About 18 months ago I did a full treatment of the fourth of St. Thomas's "Five Ways" of demonstrating God's existence, for my blog, Tollat Summam, which was the first draft of a commentary on the first few questions of the Summa. I'm currently re-drafting the commentary, amending the various inadequacies of the earlier version, and have, for the past month and a little more, been stuck on Ia q.2 a.3, which is incredibly dense. Thus far I have constructed expositions of the first three ways, which, while still in need of some further tweaking, are at least satisfactory. The Third Way in particular was extremely difficult to work through. The Fourth Way is more difficult than the Third, so now that I come to it, I feel a little dismayed. ("More? Really?" some part of my psyche seems to say.) The five ways are foundational for the subsequent questions in the first half of the treatise de Deo Uno (Ia qq.2-26). To skip any of them, or do them imperfectly with the intention of substantial later revision, would invite massive difficulties in the way of pedagogical re-ordering and the re-composition of sections of text. In this post I would like to give a quick run-up to the fourth way, for my own purposes. However, I suspect that what follows will be useful to others as well.
I. The Notion of Theology
St. Thomas begins the Summa by defending the existence and scientific character of theology. Theology is weird as a discipline for two primary reasons: first, its subject is incapable of being really defined in the present life and seems superfluous when one considers merely the mundane lines of inquiry suggested by the ordinary sciences; secondly, its data are largely non-empirical. When theology works off of experience, it does so in a highly speculative way, but much of its content is incapable of empirical verification, and is based on faith in divine revelation.
Because of the weirdness of theology, the first difficulty the theologian has in doing theology is specifying what exactly he's talking about. Is one talking about anything at all? What is signified by the word "God"? Is there anything signified by the terms of this discourse?
II. The Idea of Proving God's Existence
Thomas has a habit of asking "is there such a thing" before he asks "what sort of thing is it". This habit seems strange, but it makes sense once one acclimates oneself sufficiently to his thought. Why? Because Thomas is interested in scientific investigations and demonstrations, and science concerns the nature of existing things. It is no use to define an abstraction and then subsequently try to pidgeonhole things into it. One should start by looking at what is, and then try to figure out what it is. This spares one a host of problems caused by rationalizing prejudices and what the phenomenologists might call "totalizing intentionality".
So, when Thomas demonstrates the existence of God, he is saying "what is" that we're going to be talking about. The goal is to jump-start the rational investigation of the Divine Essence by showing a number of indirect ways of specifying what aspect of reality we're looking at. The five ways act as the substrate upon which all subsequent discussions in the first eleven questions of the Prima Pars are built. "Hoc dicimus Deum" may seem like a cutesy conclusion to a complex metaphysical argument, but St. Thomas's saying, which is the general conclusion to his demonstrations, serves two purposes: first, it grounds the indirect evidence in the common usage of the name, making clear that what ordinary people would call "theology" is in fact being done in the investigations performed—"We're talking about just that sort of thing here too." The second function of the "hoc dicimus" is to serve as a definition of the term as used in subsequent investigations. When St. Thomas uses the word in qq.3-11, what he means is "this sort of thing", i.e. an unmoved mover, which is a first efficient cause, which is absolutely necessary and permanent, which possesses maximal actuality and perfection, and which orders the natures of unintelligent things toward their fruition. The goal, and this is extremely important for properly reading his text is not apologetic, but illuminative. The goal is not primarily to answer the protests of atheists, but to ground the discourse of theology by identifying its subject. We can think of qq.3-11 as bookended by two major investigations. On the front end (q.2), there is the question of what in reality we are talking about as theologians; on the back end (qq.12-13), there is the equally ticklish question of how our petty minds could know—and how our theological talk could be capable of analyzing—such a reality.
III. The general form of these demonstrations
In q.2 a.2, Thomas lays down the principles which must be used in any demonstration of God's existence. He distinguishes between demonstrations propter quid—demonstrations of a reasoned fact—and demonstrations quia—demonstrations of fact. A demonstration propter quid proceeds from something which is objectively prior, to another thing (fact, effect, event, end, being) which is consequent upon it. These demonstrations give us a fact, but because they give us something which is the basis of the fact, we understand the fact better: it is understood with at least some of its reasons for being so. Demonstrations quia on the other hand, proceed from an observed consequence to the inference of the existence and, indirectly, the nature of the cause. Such and such a thing happened, and therefore whatever caused it must belong to the genus of things whose proper effect such and such a thing is. Inferences of this variety are weaker, because the precision with which the cause is specified in the demonstration depends on the uniqueness of the effect and the kind of nature necessary to produce it. Because we cannot know God directly through our senses, and because nothing is objectively prior to God, it is impossible to demonstrate his existence using a demonstration propter quid. Any demonstration therefore, must proceed to God by identifying him through his proper effects. The key, then, for forming any demonstration of the existence of God is to identify a feature of the known world which cannot be explained without recourse to some being which is outside the order of mobile, contingent beings. Genuine demonstrations of this kind cannot take the form of "God of the gaps" arguments, because the latter simply point to the lack of an explanation, where any number of alternative and reasonable explanations are theoretically conceivable. A "God of the gaps" argument says "no one has explained the mechanism behind this material phenomenon, therefore God did it". A genuine demonstration of God's existence says "the nature of this observed feature of the universe cannot be explained by anything other than a radically transcendent cause." All of St. Thomas's arguments have this latter form.
IV. The First Way
The first way is based on the nature of motion (i.e. transformation or change, not strictly locomotion). Motion is the reduction of potency to act. Everything which undergoes motion is in potency to that which it is becoming, and therefore cannot actually be what it is becoming in the way it is becoming it, since potency and act are mutually exclusive by definition. But potency is not self-actualizing. In order to become, something must be caused to become by something which already is. Therefore whatever undergoes motion must be moved by something other than itself. But, to consider the chain of movers which are presently actualizing any particular thing in a given moment, it is impossible that this essentially subordinated simultaneous chain of movers should be infinite and without a first cause, since then every mover would be in potency to that which it is becoming, and the entire change would be an instance of self-actualizing potency, or non-existence causing itself to become real. Therefore in any change, there must be some first mover, which is not in any way moved, and forms the basis of motion. "God" is the word used for such an unmoved first mover. So, when we speak of "God" we speak of this sort of thing, which evidently exists.
V. The Second Way
The second way is based on the order of efficient causes. Evidently some things cause other things in such a way that their ability to act as causes is dependent on some higher thing. For example, one giraffe begets another giraffe, but this ability of giraffes to reproduce is essentially dependent on the activity of whatever caused giraffes to exist as a species. Every successive generation of giraffes is merely a mediator or communicator of the form of giraffe to some other thing. Now, nothing in a particular species or genus can be the cause of absolutely everything of that kind, because obviously then it would have to be cause of itself. Therefore whatever kinds of things are caused to exist must refer back to some other kind of thing which caused them to exist. And when we consider this chain, again we realize that it cannot go on to infinity, because in this case all of the accidental "mediator" causes which are transmitting their forms or kinds to each other, cease to function as mediators, there being no ultimate origin or reason for the particular nature or existence which they pass on. So in any series of efficient causes there must be something which is first, which is uncaused, ungenerated, and which acts as the cause of other kinds of things.
VI. The Third Way
The third way is, like the second way, focused on the fact that things are generated, but deals mainly with the question of permanence and the possibility of nonexistence. Thomas asks whether it is possible that everything is generated and corruptible, and nothing has the absolute permanence which he calls "necessity". If everything were this way, then the fundamental matter and primary stuff of which the universe is composed would have not existed at some point in the past. Because nothing comes from nothing, if absolutely nothing had existed in the past, nothing would exist presently, which is absurd. And had there been some prior cause in the past which created the whole mess of stuff of which the universe is composed out of nothing, such a thing would necessarily exist of its own nature (being able to cause existence simply and absolutely), and therefore could not be contingent. So there must be something which is necessary and always exists, either conditionally (based on some other thing's necessity) or of its own nature simply. But if there is a conditionally necessary being which depends on some other conditionally necessary being, etc., we can be sure that there is not an infinite chain of dependent necessary beings, because, taken as a simultaneous composite, the chain would not suffice to explain its own necessity. Therefore there must be some necessary being, which is absolutely permanent and the existence of which is due to its own nature and nothing else.