Stanza 3 Summary

Get out the microscope, because we’re going through this poem line-by-line.

We all have reasons
for moving.

This the first time the speaker uses the word "We" instead of "I." So far up to this point, he's been describing his thoughts and feelings about presence in the world.
And for him, existing ain't all it's cracked up to be. He feels empty, or is in constant fear of not existing.
But when the speaker says "We," he's including everyone that exists. It's a general "We," like, all of us. This opens the poem up to include everyone in the world, instead of continuing to harp on this one guy's bummer of a life.
The speaker could be implying that we all have different motivations for living, or staying alive. While he seems to be obsessed with the idea of duality (being present and absent at the same time), he's saying that all of us have different motivations in life. These "reasons for moving" are the central focus of the poem, just like they're central to how we understand our place in the world.

I move
to keep things whole.

Finally, the speaker completes his "reason for moving." Apparently it's to keep things whole.
His presence is an empty space in the landscape that keeps it from being whole.
Based on what we've gleaned from the rest of the poem, the speaker seems to thing that his presence fragments or breaks apart "things" wherever he stands.
So moving allows those things to be put back together.
Well that's nice for the things, but what about the speaker?
To be fair, we don't actually know what these things are. Sure, the speaker could be talking about the air in, say, the field. But he could also be talking about himself.
The speaker could be moving because he wants to keep himself whole—maybe wholeness for the speaker is that embodiment of being both present and absent.
Or, maybe wholeness is impossible for the speaker, and it's only the natural world that can be "whole." And if he doesn't move, then the world can never be whole, which would be terrible, so he keeps moving.
Or, maybe he's making a joke, and being sly about his otherwise anxiety-inducing existence as "absence" in the world.
It's hard to pin down one reading of the poem, and that's the great part of well-written poems: they refuse to be pinned down by one singular interpretation, but instead open up the world and our experience of the world into its complexity and uncertainty.
And if this narrator is feeling anything, it's a little complexity and uncertainty. And invisibility, of course.