It opens up:
"A poem should be palpable and mute
Like a globed fruit."
So he's proceeding by paradox, since no poem is either palpable or mute.
It ends like this:
"A poem should not mean
Another paradox, since words are born to carry meaning.
However, he's also stating a version of a major movement in modern poetry, Imagism, in which poetry properly presents images, rather than making statements or music.
Perhaps it will come as no surprise that John Cage, who wrote the silent piano piece, reportedly liked this poem:
"Ars Poetica," John Cage suggests, is the best piece of propaganda the imagist movement ever had. It is not an imagist poem, he says, because, first, it is almost impossible to write one, and second, it is too didactic; there is too strong a message.I like MacLeish's poem as well.
But at the top of my lungs I yell
That poets must be allowed to tell