The worst aspect of writing poems - besides the embarrassment of expressing the very things you might express in a poem above all else and all other places - is cryptomnesia. Like, many of my best lines - I'm pretty sure they aren't mine. I'm just not sure where I saw them. And when I wrote them, I actually wrote them, word for word, sometimes through multiple iterations. I went through the whole process. I snuffled around for something, cobbled, and then tinkered. It's just when I was done I was sure I'd read that somewhere. Sometimes I even have an idea about what the rest of the poem or piece was like, but I can't find it. I can never find it.
If I *have* seen these lines before, I reinvent them without at first realizing it - but maybe that's not unlike intentionally reconstructing a memory piece by piece. It's easier to work your way to some place if you already sense that place exists. Stumbling blind back toward a feeling given to me (right now unawares) by a line I once heard is not exactly writing it, but it's sort of rewriting it.
Also, my memory is not good. I can't memorize a poem to save my life. I mean, ok, I can, but it takes constant practice and I soon forget. Words drift fast in my mind. I don't quote movies because I can't. (Maybe a dozen lines ever, and often paraphrasing.)
So... I tend to trust that I'm not remembering entire phrases and sentences wholesale without even realizing it.
But I probably am occasionally. And partly realizing, or suspecting.
Recently I lifted Emerson's "there is no screen or ceiling between our heads and the infinite heavens" (one of my very favorite clusters of words) for a poem, but I dropped a reference to him as a "shaman" and a "priest." That's an example that was fully aware all along and I knew the source. Hopefully I'm transparent enough about it there without disrupting the flow. Whether that's ok is an open question to me, so I'm making a note of it here. I didn't use quotation marks but maybe should.
Anyway, that aside if it's an exception, I never mean to plagiarize.
Actually, I wrote a poem about falling in love while falling in love - you're already falling, and then you fall mid-fall into a deeper layer of falling. I'm certain I stole that image from somewhere. I feel it's important to say that. It isn't my image. But I don't know where I read it, other than it was a poem. Probably one published in The Paris Review. I'm subscribed to their daily poetry archive emails but only very occasionally read them.