In order to understand why and in what ways St Thomas says that our knowledge of God is analogical, there are some prior considerations that must be understood first. The Summa Theologiae is nothing if not spectacularly orderly, such that later articles depend upon what is developed in earlier ones. We shall see this as we proceed, but it seems like a good idea to mention this up front. It is easy to question or misunderstand some things that he writes if the reader ignores or is unaware of the foundations upon which it stands. I am quite certain that for the present subject it would be helpful if I could make reference to his Commentary on the Metaphysics (available online here). Unfortunately I haven’t read it yet. Nevertheless, the Summa is sufficiently orderly and self-contained that we should be safe from misrepresenting him by omission.
Aquinas makes his major case for the fact that our knowledge of God is analogical in I q.13, and particularly in article 5 (“Whether what is said of God and of creatures is univocally predicated of them?”).
Whatever is predicated of various things under the same name but not in the same sense, is predicated equivocally. But no name belongs to God in the same sense that it belongs to creatures; for instance, wisdom in creatures is a quality, but not in God. Now a different genus changes an essence, since the genus is part of the definition; and the same applies to other things. Therefore whatever is said of God and of creatures is predicated equivocally.
Things cannot be said of us and of God in the same exact literal sense, and the reason for this has at least partly to do with God’s perfections in contrast to ours. God is said to be perfect by virtue of the fact that he is the First Mover or “first active principle” (see I q.4 a.1). Why does he argue this way? When we think of things as perfect rather than imperfect, we typically do so in terms of defects which the latter possess that the former do not: so that perfect whiteness lacks any tinge of color at all that takes away from its whiteness, or perfect behavior might be said to lack any misbehavior that makes a child naughty. This is not how Aquinas approaches the question. Instead, he speaks of it in terms of act and potency.An illustration that makes better sense of what he means might be to think of a lamp that is controlled by a dimmer switch. When the dimmer switch is turned way down, the light produced by the lamp is imperfect because it lacks its ordinary full brightness: it is actually dim, but it possesses the potentiality to be brighter than it actually is. But when the dimmer switch is turned all the way up, the light produced by the lamp can be said to be perfect because it cannot be any brighter than it is: it has no potency for greater brightness; in this respect, it is fully in act. So that which is completely actual, or in act, lacks any potency to be more in act.
In this way we see that perfection necessarily lacks any potentiality to be more perfect, just by definition. Now there is no potentiality in God at all. He is completely actual, as Aquinas argued earlier in I q.3 (the whole article, really). So when he says that God is perfect, this is what he has in mind: God is fully in act; there is no potency in Him at all.
A second connected reason for the fact that we cannot speak strictly univocally of God and creatures has to do with the fact that God is absolutely simple: there is no composition of any sort in Him at all. This is something that Aquinas establishes in I q.3, and it finds expression in I q.13 a.5 thus:
wisdom in creatures is a quality, but not in God.
This is a point that he addresses specifically in I q.6 a.3, where he concludes:
Hence it is manifest that God alone has every kind of perfection by His own essence; therefore He Himself alone is good essentially.
What this means is that God is His goodness; He is His wisdom; He is His justice; etc. But this is not how creatures are at all. As Thomas says, these are qualities that we may possess. So when we use these terms of us and Him, properly speaking they cannot mean the same thing.
Along the same lines, it cannot be the case that our knowledge of God is univocal because He is not in the same genus as we are. In fact, He is in no genus at all, as St Thomas explains in I q.3 a.5. Genus is necessarily part of the definition of that which it contains, just by the meaning of the terms. But God is in not part of any genus. Consequently nothing that defines the genus of which we are a part can be said to define God as well, or He would be a member of our genus as well. And so Aquinas concludes that “whatever is said of God and of creatures is predicated equivocally.”
But that is not the end of his argument. He continues:
I answer that, Univocal predication is impossible between God and creatures. The reason of this is that every effect which is not an adequate result of the power of the efficient cause, receives the similitude of the agent not in its full degree, but in a measure that falls short, so that what is divided and multiplied in the effects resides in the agent simply, and in the same manner; as for example the sun by exercise of its one power produces manifold and various forms in all inferior things. In the same way, as said in the preceding article, all perfections existing in creatures divided and multiplied, pre-exist in God unitedly. Thus when any term expressing perfection is applied to a creature, it signifies that perfection distinct in idea from other perfections; as, for instance, by the term “wise” applied to man, we signify some perfection distinct from a man’s essence, and distinct from his power and existence, and from all similar things; whereas when we apply to it God, we do not mean to signify anything distinct from His essence, or power, or existence. Thus also this term “wise” applied to man in some degree circumscribes and comprehends the thing signified; whereas this is not the case when it is applied to God; but it leaves the thing signified as incomprehended, and as exceeding the signification of the name. Hence it is evident that this term “wise” is not applied in the same way to God and to man. The same rule applies to other terms. Hence no name is predicated univocally of God and of creatures.
The thrust of this argument depends upon two things we have already adumbrated above: the simplicity of God, and the fact that His essence is His existence. These are contrasted with how things are in creatures. Perfections in us are “divided and multiplied” as is obvious from observation, but when we say them of God “we do not mean to signify anything distinct from His essence” precisely because there is no composition in Him at all. Furthermore, we know what it means to say that a creature is (for example) “wise,” because the word refers first in our understanding to our fellow creatures, so that (as he says) it “in some degree circumscribes and comprehends the thing signified. In contrast, though, this is not so when we apply the term to God. This is because in reference to Him it would require saying something about His essence (which we certainly do not comprehend), which is not at all the case with creatures, and because He is infinite (on both of which counts He necessarily “exceeds the signification of the name.” This is true of other words as well, and so the same conclusion follows.
[Aside: at the time of this writing there is a transcriptional error on the page for this article at New Advent. There is a second paragraph beginning “I answer that…” that actually belongs to this article.]
So names are not used of God and creatures univocally, but on the other hand they are not used strictly equivocally either:
Neither, on the other hand, are names applied to God and creatures in a purely equivocal sense, as some have said. Because if that were so, it follows that from creatures nothing could be known or demonstrated about God at all; for the reasoning would always be exposed to the fallacy of equivocation. Such a view is against the philosophers, who proved many things about God, and also against what the Apostle says: “The invisible things of God are clearly seen being understood by the things that are made” (Romans 1:20). Therefore it must be said that these names are said of God and creatures in an analogous sense, i.e. according to proportion.
We already know (as he says) that philosophers have demonstrated various things about God. So it cannot be the case that we know nothing about God, and so it cannot be the case that names are applied strictly equivocally. Furthermore, to say this would be contrary to Scripture (as he points out). Consequently, since we cannot speak univocally about God (for the reasons he has provided), and because it is not the case that we speak strictly equivocally about Him either, there must be another alternative. And he says that this alternative is that they said analogously or “according to proportion.”
Now names are thus used in two ways: either according as many things are proportionate to one, thus for example “healthy” predicated of medicine and urine in relation and in proportion to health of a body, of which the former is the sign and the latter the cause: or according as one thing is proportionate to another, thus “healthy” is said of medicine and animal, since medicine is the cause of health in the animal body. And in this way some things are said of God and creatures analogically, and not in a purely equivocal nor in a purely univocal sense. For we can name God only from creatures (1). Thus whatever is said of God and creatures, is said according to the relation of a creature to God as its principle and cause, wherein all perfections of things pre-exist excellently. Now this mode of community of idea is a mean between pure equivocation and simple univocation. For in analogies the idea is not, as it is in univocals, one and the same, yet it is not totally diverse as in equivocals; but a term which is thus used in a multiple sense signifies various proportions to some one thing; thus “healthy” applied to urine signifies the sign of animal health, and applied to medicine signifies the cause of the same health.
(Fr. Garrigou-Lagrange has helpfully addressed this subject as well, and his analysis may be found here. I am not going to go into a complete discussion of it now because this post is too long already. But it is well worth reading.)
When therefore a man thinks that God is good or eternal or almighty, he not only means something different from what God means by good or eternal or almighty, but, worse (if anything can be worse) he means something different by saying that God is.
When he says this, he is not saying anything that is relevant to what Aquinas says about how we can speak truthfully about God. Because (as Aquinas says) we do not speak merely equivocally about Him (in which case the criticism just given would be true), but rather “in an analogous sense, i.e. according to proportion.”
And when he says in the same place:
Since as temporal creatures we cannot know the eternal essence of God, we cannot know what God means when he affirms his own existence. Between God’s meaning of existence and man’s meaning there is not a single point of coincidence.
When he says this, he is once again saying nothing about Aquinas’ views, because Aquinas affirms that we can indeed see God’s essence, although of course we cannot comprehend itprecisely because we are finite and He is not.
And when our gentleman says (again in the same place):
Weaver says, “The word analogical, so far as I can find, is not used by Van Til to apply to terms, but to the process of reasoning it is based squarely upon the doctrine of the creation of man in God’s image.”[Festschrift, 304-305] For some strange reason Van Til actually thinks this is the only way that man can have knowledge of God. Clark says, “if Van Til means nothing else by the term analogical than the dependence of all knowledge on knowledge of God, what is the point of Weaver’s criticisms?”[Ibid,463 ] Van Til’s appeal to analogy merely disguises the skepticism, it does not remove it. Van Til’s system is supposed to preserve the idea that man cannot have knowledge apart from God. We heartily agree. The word “apart” though is ambiguous. Does “apart” mean apart from effectual calling, apart from common grace, apart from a priori structures? Even Clarkians believe that man must have a priori structures to have knowledge of God, even a lost man must have them and does have them. Clark’s take on Van Til was that he was inconsistent and at times said things that were on the right path but then would go back on them. Who knows? Van Til’s writings are so cryptic, it’s difficult to determine what he believed.
Well, when he says this, he is obviously talking about Van Til and not about Aquinas. So how this is supposed to constitute a criticism of what Aquinas meant is a mystery to me, since Van Til was no Thomist and Aquinas was obviously not Vantillian.