I'm not sure when Jarrell delivered a lecture titled "The Obscurity of the Poet," but I believe it must have been some time in the late 40's or early 50's at a Harvard conference called The Defense of Poetry. I found the lecture in a volume called Poetry and the Age, pub date 1953.
"Art matters not merely because it is the most magnificent ornament and the most nearly unfailing occupation of our lives, but because it is life itself. From Christ to Freud we have believed that, if we know the truth, the truth will set us free: art is indispensable because so much of this truth can be learned through works of art and through works of art alone—for which of us could have learned for himself what Proust and Chekhov, Hardy and Yeats and Rilke, Shakespeare and Homer learned for us? and in what other way could they have made us see the truths which they themselves saw, those differing and contradictory truths which seem nevertheless, to the mind which contains them, in some sense a single truth? And all these things, by their very nature, demand to be shared; if we are satisfied to know these things ourselves, and to look with superiority or indifference at those who do not have that knowledge, we have made a refusal that corrupts us as surely as anything can."
He goes on eloquently to deplore a democracy that shares material but not spiritual goods, and concludes with this:
"If a democracy should offer its citizens a show of education, a sham art, a literacy more dangerous than their old illiteracy, then we should have to say that it is not a democracy at all . . . Goethe said: The only way in which we can come to terms with the great superiority of another person is love. But we can also come to terms with superiority, with true Excellence, by denying that such a thing as Excellence can exist; and, in doing so, we help to destroy it and ourselves."
He ends by quoting Proust:
"All that we can say is that everything is arranged in this life as though we entered it carrying the burden of obligations contracted in a former life; there is no reason inherent in the conditions of life on this earth that can make us consider ourselves obliged to do good, to be fastidious, to be polite even, nor make the talented artist consider himself obliged to begin over again a score of times a piece of work the admiration aroused by which will matter little to his body devoured by worms, like the patch of yellow wall painted with so much knowledge and skill by an artist who must for ever remain unknown and is barely identified under the name Vermeer. All these obligations which have not their sanction in our present life seem to belong to a different world, founded upon kindness, scrupulosity, self-sacrifice, a world entirely different from this, which we leave in order to be born into this world, before perhaps returning to the other to live once again beneath the sway of those unknown laws which we have obeyed because we bore their precepts in our hearts, knowing not whose hand had traced them there—those laws to which every profound work of the intellect brings us nearer and which are invisible only—and still!—to fools."
You can find this essay and other greats in both of these books: