Critical to Saint Thomas Aquinas' doctrine as expressed in the Summa Theologica are the notions of God's timelessness and omnipresence. For Aquinas, both of these are inextricably bound up with His existence; but as both are slippery ideas to grasp hold of, it is important to understand precisely what he thought timelessness (or eternity) and omnipresence to be. Eternity, for instance, is too easily confused with time, thought of as a series of moments that simply hasn't got a beginning or an end for one reason or another, just as omnipresence can be misconstrued as simply the property of physically being in all places at once. To reduce Aquinas' doctrine to such simple terms does him a grave disservice indeed, and is a dangerous endeavour, to the point of risking complete misinterpretation. In his essay, "Aquinas on God's Omnipresence and Timelessness", commentator Richard R. La Croix falls prey to just that -- through over-simplifying omnipresence and eternity, and thence proceeding to the claim that one by necessity precludes the other, he invents a logical impossibility where there is no grounds for one to exist.

In considering Aquinas (and also Augustine before him), it is important to realise that time and eternity are not the same thing, nor are they even particularly similar. "Time" as we can understand it is defined as the progression of moments. We can understand this definition more effectively by thinking in terms of movement (or of improvement, for temporal things in nature are perfectible): for "since succession occurs in every movement, and one part comes after another," we can perceive time in "nothing else but the measure of before and after in movement" (Summa, I.10.1). Eternity does not consist of such a series of moments -- instead it is complete in and of itself. Aquinas exhorts us to consider "the uniformity of what is outside of movement" to understand what he means; time cannot be measured in terms of the movement of an object that is immutable, and eternity is similarly unchanging (Summa, I.10.1). A necessary consequence following from this immutability is that eternity cannot have a beginning or an end -- its fundamental unity, its lack of moments, precludes them (Summa, I.10.1).

Time is defined before eternity is merely so that we have something to compare it with, as a point of reference, since "[s]imple things are usually described by way of negation" -- eternity is that which is lacking in time, just as Euclid describes a point as being that which lacks extension in any direction (Summa, I.10.1) Similarly, Aquinas' intention is not to draw our attention to the ways in which time and eternity might be the same, but rather to the fact that they are (and must be) fundamentally different. Then, where time only has one definition (that is, the straightforward procession of moments, one after another), there are two ways in which we can conceive of eternity:

first, because what is eternal is interminable -- that is, has no beginning nor end (that is, no term either way); secondly, because eternity has no succession, being simultaneously whole. (Summa, I.10.1)

The second doctrine under consideration is that of the omnipresence of God. For Aquinas, the being in place of something (from which he derives omnipresence -- being in all places at once) also has two different possible interpretations. Firstly, it can be thought of in the sense that "one thing is said to be in another" -- that is, anything that takes place in something else is said to be "within" it, even a given thing's "accidents" or its antecedent causes, which are not within it like corporeal objects (Summa, I.8.2). The deliberate placing of something within something else is the second interpretation; simply and intuitively enough, Aquinas says that "things placed are in a place", and though this reading is just as valid as the first, it in itself doesn't encompass the entire meaning (Summa, I.8.2). In the case of God we can extend both definitions to everything and have them prove true -- this is omnipresence, being everywhere at once; and by this doctrine, God is as transcendent of place as He is of time.

To demonstrate God's omnipresence, Aquinas looks back to his "five ways" of proving that God exists in the first place (which, for the purposes of this discussion, is a presupposition -- but not a particularly important one, since though it was established earlier on in the Summa, whether one has faith or not bears no relevance to an examination of possible logical fallacies within the argument itself). In accordance with the first interpretation of presence, Aquinas cites God's presence within things as that which provides them with "being, power and operation [...] existence and locative power"; or as their efficient cause, in Aristotelian terms (Summa, I.8.2). Secondly, God fills every place, just as it is written in Scripture ("I fill heaven and earth" from Jeremiah 23:24, as cited in I.8.2) -- but since God is incorporeal, His presence does not preclude the presence of other things in the same space. The two halves together apply to all things, such that "by the very fact that He gives being to the things that fill every place, He Himself fills every place" (Summa, I.8.2).

For Aquinas, God's timelessness and His omnipresence are not mutually exclusive; both are equally important in constituting His nature. But for La Croix, there is difficulty in reconciling them, and he objects as follows:

If God is indeed omnipresent then it would appear that he must have been in the United Nations building yesterday as well as the day before yesterday. And if God was in the United Nations building both yesterday and the day before, then it would appear that he is in time and that temporal predications do actually apply to him. So, it would appear that God is not a timeless being if he is omnipresent and that two doctrines crucial to the theology of Aquinas are logically incompatible. (La Croix, 391)

On the contrary, if God is truly beyond the limits of time and place, then it ought not to matter where or when He is; according to doctrine, He transcends such petty boundaries. The problem with La Croix's argument -- and the reason that it doesn't stand up to more-than-superficial scrutiny -- is that it relies heavily on only one-half of Aquinas' definition of eternity, and fails to give the other half the priority that it deserves; similarly, it treats only one-half of the argument for omnipresence.

According to La Croix's example, and following the thread of his logic, the fact that God can be in the United Nations building (or anywhere else, for that matter) for several days running means that He is subject to the constraints of time, and that He is subject to the same physical changes that occur in the physical world as we can know it. For instance, God could not have been in the United Nations building before it was built, nor could He continue to be inside it if at some future date it was torn down -- and thus this sort of literal, physical omnipresence means that He, too, can experience beginnings and endings, and is therefore by definition not eternal (La Croix, 399).

By one half of Aquinas' definition of omnipresence, or "being in everything", this is partly true. It is true that God is in the United Nations building, filling it as He fills Heaven and earth according to Jeremiah, for as long as it exists; and it is true that the building -- or any other object -- is subject to temporal laws and therefore has a beginning and an end. But as part of the same definition, for Aquinas, God's presence in anything can be seen in its efficient cause. The very existence of the United Nations building, its "being, power and operation", are proof enough of God's presence in it. (This is similar in principle to the argument from design, popular in early modern natural theology; for instance, William Paley used the analogy of a pocketwatch found on the ground, perfectly suited for its intended use, to demonstrate God's hand in the adaptation of natural things to their environments and purposes.) He need not walk into the building in order to be there, nor would He be gone from it were He to walk outside, since by its very existence it is permeated with His spirit. Aquinas uses the analogy of a king, who is present throughout his entire kingdom "by his power, although he is not everywhere present" (Summa, I.8.3). So too is God in everything that He creates, regardless of whether we can see Him at any given moment -- seeing His creations and thereby intuiting His presence in them is enough proof in itself, just as William Paley could know that the pocketwatch had the mind of a craftsman at work behind it and in it, and similarly so too did other things perfectly suited to their functions, like the human eye.

That the United Nations building has a definite beginning and end is not proof that God is subject to the same conditions. This reaches back to Aquinas' dual definition of eternity: that which has no beginning or end, and that which consists of all moments held together within a single moment, immeasurable and whole in itself. Insofar as all moments are simultaneous to God, who is outside the "literal temporal predications" (La Croix, 391) to which all other creatures and things are subject, whether it is yesterday, today, or tomorrow is inconsequential -- and one cannot eliminate God's presence simply by destroying something which He inhabits, just as one cannot oust a king by razing a building in some far corner of his kingdom.

Eternity as all moments contained in one was a notion that Aquinas borrowed from Saint Augustine; and in following it through to this particular origin, it becomes clearer. For Augustine, as for Aquinas, time and eternity are markedly different. Where Augustine gives time a sort of mathematical signification, described as measured intervals whose duration cannot be determined until they have ceased to be, eternity lies outside both the past (that which has ceased) and the future (that which has not yet occurred) -- it is a sort of permanent present, which "transcends all distension between past and future, all the fleeting transience of time" (Confessions, XII, xxii). Temporal limits are for things that can be changed; and just as for Aquinas, time is measured by things which are mutable and thus subject to improvement or perfectibility. Augustine's eternity is the realm of that which is already perfect -- that is, God.

[For Augustine,] time and eternity are absolutely incompatible. Their differences are absolute. Time implies change, movement, transition, succession, imperfection, and improvement. Eternity is all that time is not. It is the immutable, quiescent present, the simultaneous unison of that which unfolds in time. (Hausheer, 509)

Thus is eternity tota simul, without future or past; and the relentless and inexorable procession of yesterdays and tomorrows belongs to the United Nations building, not to God. It is because the former is imperfect, perfectible, that it is subject to such temporal constraints as a beginning and an end -- and since God is independent of time because of His perfection, His eternal nature transcends them:

Being a totally realized perfection, God is wholly independent of time. He is an immobile eternity. His life is not an ascent to still higher perfections. Neither is it a descent to a lower world. It is a process without external aims. Its process is self-concentrated, circulating on its own horizontal level. (Hausheer, 509-510)

The fact that things in which God exists begin and end within time does not mean that God Himself is subject to those same limitations. La Croix's argument falls down here because he mistakes time for eternity; this confusion is implicit to the very foundations of his argument. Where he says that God's having been in the United Nations building both today and yesterday precludes his timelessness, he forgets that today and yesterday are already the same moment in God's experience. Furthermore, he falls into the trap of limiting his interpretation of omnipresence to simply physical presence in all places at the same time, and this is an over-simplification of Aquinas' doctrine. God can be physically present within a place, certainly -- as per the second half of Aquinas' "being in place" definition -- but he is also all around it at the same time, and in every other place as well; getting rid of God is not as simple as getting rid of one place that He inhabits.

God is in all things by His power, inasmuch as all things are subject to His power; He is by His presence in all things, as all things are bare and open to His eyes; He is in all things by His essence, inasmuch as He is present to all as the cause of their being. (Summa, I.8.3)

Thus, Aquinas' doctrine of omnipresence is not incompatible with eternity or timelessness. In fact each is a natural continuation of the other, as any being that is not subject to mutability owing to concrete physical presence is outside of time.

Hausheer, Herman. "St. Augustine's Conception of Time". The Philosophical Review, Vol. 46, No. 5. (Sep., 1937), pp. 503-512.
La Croix, Richard R. "Aquinas on God's Omnipresence and Timelessness". Philosophy and Phenomenological Research, Vol. 42, No. 3. (Mar., 1982), pp. 391-399.
Thomas Aquinas. Summa Theologica. Benziger Bros. ed., 1947, trans. the Fathers of the English Dominican Province. Accessed 18 April 2005. http://www.ccel.org/a/aquinas/summa/home.html
Saint Augustine. Confessions, trans. Henry Chadwick. Oxford: Oxford University Press, 1991.