From The Necessary Angel: Essays on Reality and the Imagination, Wallace Stevens:

Poetry is the imagination of life.  A poem is a particular of life thought of for so long that one’s thought has become an inseparable part of it or a particular of life so intensely felt that the feeling has entered into it.  When, therefore, we say that the world is a compact of real things so like the unreal things of the imagination that they are indistinguishable from one another and when, by way of illustration, we cite, say, the blue sky, we can be sure that the thing cited is always something that, whether by thinking or feeling, has become a part of our vital experience of life, even though we are not aware of it.  It is easy to suppose that few people realize on that occasion, which comes to all of us, when we look at the blue sky for the first time, that is to say: not merely see it, but look at it and experience it and for the first time have a sense that we live in the center of a physical poetry, a geography that would be intolerable except for the non-geography that exists there—few people realize that they are looking at the world of their own thoughts and the world of their own feelings.  On that occasion, the blue sky is a particular of life that we have thought of often, even though unconsciously, and that we have felt intensely in those crystallizations of freshness that we no more remember than we remember this or that gust of wind in spring or autumn.