If this Be but a vain belief, yet, oh! how oft – In darkness and amid the many shapes Of joyless daylight; when the fretful stir Unprofitable, and the fever of the world, Have hung upon the beatings of my heart – How oft, in spirit, have I turned to thee, sylvan Wye! thou wanderer thro' the woods, How often has my spirit turned to thee!
At eight lines, this is the shortest stanza in the poem, but it's all one long, complicated sentence. But never fear, Shmoop is here to parse it out with you…
The sentence has a big, almost parenthetical chunk in the middle, dividing it up (between the dashes at the end of line 50 and at the end of line 54). Try reading the sentence leaving out the middle section between the dashes, and then go back and read the middle part.
The speaker starts with the hypothetical worry that his whole theory (about how it's possible for the memories of beautiful things to lead you to a state where you understand important truths about the world) is totally bogus – a "vain belief."
Looking to the last few lines of the stanza, the speaker says that, even if it is bogus, who cares? Whether it is true or not, he still often ("oft") called out to the "sylvan," or wooded, river Wye "in spirit."
Back to the middle section: the speaker describes when, or under what circumstances, he used to cry out to the river Wye "in spirit." It was when everything seemed dark and "joyless," even in the "daylight," and when the "fretful stir," or anxious bustle, of the world was really getting him down.
So the speaker seems to be saying that it doesn't matter how bogus the whole "we see into the life of things" idea really is. It worked for him when he was depressed, and that's what matters.