Imagine yourself walking along a very long road. This road stretches far into the distance, beyond the horizon. You walk and walk and walk, your entire life, never finding the end. In your final days, you ponder whether the road is in fact infinitely long. What you know for sure is that it is too long for you to traverse in your lifetime. But does that mean the road is infinite, or simply that you didn’t live long enough? If you could continue walking another 500 years, would you reach the end?

Infinite means “not finite”. We can’t observe infinity because we are—or more precisely, our perception is—finite. Imagine yourself walking along the road again, but this time you are immortal; you will live forever. Now can you figure out whether the road is infinite? Not quite. When would you know for sure? After the first 1,000 years? After the first million years? You would never know that the road is infinite, even if you lived forever and never stopped following it. (One might be tempted to say, “I would know when I reached the end of my infinite life and the road was still going”; but upon saying the words it should become clear why this isn’t possible.)

The only way we could know if something is infinite would be by having the ability to “see infinity”; but we don’t have this ability. There are bounds to our perception, in every sense.

We can only see so far. Even with the most powerful telescopes, we cannot see beyond the stars that are close enough for their light to have reached us since they first formed.

We can only observe phenomena that occur within timescales we can perceive. For most of us, this means a human lifetime—hardly the blink of an eye compared to the age of the universe (at least as far as we know). But even a lifetime is far longer than anyone can practically observe anything. This is limited further by our attention spans.

What we can touch is even closer still—objects roughly our own scale, made of the kinds of matter our nerves are tuned to register. We don’t feel atoms, even though we are made of them; we don’t feel the curvature of the planet under our feet, even though we are pressed against it constantly. Beyond a narrow band on either side of our own size, the world is no longer something we can place our hands on.

These bounds don’t just keep us from settling infinity. They keep us from settling almost anything that lies beyond what we can directly experience. The Big Bang, the fate of the universe billions of years from now, the galaxies whose light is so old that by the time we see it, it may be gone—each of these is the product of extrapolation, an extension of what we know about local physics projected out by orders of magnitude beyond anything we have actually checked. We have very good reasons to believe many of these extrapolations are correct. We do not, and cannot, know that they are.

This is probably easy to misread as a complaint against science. It isn’t. Reasoning carefully past what we can directly observe is one of the most extraordinary things our species does, and it has served us well. The lens we use to see further is one of the most powerful tools we have. But it is still a lens. And a claim about the universe ten billion years from now is not in the same epistemic neighborhood as a claim about what will happen if I drop this cup.

Back to the road. We walk along it, with no view of either end. We can be quite confident about the patch of road directly under our feet—where it is solid, where the cracks are, what it is made of. We can be reasonably confident about what we can see ahead, dimming as the distance increases. About what is far beyond the horizon, in either direction, we can really only say what seems consistent with the part we can see. Sometimes that is enough. Often it has to be. But it is worth remembering, when we make our most confident pronouncements about what is out there, how small the lit portion of the road actually is.