It's a modern poem, so you know "Ars Poetica" is likely going to delve into the tricky world of reality and our frequent, but futile, attempts to define the world around us. And yet, MacLeish keeps things relatively simple for us here. Reality, in his poem, is about "being" rather than "meaning." So put away that calculator and dust off your imagination already.
Reality, in MacLeish's poem, is a matter of perspective and "being," rather than a concrete depiction of the world around us. Trippy, we know.
Time for a cliché: a picture says a thousand words. If we're going to try to define reality in MacLeish's poem, it makes sense to do so via imagery, rather than so-called "truths."