In Archibald MacLeish’s “Ars Poetica,” the first of numerous paradoxes appears at once, when the speaker declares that

A poem should be palpable and mute
As a globed fruit . . .

A poem, that is, should have an almost physical weight and presence. It should have an almost perfectly symmetrical shape and structure, seeming perfect when viewed from any perspective. Yet the idea of a "mute" poem seems paradoxical. Perhaps the speaker implies that a poem should be subtle and should communicate by showing rather than by telling. Rather than openly stating some simple, overt meaning, a poem should communicate indirectly, by suggesting.

Paradox continues in lines 3-4, where the speaker declares that a poem should be

Dumb
As old medallions to the thumb . . .

Here again we are invited to wonder how a poem can be “Dumb” (that is, silent), but line 4 suggests once more that a poem should have an almost physical presence, so that it is not so much heard as felt.  The paradox is emphasized one more time in lines 5-6, which suggest that a poem should be

Silent as the sleeve-worn stone 
Of casement ledges where the moss has grown --  . . .

Again, the paradox of silent poetry is suggested, but once more the second line of the pair suggests that that poetry should be almost tactile in its effects: it should communicate almost as much through touch as through words.  By using image after image to suggest his meaning, the speaker in a sense exemplifies the very de-emphasis on bald, blatant statement that he seems to advocate. Rather than telling us precisely what a poem should be and how it should function, the speaker merely suggests meanings, merely implies them, encouraging readers to interpret his meanings rather than plainly and openly stating them himself.

The poem itself, therefore, conveys some of the mystery it celebrates.  Lines 7-8 complete the first section, leaving us with one more vivid image and one more paradox (the idea of a “wordless” poem).

Paradoxes again dominate the second section. If section 1 had extolled poetry that was somehow silent and wordless, section 2 celebrates poetry that is somehow motionless and yet still moving. Here, though, the paradox is less puzzling: everyone knows that the moon seems to move, yet everyone also knows that the moon seems stationary at any given moment. The moon seems to move so slowly that its apparent movement seems almost imperceptible. A poem, the speaker suggests, should likewise be so subtle, so understated, that its effects should be neither obvious nor overt. They should creep up on us, as the moon seems to do. Just as the moon seems to move, ever-so-slowly, behind a tree, so should a poem achieve its effects just as subtly, indirectly, and elusively.

Finally, in the third section, the speaker suggests once more that a poem should work through indirection and implication rather than through overt and blatant statement. It should communicate through symbolism, not through plain, simple declaration. Thus, the whole “history of grief” can be suggested and symbolized by “An empty doorway and a maple leaf” (19-20), just as love can also be suggested symbolically (21-22). A poem, the speaker finally (and somewhat paradoxically) declares,

should not mean
But be.  (23-24)

In other words, it should almost seem an object rather than a statement, almost a thing itself rather than a merely simple thought.