A common reading at the funeral Mass is this powerful one from the Book of Ecclesiastes:
I have considered the task that God has appointed for the sons of men to be busied about. He has made everything appropriate to its time, and has put the timeless into their hearts, without man’s ever discovering, from beginning to end, the work which God has done (Eccl 3:10-11).
Somewhere in our hearts is something that the world cannot, and did not, give us. This passage calls it “the timeless.” We also refer to it as eternity or even infinity.
But where did this concept come from? The world is finite. Time here is serial. Things have a beginning, a middle, and an end. We do not experience anything of the timeless. Rather, everything is governed by the steady ticking of the clock. Every verb we use is time-based. Everything is rooted in chronological time. Yet somehow we can grasp the timeless. Yes, we do know it.
The experience of “forever” does not exist in this world, but it is still there our minds and hearts. There is no way to travel through time here in this world. Yet instinctively we know that somehow we can. Science fiction and fantasy novels often feature going back to the past or into the future. The world could not teach us this because we are locked in the present and have never actually travelled in time. Somehow, though, we know that we can do it.
The word “eternity” comes from the Greek word “aeon,” which means “the fullness of time.” It is not just a long time; it is all time: past, present, and future all at once. If you look at a clock you’ll notice that at the center dot, 10 AM, 2 PM, and 6 PM are all the same, even though 10 AM is in the past, 2 PM is now, and 6 PM is in the future. This is aeon, eternity, the fullness of time; this is timelessness.
From whence did we get such a concept? The world cannot give it, for the world does not have it. Nothing can give what it does not have. The world is finite, limited, and time-bound—not timeless. It cannot, therefore, give what is infinite or “timeless.” If we have such a notion it must come from outside time or in the fullness of time, something that contains the full sweep of time all at once. But what is outside time?
Our five senses are locked in space and time, but the senses feed these temporal/spatial elements to the intellect, and the intellect grasps truth that transcends space and time. Some ancients, such as Aristotle, argued persuasively that the spiritual capacity to grasp the truth allows man to view the temporal/spatial world as pointing to something beyond itself: e.g., Aristotle deduced that there must be an unmoved mover, outside of time; in his metaphysics, Aristotle sees God as His own self-thinking thought. Such deductions arise within man’s spirit when it grasps the significance of creation. The Church fathers observed that all creatures in the physical world exist in time, but because creatures cannot be their own cause, their mere existence in time points to something outside of time, to God, the creator. The creature properly viewed, always points to the creator. That which is caused, always points to that which is un-caused; all composites point to the principles, and the ultimate principles of composites are always outside of time.
Time out.
Aliens don’t exist, yet stories about aliens doesn’t mean they exist. In the same way, conceiving of eternity does not mean eternity exists. Eternity exists because it is Dogma that God is Eternal. Not everyone believes Dogma, so we cannot assume everyone believes in eternity. Indeed, such would be a false equivalence fallacy.
I think it is you who have broken your own rule. The argument is that that whatever we can imagine exists. Imagining aliens is merely a control of things that clearly do or can reasonably exist in a universe such as ours. They have human-like qualities, bodies, and powers which, while preternatural are not per se supernatural. So aliens, to use your example are a construct of reasonably existent beings rooted in what is already about us. But eternity is not something found anywhere in the created order. It is an idea or matter wholly construed by the human soul.