One of the big ideas of Romanticism is the notion that the spiritual vision – the imagination – can hold greater truths than those given by our senses. We can never fully express what goes on in our imagination, but the notion of an "inner eye" captures the sense of reality that it gives us. Anyone who has ever read a good book or had a crazy dream knows that the imagination can seem really real.
Wordsworth is all about that "inner eye." For example, go check out "Tintern Abbey" on Shmoop, in which he describes the same phenomenon as at the end of "I wandered lonely as a Cloud." After visiting an old ruined church, he goes home and remembers the images in his imagination, just as he remembers the daffodils here. He writes that, "with an eye made quiet by the power of harmony, and the deep power of joy, we see into the life of things."