Lines 1-6 Summary

Lines 1-2

The poem of the mind in the act of finding
What will suffice.

• This first line comes at us out of nowhere, and you gotta get your think on to figure out where the poem's heading with it.
• Basically, all we know at this point is the title, "Of Modern Poetry." So we can assume the poem's probably going to talk about what modern poetry is and how it works. That means we can think of this opening line as Stevens' thesis statement, which is like a short summary of what he means by "modern poetry."
• You might also notice that Stevens begins this poem using enjambment, and goes on to use enjambment in almost every line for the rest of the poem. It's crucial that he makes the enjambment on the word "finding," because if modern poetry is about the mind in the act of finding, it's crucial that Stevens gives this entire poem a sense of "searching" for something. By using enjambment to end line 1 on the word "finding," Stevens sets the stage for the entire poem as a continued act of searching. Every time we seem to reach a conclusion, enjambment causes a line to fall into the next one, which keeps up our sense of searching for something. 
• For starters, modern poetry is "The poem of the mind." It tends to focus on how our minds work more than the beauty of physical things like flowers and such. Even if a modern poem did focus on flowers, Stevens would say it's supposed to focus on how much the mind enjoys flowers, more than the flowers themselves. 
• But Stevens doesn't stop with just a poem of the mind. He's talking about a poem of the mind "in the act of finding." In this sense, poetry needs to talk about something more specific than just the way our minds work. It needs to talk about something that we're all looking for, something that poetry will help us find. We're still not sure what that thing is, but Stevens' vagueness suggests that it's something huge (like maybe the meaning of life).
• What is poetry supposed to show our minds in the process of finding? Well to put it super-vaguely, Stevens says that the thing our minds are supposed to be finding is "What will suffice." "Suffice" is basically a fancy word for saying "good enough." So poetry is supposed to help our minds find something that is "good enough" for us. Still no word on what that is yet. It seems as though Stevens wants each of us to decide for ourselves what our minds are looking for in poetry and what will be "good enough" to satisfy us.
• In a nutshell, the start of this poem implies that when we read poetry, our minds are looking for some sort of deeper satisfaction. At this point, it's up to us to decide what this satisfaction is. But Stevens puts no limits on how deep we want to take it. The satisfaction we're looking for in poetry might be satisfaction with our lives in general. Maybe poetry is something that's "good enough" to make getting out of bed seem worthwhile.

Lines 2-4

It has not always had
To find: the scene was set; it repeated what
Was in the script.

• You can tell from the grammar of this next part that "it" here refers to modern poems, which are supposed to be about our minds finding something. Notice how that falling-off feeling created by the enjambment makes your eyes feel like they're "searching" for something?. Stevens steps in here and says that poetry hasn't always had to do this. Back in the old days, "the scene was set" for poetry, and poetry just "repeated what/ Was in the script."
• Now just think about all those old classical sonnets your teacher might have made you read in school. Notice how they all basically follow the same pattern and talk about the same things, like love, the sun, roses, and such? That's exactly what Stevens is talking about here. He says that in the past, poets didn't face the same challenges modern poets do. Everyone loved a good sonnet about how love is like a rose. So the "stage was set" for poets to just "follow the script" of traditional poetry.

Lines 5-6

                         Then the theatre was changed
To something else. Its past was a souvenir.

• What's all this, then? It turns out that all those old poets talking about love and roses had the rug pulled out from under them.
• Something in the modern world changed "the theatre" of poetry. So we can assume that the metaphor of the "stage" isn't so perfectly set for these poets anymore.
• So what happened to change the world of poetry? Stevens isn't totally clear on this point, but we can probably assume from his later lines that World Wars I and II had a bit to do with the shift. After all, it ain't so easy to talk about love and flowers when tens of millions of people are lying face first in mud with bombs exploding all around them. The ware wasn't just a big deal for Stevens, but pretty much anyone writing during the "modernist" movement.
• In this harsh new reality, the old poetry of love and roses doesn't seem totally real to us anymore. It's like a metaphorical "souvenir" or keepsake from the past. When you think of souvenirs, you might think of cheap little plastic trinkets or knick-knacks that remind you of where you've been, but in a fairly superficial way. Well, for Stevens, that's the case with classic poetry. It reminds us of the past, but love and roses seem cheap and gimmicky in the modern world.