissed as based on a fallacy of equivocation. The standard objection (going back at least to Cajetan, I believe) is that Aquinas equivocates on the phrase "removing the first cause." If we have a finite chain and we hypothetically remove the first cause from the series, it is obvious that none of the intermediate causes can act. Aquinas asserts that if we consider any infinite regress, we have a situation from which we have (in a sense) "removed the first cause". But, as critics point out, in this case there never was a first cause to be "removed", and so the cases are incomparable. However, Aquinas real point is simply to claim that intermediate causes are never causes in their own right but are always wholly parasitic on the first cause. Given that assumption, infinite causal regresses are indeed impossible.
 
The other strategy for dealing with infinite regresses was invented by Avicenna, followed by Scotus, Leibniz, and many subsequent thinkers (including me in 1997). This is the Aggregation Strategy. The Aggregation Strategy concedes, for the sake of argument, that infinite regresses are possible. However, the Strategy insists, if a regress consists entirely of contingent (or finite) things, then we can aggregate the whole series into a single entity and insist on a cause for it. Start now with a single finite thing, and consider all of the finite causes of that thing (whether immediate or remote). Either this series terminates in an uncaused thing, or else it constitutes an infinite regress. In the latter case, we can demand a cause for the whole series. This cause must be infinite, since any finite cause of the series would be a cause of the original entity and so would already be included in the series itself. A member of the series cannot cause the whole series. An infinite thing cannot be caused. And so we reach an uncaused first cause.
 
Aquinas was aware of this strategy, through his close reading of Avicenna. Why didn't he adopt it? I think he was worried that not all infinite series can be aggregated into a single entity. If an infinite series consists of entities of the same species (or a finite number of species), then it has the characteristic that Aquinas labels being accidentally infinite. An accidentally infinite series can be aggregated and must be caused as a whole. This is why Aquinas can concede that accidentally infinite series might exist without losing the force of his first cause argument. However, if an infinite series consists of entities of an infinite number of species, with the species climbing progressively higher and higher in the Great Chain of Being, the Aggregation Strategy could not be convincingly applied. Aquinas would call such a series essentially infinite, and he must (if the Second or Third Way is to work) deny the metaphysical possibility of such a series. This is why he appeals to Aristotle's argument and what I call the No-Intermediate-Real-Cause thesis, which should now be applied only to series that rise "vertically" through the ontological order of species.

​Aquinas's second way, the way of efficient causation, had perhaps the most influence on subsequent natural theology. It has counterparts in the work of Scotus, Leibniz, and Samuel Clarke (to name a few), and it is the argument that Kant labels "cosmological" in the First Critique. I am going to assume the argument that Aquinas develops in chapters 3 and 4 of De Ente et Essentia is an elaboration of the second way. The argument's form is quite simple:
 
1. Some things actually exist (as known primarily by sense perception).
2. Every finite thing (i.e., thing for which there is a real distinction between essence and existence) that actually exists has an actually existing cause of its existence.
3. Causes are prior to their effects in the order of actual existence.
4. Every chain of causes has an essential structure.
5. The essential structure of every chain of causes is finite (has terminal, uncaused nodes).
6. Therefore, there exists at least one actually existing infinite being, and every finite thing is caused by one or more such beings.
7. There can be no more than one infinite being.
8. Therefore, there exists exactly one actually infinite being, which is the ultimate cause of every finite being.
 
If something has actual existence, and this actual existence is something distinct from its essence, then there is some part of the thing's essence which is disjoint from its existence. The finite thing's existence depends on a joining or combining of its existence with the remainder of its essence, and this joining or combining must have some explanation. It cannot be explained by the remainder of the essence, since, it it were, the thing would be a cause of itself (contrary to premise 3). It cannot be caused by the thing's actual existence, since a thing's existence cannot be prior to any part of its essence, since every 'act' or 'event' of existence depends for its very identity on the essence that it actualizes. Hence, the only possible explanation for this joining or combining must appeal some distinct entity (its efficient cause).
 
In contrast, if a thing's existence is identical to its essence, no explanation of its existence is needed or even possible. The essence of a thing constitutes its possible existence, and if a thing's existence is identical to its essence, then that thing's actuality is identical to its possibility. Hence, we cannot sensibly ask how or why its possibility has been actualized. For such a thing to be really possible is for it to be actual, and vice versa. If such a being exists at all, it must exist with absolute necessity. For this reason, the causal principle (premise 2) must be limited to finite things.
 
Premise 7: There can be only one infinite thing (so defined). Suppose that there were two. We can ask whether it is possible for one to exist without the other. If so, then one or the other exists only contingently, which we've have shown to be impossible. So, it must be necessary for both to exist and to relate to each other as distinct entities. Is this relation of distinctness contained in the essence of one or both? It couldn't be just one, since, if it were, the other would depend for its existence on the other. So, the relation must be contained in both essences. But this would introduce some complexity into both essences, which means that we could no longer identify either essence with a simple act of existence. In addition, there would have to be some explanation of the symmetry. Each would have to depend essentially on the existence of another, again contrary to their simplicity and uncausability.
 
This leaves us with premises 4 and 5, which jointly rule out the possibility of an essentially infinite regress.  I will take up this crucial question next time.

Thomas Aquinas, building on arguments found in Books 7 and 8 of Aristotle's Physics, argues in The First Way (in Summa Theologiae I q2 a3, and in a parallel passage in Summa Contra Gentiles I.13) for the existence of an unmoved mover. However, there is an evident gap between such an unmoved mover and God. At the very least, Aquinas needs to show that the unmoved mover is absolutely unmovable (in all respects). If he can establish that, then he can conclude that the First Mover must exist outside of time. And, in order to exist outside of time, the First Mover must lack all passive potentiality (i.e., be a being of Pure Act).
 
Aristotle and Aquinas are both well aware of this gap, and they have a definite strategy for filling it. The argument goes something like this:
1. Assume (for contradiction) that the First Mover is changeable in some respect.
2. Necessarily, time passes if and only if change occurs.
3. All motion in fact depends on the activity of the First Mover (established by the main argument of the First Way).
4. If the First Mover were changeable in any respect, then it could be in a state in which it failed to cause any motion.
5. To be in such a state, the First Mover would have to be in that state for some period of time (since nothing can be in a state in a single instant).
6. Since all motion in fact depends on the activity of the First Mover, if the First Mover were in a state in which it failed to cause any motion, there would be no change during the period in which it is in that state.
7. If there were no change during that period, time would not pass during that period.
8. If a period has a temporal duration, time must pass during it.
9. There would be a possible state of the world during which time both does and does not pass. Contradiction.
10. So, the First Mover cannot change in any respect.
 
The crucial premises are 4, 5, and 6. Let me take 5 and 6 first.
 
Premise 5. This is based on Aristotle's resolution of Zeno's paradoxes in the Physics. Instants of time are not parts of time--they are only boundaries of such parts. Hence, nothing happens during an instant. Nothing can be in a state of activity or inactivity for only an instant. Instants can only mark the beginning or end of a period of activity or inactivity.
 
Premise 6. This depends on a kind of subtraction principle. If all change in the actual world depends on the First Mover and there is a possible state of the First Mover in which it would cause no change, then there is a possible world where no change occurs. We can simply subtract the activity of the First Mover from the actual world without being forced to add any new source of motion.
 
So, the crucial assumption is premise 4.  Suppose the First Mover is changeable in some respect. Why think that it must be changeable into a state in which it would cause no motion at all? Why couldn't it have a nature such as to cause motion in every possible internal state, while admitting of more than one such possible state?
 
There is some plausibility to the idea that the activity of a thing must depend on the thing's internal state, and that this dependence entails that there be some internal state in which no activity would result. However, this seems far from airtight to me.
 
I think there's a better strategy for defending premise 4--one that is not explicit in the texts of either Aristotle and Aquinas, but which lies quite close to their conception of time and motion.  If something is changeable in any respect, then it lies within time. If a thing lies within time, then its natural activity through time depends on the metaphysically prior passage of time. So, it is impossible for the activity of something changeable to be the ground for the passage of time itself. Yet, that is exactly what the First Mover must do. Hence, the First Mover must be absolutely unchangeable.

I've been working a lot lately on Aquinas's First Way, the argument from motion, which builds on Aristotle's arguments in Books 6, 7, and 8 of the Physics, and which Aquinas develops at length in the Summa Contra Gentiles, Book I, chapter 13. Aquinas calls it the "most manifest" way of proving God's existence, but it has not been popular with commentators or critics. Sir Anthony Kenny is thoroughly dismissive of it in his book on the Five Ways. He quotes Suarez, who wrote: "Taken by itself, this argument is shown in many ways impotent to prove there is anything immaterial in reality, let alone that there is a first and uncreated substance." (Disputationes metaphysicae XXIX, I, 7)
 
The basic argument is quite simple:
1. Some things are in motion (experience change).
2. Everything that is moved is moved by something else (no self-moving).
3. A chain of movers cannot regress to infinity.
Therefore, there must be at least one unmoved mover.
 
Almost everyone accepts premise 1, so all of the action concerns premises 2 and 3. In addition, the argument faces a serious "gap" problem: how does one get from an unmoved mover to a "first and uncreated substance" (as Suarez puts it)? By paying careful attention to the arguments, and by exercising a little imagination and creativity, we can rehabilitate the First Way into an argument that deserves consideration alongside the many other sound theistic proofs that have been crafted recently.
 
Before getting into the details, we have to consider first what Aristotelians like Aquinas assume about the nature of change and time. There are essentially only two options here: either time is fundamental, and change is definable in terms of time (Russell's at-at theory of change), or change is fundamental and time is definable in terms of change (time is the "measure" of change). There are very strong considerations in favor of the second, Aristotelian option. At-at theorists have never been able to develop a successful explanation of the direction of time or of causation. See, for example, Huw Price's Time's Arrow and Archimedes' Point, or Alex Pruss's critique of David Lewis's counterfactual-conditional account of time's arrow. In addition, at-at theorists cannot explain how we are able to measure the true duration of processes, without making the ad hoc assumption that each kind of process has a fixed velocity (like the speed of light). Aristotle's option avoids both of these defects.
 
Famously (or, infamously, depending on your point of view), Aristotle defines change in Book III of the Physics as the actuality of the potential qua potential. Aquinas does a good job of unpacking this somewhat cryptic statement in his commentary on the Physics (Lectures 2 and 3 of Book III). Potentiality is, for Aristotle, something real and irreducible. It is a feature of all natural things, a kind of "natural intentionality" as David Armstrong and George Molnar put it. When a thing has a certain potentiality, it is pointing in a specific direction to a particular, non-actual situation. Motion occurs when such a potentiality is partially but not completely actualized.
 
Take a stone that is in the process of becoming hotter. Let's say that the stone is currently lukewarm. The stone has both the potential of being colder and the potential of being hotter, but only one of these two potentialities is now partially actualized, namely, the second of the two. That is what constitutes the stone's becoming hotter. Once the stone has reached its equilibrium state, it will have fully actualized that potential and will no longer be in motion (change). At that point in time, neither of the stone's potential will be partially actualized. Both will exist only in a state of perfect potentiality.
 
All change is, therefore, inherently directional. It is always change toward some unrealized state or states. Time passes as change occurs, and time itself is therefore also directional, pointing from the terminus ab quo and toward the terminus ad quem of the process of change. Moreover, the measure of time consists in the completion of certain standard processes, like the movement of light across a fixed distance. Thus, there is no mystery about the arrow of time, nor about the fixity of the velocity of these standard processes. In addition, the Aristotelian option yields the impossibility of time travel, since this would involve making the end of a process into its beginning.
 
Time passes because change happens, and not vice versa. Once we grasp this, we see that any law of inertia is completely irrelevant to the argument from motion. A law of inertia dictates that a body will continue to move in a straight line and at a constant speed as time passes. The inertial motion of the body thus depends on the movement of time and cannot be the explanation for the passage of time. The passage of time requires the continuous occurrence of change that is independent of time, in a way that no merely inertial motion can be.
 
Thus, the First Way points us toward a crucial metaphysical question: what is the source or explanation of this time-independent change?

As I explained in an earlier post, the use of prime matter as a way of grounding numerical distinctions between substances in the same infima species results in an attractive form of "moderate realism," with a powerful explanation both for specific sameness and for intra-specific plurality. There is, however, an obvious alternative: the use of Duns Scotus's haecceities, which are properties of primitive thisness. On Scotus's picture, what individuates Socrates and Plato is not their respective prime matters but two special properties, one of Socrateity and another of Platonity, properties that, as a matter of metaphysical necessity, can only be instantiated by Socrates and by Plato (respectively), and which Socrates and Plato instantiate whenever they exist.
 
Is there any way to evaluate the two proposals? I think there is: we have to look at the problem, not only of individuating whole substances, but also of individuating all of the quantitative or material parts (both actual and potential) of substances. Consider, for example, a perfectly symmetrical starfish. Each sector (one-fifth) of the starfish is (let's suppose) qualitatively and functionally interchangeable with the other four sectors. The sectors are conspecific as parts of the whole. Thus, we must individuate them --find something that can ground their mutual distinction. For Thomists, this is no problem. The prime matter that individuates the whole starfish is infinitely divisible: it contains five non-overlapping parts that can also individuate the five sectors from one another. There is no redundancy involved: the same thing that (as a whole) individuates the whole starfish from other starfish in the same species also individuates (as the sum of five parts) the five sectors from each other.
 
Scotists will have to posit haecceities, not only for the whole starfish, but also for each of the five sectors (with six haecceities in total). Consider a perfectly homogeneous bronze sphere. We will need haecceities for every part of the sphere--for every hemisphere that it contains, for example. Thus, we will need an infinite number of different haecceities for each such sphere (assuming the continuity of matter).
 
So far, this seems like a tie. Both Thomists and Scotists require an infinite number of individuators. In fact both require the same infinite cardinality, probably aleph-one, the number of connected, smoothly-bounded spatial regions. There is, however, a crucial difference. The Scotist account involves an infinite number of redundancies that are avoided entirely by the Thomist account. Consider again our starfish. Once we have the five haecceities for the five sectors, the sixth haecceity (for the whole) is entirely superfluous. The five sector-haecceities suffice to individuate the whole starfish from all other starfish. In contrast, the Thomists have just one thing (the whole mass of prime matter) that simultaneously individuates the five sectors from each other and the whole starfish from other starfish.
 
To match the elegance of the Thomist approach, the Scotist would have to posit that each haecceity is composed of an infinite number of "smaller" haecceities, each corresponding to a proper part of the whole. However, it is far from clear that properties can be composed in this way. And, if we suppose that they can, then the whole difference between haecceities and prime matter seems to disappear. The difference threatens to be merely verbal.

I've recently discovered that my views on prime matter as the universal individuated can provide new support for the real distinction between essence and existence. As I've explained in earlier posts, I believe that packets of prime matter are the ground for any numerical distinction among substances of the same species, whether those substances exist at the same time or at different times. Imagine, for example, a sempiternal world with an infinite past and infinite future. Let's suppose that Nietzsche's eternal recurrence is realized in this world, so that there are an infinite number of distinct but indiscernible Napoleon Bonapartes, one for each cycle. Each temporal stage one each Bonaparte has its own, unique packet of prime matter. It is the distinctness of these packets that ground the distinction between different Bonapartes, and it is the distinction between the Bonapartes that distinguishes the accidents of time during which they exist.
 
This individuating role of prime matter extends also to merely potential beings. Consider, for example, some merely potential pair of identical twin daughters of my wife and me (we did not in fact have any such twins). There are surely some potentially existing pairs of twins, and, within each pair, each twin must be numerically distinct from the other. This numerical distinction between potentially existing daughters must be grounded by the numerical distinctness of the potentially existing packets of prime matter associated with each daughter at each moment of her potential existence.
 
Now, each of these potentially existing daughters is potentially a human being. This modal fact about the twins must be grounded in the potential existence of two substantial forms of the human species. Thus, there must be potential substantial forms as well as actual ones. What is the difference between the two sorts of forms? The actual forms transmit actual existence to their substances, while merely potential forms do not. Hence, there must be acts of existence (actus essendi) which are present in the one case and absent in the other. Thus, we get a real distinction between essence (form + matter) and existence (acts of esse).
 
Those (like Averroës) who reject the real distinction must suppose that every form is an actual form, a source of actual existence. This means that they must deny that merely potential beings are in any sense real. In particular, they must deny that there are potentially existing twin daughters (as described above). They must say, instead, that there could have been such daughters. However, they cannot provide any ground or truthmaker for such a modal truth.