In one sense, time is simple. It’s measured by the steady tick-tick-tick of a second hand, like a virtual yardstick, at the same rate, never stopping. That’s the time we depend on when we say we started cooking the roast an hour ago or need to be in Peoria at 10 am on Tuesday.

And who would argue about its nature? Extend the roast span back six thousand years or the trip to Peoria ahead by a million and we still see the same tick-tick-tick yardstick measuring it, just for a longer stretch in each direction.

If we had a permanent, perpetual clock, powered by uranium or something that would be enough for billions of years, and we put it in a lead-lined box with massive steel walls and placed that deep in a cave, we could feel confident in the knowledge the time would always pass, physically, at the same rate. Imagine someone put that clock in the cave thousands of years ago. It is never altered in its function; it will never be acted upon by an outside force. That’s how we usually think of time.

We might fall asleep in the afternoon and wake up in the middle of the night. We might forget to look at the calendar one morning and think it’s Thursday when it’s actually Wednesday. We might glance at a book on the shelf in our office that we bought at a conference once, intending to try a new way of doing things that we completely forgot about, and say to ourselves “Was that really ten years ago?” Time can be fudged. It can be forgotten. But despite any temporary warps, our view is rarely shaken that all continues to proceed at the steady tick-tick-tick pace of the clock in the cave.

Einstein obviously said if we took a super fast train to Alpha Centauri we’d come back 50 years younger than our grandchildren, but I don’t know about the relative time of the physicists. I’m just talking about the time we live in here.

But there is a second sense of the “time” we live in where it’s more complicated. When a book reminds us of a past that feels like it could’ve been yesterday, or we realize a plan will never come to pass because of the late hour, our understanding of life changes because the span we imagined for it becomes compressed. Looking in the mirror can have the same effect. Having someone die might be the best example.

Even those of us who are busy with the schedule of a working life where we’re always conscious of the clock also live within the other time frame. It’s not like the external yardstick running alongside us, but more like how we think of ourselves. We might be rushing to and fro for meetings but we also think there’s a big picture in which it pays off, or if we are more the depressive type the big picture may simply mean when it’s over.

But regardless of our level of hopefulness, most of us have both an awareness of time immediately progressing, and a concept of a time frame that we live within, and the latter one is part of what we think we are and what we think life is all about.

If that latter point is not evident, please follow along with me for a second.

Without getting into theological details, it’s safe to say that the “meaning of life,” technically, in the most generic sense, for most of us is “a period of time in which we can do stuff.”

What does it mean to be alive? It means I can sense that I am able to act, even if the act were as basic as seeing or thinking or breathing. The opposite state from that would be, presumably, the “meaning of death” (although I suppose that may be worthy of further discussion sometime).

On top of knowing we are alive, at a basic level we also have a sense of the time frame we live within—within which we are aware of our ability to act. Awareness of this period of existence, even when our awareness is hazy or wrong, is part of how we think and probably one of the characteristics that sets us apart as a species.

Our relationship to that “period” changes. As children, we may have little awareness of it or be vastly mistaken about it. Little kids probably look at the time frame of life similarly to how puppies do, as a stream of sensations that stops and starts for no reason. But like old dogs older people learn there are blocks of time for doing certain things.

The trip to the store; the time before graduation; the apprenticeship; the road to retirement; retirement—each of these are like pieces of a mental template that consciously and subconsciously forms what our life is and who we are in our own minds.

Some of us, I think, have unconventional time templates. Hard-charging business people, I imagine, see their life spans as a series of finish lines they cross one after another, breaking the tape, lifting their arms in triumph, shouting at the sky, then going to bed before waking up and doing it all again. Or so I imagine. Artists may see life measured out from one project to the next. Yogis may live in the eternal now.

But even in the most unconventional of lives, I think, there the two types of time: the one we can all measure together, and the one that forms the boundaries of our personal vista. The tick-tick-tick, and the frame of time that is like our arm span or how far away our voice can be heard.

That big picture which includes the impression we make on the world also includes how far ahead we see. Conceiving of “life” means conceiving of its extent with us in it. The variability in that is akin to whether we are about to set across the sea on a piece of driftwood or aboard the largest ocean liner ever built. So the meaning of life can vary based on the frame we envision, which can change drastically because of events and as a result of our own shifting awareness. The cast can go from bright to ominous, for instance, if the far edge suddenly pops up right in front of us.

A shortened span of life is a terrifying thought, but for the pull of imagination that lets us step back and see that border as having two sides, as merely a fence across a landscape that extends outward in all directions, above and below and beyond without limit from the speck with the clock in the cave.