Defined by Boetius as possession, without succession and perfect, of interminable life
Eternity (oeternum, originally oeviternum, aionion, aeon-long) is defined by Boetius (De Consol. Phil., V, vi) as "possession, without succession and perfect, of interminable life" (interminabilis vitae tota simul et perfecta possessio). The definition, which was adopted by the Schoolmen, at least as applying to eternity properly so called, that of God, implies four things: that eternity is (I) a life, (2) without beginning or end, (3) or succession, and (4) of the most perfect kind. God not only is or exists, but lives. The notion of life, like all notions however abstract or spiritual, is, when applied to God, but analogous. He not only does not live precisely as anything else with which we are acquainted lives; He does not even exist as anything else exists. Our notions of life and existence are derived from creatures, in which life implies change, and existence is something added to essence, thus involving composition. In God there can be no composition or change or imperfection of any kind, but all is pure act or being. The agnostic, however, is not thereby justified in saying that we can know nothing and should predicate nothing of God. It is true that, however we conceive Him or in whatever terms we speak of Him, our ideas and terminology are utterly beneath and unworthy of Him. Yet, even while arguing in this way, the agnostic thinks and speaks of Him as really as we do; nor can he or we do otherwise, compelled as we are to trace things back to their first cause. Yielding to this necessity, we can but think and speak of Him in the highest and most spiritual terms known to us; not merely as existing, for instance, but as living; correcting at once, as far as we can, the form of our thought and predication, by adding that the Divine life is perfect, free from the least trace of defect. That is how and why we represent the Divine existence as a life. It is a life, moreover, not only without beginning or end but also without succession—tota simul, that is without past or future; a never-changing instant or "now". It is not so difficult to form some faint notion of a duration which never began and shall never end. We hope that our own life shall be endless; and materialists have accustomed us to the notion of a series stretching backward without limit in time, to the notion of a material universe that never came into being but was always there. The Divine existence is that and much more; excluding all succession, past and future time—indeed all time, which is succession—and to be conceived as an ever-enduring and unchanging "now".
In forming this notion of eternity it is well to think of the Divine immensity in its relation to space and extended things. One may conceive first a broken straight line—a line of separate dots; then a continuous line within two limits, beginning and end. The line can be, but is not, divided into parts, shorter lines or dots, and the whole is finite both ways. It is like and yet unlike a finite spirit; like, since it has no actual parts or divisions and is limited; yet unlike since it may be divided, whereas a spirit cannot be divided. Spirit exists whole and entire wherever it exists at all; and though it may fill the space occupied by a human body, let us say, it is whole and entire in every possible part of it; not quite unlike the continuous line. If we further think of the end or limits of the line as removed, of the earth's axis, for instance, as extending indefinitely into space, the line is not only continuous or unbroken but infinite, without end or beginning, yet still divisible; like, but so unlike, the immensity of God. For God is a spirit, and as the human soul fills the space occupied by the body to which it is united, yet is whole and entire in every possible part of that space, so God fills all space whatsoever, extending without limit in all directions, and yet is whole and entire everywhere, in the smallest conceivable point, in the very loose or improper sense in which we may think or speak of God as being "whole". Even the spatial relations of the soul to the body are coarse as compared to those which God's existence bears to that of creatures and the spaces in which they exist or may exist. For however free from extension created spirits may be, they are not incapable of real internal change, real motion of some kind within themselves; whereas God, filling all space, is incapable of the least change or motion, but is so truly the same throughout that He is best conceived as an infinitely extended point, the same here, there, everywhere.
If, now, we apply to the time-line what we have been attempting in that of space, the infinite, unchangeable point which was immensity becomes eternity; not a real succession of separate acts or changes (which is known as "time"); nor even the continuous duration of a being which is changeless in its substance, however it may vary in its actions (which is what St. Thomas understands by an oevum); but an endless line of existence and action which not only is not actually interrupted, but is incapable of interruption or of the least change or movement whatsoever. And as, if one instant should pass away and another succeed, the present becoming past and the future present, there is necessarily a change or movement of instants; so, if we are not to be irreverent in our concept of God, but to represent Him as best we can, we must try to conceive Him as excluding all, even the least, change or succession; and his duration, consequently, as being without even a possible past or future, but a never beginning and never-ending, absolutely unchangeable "now". That is how eternity is presented in Catholic philosophy and theology. The notion is of special interest in helping us to realize, however faintly, the relations of God to created things, especially with regard to His foreknowledge. In Him there is no before or after, and therefore no foreknowledge, objectively; the distinction which we are wont to draw between His knowledge of intelligence or science or prescience and His knowledge of vision is merely our way of representing things, natural enough to us, but not by any means objective or real in Him. There is no real objective difference between His intelligence and His vision, nor between either of these and the Divine substance, in which there is no possibility of difference or change. That infinitely perfect substantial intelligence, immense as it is eternal, and withal existing entire and immutable as an indivisible point in space and as an indivisible instant in time, is coextensive, in the sense of being intimately present, with the space-extension and the time-succession of all creatures; not beside them, nor parallel with them, nor before or after them; but present in and with them, sustaining them, cooperating with them, and therefore seeing—not foreseeing—what they may do at any particular point of the space-extension, or at any instant of the time-extension, in which they may exist or operate. God may be considered as an immovable point in the center of a world which, whether as a more or less closely connected group of granulated individuals, or as an absolutely continuous ether mass, turns round Him as a sphere may be supposed to turn in all directions round its center (St. Thomas, Cont. Gent., I, c. lxvi). The imagery, however, must be corrected by noting that while in the time-line God's duration is an ever-enduring point or "now", his immensity in the space-line is not at all like the center of a circle or sphere; but is a point, rather, which is coextensive with, in the sense of being intimately present to, every other point, actual or possible, in the continuous or dis continuous mass that is supposed to move around Him.
Bearing this correcting notion well in mind, we may conceive Him as this immovable point in the center of an ever-moving, though here and there continuous, circle or sphere. The space and time relations are constantly changing between Him and the moving things around Him, not through any change in Him, but only by reason of the constant change in them. In them there is before and after, but not in Him, Who is equally present to them all, no matter how or when they may have come into being, or how they may succeed one another in time or in space. Some of them are free acts; and almost from the time the human mind began to speculate on these questions, and wherever still there are any even rudimentary speculations, the question has arisen and does arise as to how an act can be free not to happen if, as we suppose, God's absolutely infallible foresight saw from all eternity that it was to be. To this Catholic philosophy supplies the only answer which can be given; that it is not true to say that God either saw or foresaw anything, or that He will see it, but only that He sees it. And as my seeing you act does not interfere with your freedom of action, but I see you acting freely or necessarily, as the case may be, so God sees all finite things, quiescent or active, acting of necessity or freely, according to what may be objectively real, without in the least interfering thereby with the mode or quality of their existence or of their action. Here again, however, care must be taken not to conceive the Divine knowledge as being determined by what the finite may be or do; somewhat as we see things because the knowledge is borne in upon us from what we see. It is not from the finite that God gets His knowledge, but from His own Divine essence, in which all things are represented or mirrored as they are, existing or merely possible, necessary or free. On this aspect of the question see God. When, therefore, one is asked or tempted to ask, what God did or where He was before time and place began, with the creation of the world, the answer must be a denial of the legitimacy of the supposition that He was "before". It is only in relation to the finite and mutable that there can be a before and after. And when we say, that, as faith teaches, the world was created in time and was not from eternity, our meaning should not be that the existence of the Creator stretched back infinitely before He brought the world into being; but rather that while His existence remains an unchangeable present, without possibility of before or after, of change or succession, as regards itself, the succession outside the Divine existence, to each instant of which it corresponds as the center does to any point in the circumference, had a beginning, and might have extended indefinitely further backward, without, however, escaping the omnipresence of the eternal "now" (See Billot, De Deo Uno et Trino, q. 10, p. 122).
So far for the strict or proper notion of eternity, as applying solely to the Divine existence. There is a wide or improper sense in which we are wont to represent as eternal what is merely endless succession in time, and this even though the time in question should have had a beginning, as when we speak of the reward of the good and the punishment of the wicked as eternal, meaning by eternity only time or succession without end or limit in the future. In the Apocalypse there is a well-known passage in which a great angel is represented as standing with one foot on sea and one on land, and swearing by Him that liveth forever that time shall be no more. Whatever the meaning of the oath may be, it has found an echo in our religious terminology, and we are wont to think and say that with death, and especially with the Last Judgment, time shall cease. The meaning is not that there will be no more succession of any kind; but that there will be no substantial change or corruption in what survives death, the soul; or in the body that shall have been raised from the dead; or in the heavens and earth as they shall be renewed after Christ's second coming. There is, moreover, an implication or connotation of the doctrine that in the future life of souls, whether in heaven or in hell, succession will be accidental, the act in which their essential happiness or misery will consist being continuous and unbroken vision and love, or blinded wrong vision and hatred, of God. This kind of duration is in our ordinary language spoken of as life or death eternal, by a kind of participation, in a wide or improper sense, in the character of the Divine eternity (Billot, op. cit., 119). Questions of the greatest importance have been raised as to the possibility of an eternal world, in the sense of a world of matter, such as we know, having never had a beginning and therefore not needing a first cause; also as to the possibility of eternal creation, in the sense of a being, with or without succession, having had no beginning of existence and yet having been created by God (see Creation). For other questions as to eternity see Heaven. Hell. "Eternal life" is a term sometimes applied to the state and life of grace, even before death; this being the initial stage or seed, as it were, if the never-ending life of bliss in heaven, which, by a species of metonymy, is regarded as being present in its first stage, that of grace. This, if we are true to ourselves and to God, is sure to pass into the second stage, the life eternal.