Section 3 opens with another puzzling assertion. How can something be "equal to" something that's "not true"?
The point the speaker seems to be making is that a poem shouldn't be about "truths" per se. A person shouldn't read it and think, "that's true!" Instead it should be beyond all of the truths we think we know in the physical world.
Then again, since it must be "equal to" something, we get the sense that there is a kind of truth here in the speaker's ideas. It's just not the kind of truth we can wrap up in neat little words and metaphors. Perhaps the truth is bigger (or smaller?) than all that, similar to the metaphysical kind of world the speaker has been driving at.
Notice too the kind of matter-of-fact/mathematical tone the speaker is using here in line 16. At first we think the speaker will indeed tell us something that a poem should be "equal to," in the sort of way we might hear in a math equation. But instead, in his awfully modern way, he tells us the opposite of what we're expecting to hear.
So the takeaway seems to be that a poem's truth rests in being "not true" and therefore not wrapped up in some prescribed formula. Poetry does not equal math, in other words.
For all the history of grief An empty doorway and a maple leaf.
The modern syntax here can be a bit confusing. But when in doubt, work with what you have.
So a poem can either be "for all the history of grief" or the "for" here is being used as a conjunction in an incomplete thought. Let's get crazy and consider both.
If a poem is meant to be "for all the history of grief," then we understand that a poem is indeed for the senses and emotions, since grief is a mighty common emotion. Perhaps a poem is therefore nothing more than a recording of humanity's shared histories and grievances that persist through time.
Alternatively, perhaps "the history of grief" is equivalent to the imagery of "an empty doorway and a maple leaf." Whatever comes to mind in light of these images is your own understanding of "the history of grief." So maybe poetry's response to emotions and big abstract ideas is imagery and all the ideas that come along with it.
Perhaps the empty doorway is a symbol that's meant to evoke ideas of transcendence into the more metaphysical world the speaker has been driving at. Maybe the "maple leaf" represents a readily present reminder of the physical world (either that, or Canada).
Either way, the ideas that are present here really stand out from one another, and that may be the point. We know MacLeish likes to rely on images rather than allegories in order to relate a poem to us, and these lines are a perfect example of this technique.
The connotations that come to mind based on a series of images, like this one, are indicative of our own unique humanity. And poetry, according to the speaker, should speak to that part of our humanity without all the stifling truths and such.
For love The leaning grasses and two lights above the sea—
Here we get another series of images that relate to the purpose of poetry. If a poem is "for love," then those "leaning grasses" and "two lights above the sea" should speak to us in their own way in the context of love.
And just like grief, love is a highly sensual and subjective emotion. A poem that is "for love" should therefore never limit a reader in the world of the concrete and physical. After all, love can't be measured so a poem about love should likewise never be measurable in its so-called truths.
Notice the not so typical kind of imagery the speaker is using here in association with love—again, very modern. He's not talking about ruby lips and flowing hair. He uses "leaning grasses" and "two lights above the sea" instead. So we can fill in the blanks for ourselves when imagining images like these.
Also, we notice that we don't have a couplet here. So the speaker is again blending the modern in with the more conventional devices of poetry by having a little break in the rhyming.
A poem should not mean But be.
And of course we end with the real takeaway of the whole poem. Poetry shouldn't be about "meaning," but should be about "being." What's the difference?
Think of it this way: meaning is like those math equations you can't stand, the "A+B= C" kind of idea. But "being" is way more natural than all that. It's freed from the concrete world of meanings, equations, and appearances.
And if we think about poetry being about life, our speaker's ideas here ring true (without being "true," of course). Life doesn't "mean" anything, so far as we know. But life exists and therefore it must "be." Deep, we know.
By the end, we can really dig what the speaker is talking about in "Ars Poetica." Poems are about life, life is indefinable, so poems should likewise avoid the easy routes of seemingly digestible truths and meanings. It should "be" just as free as life itself. Likewise, we should read poetry for the experience rather than any big "meaning."