The ratio of eternity consists in the awareness of the uniformity of things that are wholly outside of motion.
ST I, q. X a. 1
We have a word “eternal”. Forget for a moment if there are eternal things or not. What does the word mean, and where did the idea come from?
The awareness of death or of ceasing to be is first. Few things are more knowable than death or ceasing to be, not only because the experience of such things can be so moving (like the death of a loved one) but also because our experience of things ceasing to be is so frequent and constant (our food spoils, anything we build needs constant maintenance to ward of corruption, and growth in wisdom seems to involve a growing awareness that our time is brief). Death is somehow natural to things, for there is an inherent tendency in things to fall toward non-being. Death and corruption is simply what happens when things run their course, and seeing it happen is nothing more than a matter of letting things go their way.
Our idea of the eternal is first grounded on what does not cease to be, and our first experience of this is when we notice the larger context of things that pass away. We are right that the natural tendency in any one thing might be to corruption, but that tendency to corruption is always found within the context of a larger cycle of things, and cycles have no beginning and no end. The fruit might rot, but the tree will give more next season, the tree might die, but the earth will bring forth more after many seasons, etc.
Cycles stand behind all corruptible things and govern their rise and fall, and the most governing of cycles that is manifest to our senses is the cycle of the sun, which in one way or another is making possible all of the cycles of things on earth. In our first understanding of eternal, then, the sun is eternal, or at least the most eternal thing we know.
Since the beginning, however, there has been a sense that there is something more eternal and more governing than the motion of the sun. The search for this more eternal thing falls to science, for the scientist asks about hidden or questioned things and whatever governs the sun would have to be such. The first scientists guessed that the stars governed the sun, and they had some success in detecting very subtle cycles which do so, like the procession of the equinoxes. This success, however, made it easy to believe far more outlandish claims about the power of the stars, and charlatan astrologers sprang up as one in an endless line of pseudo-scientists.
In our own time we are more comfortable looking to a quality of something rather than a thing with the quality, like the sun. Newton guessed that the heaviness of things was what governed all motions, and he spent much of his career trying to figure out the thing that caused this heaviness. Before anyone answered Newton’s question, people started to suspect that something governed heaviness as well, or at least that there was more involved in governing things than heaviness.