In John Keats's "Ode on a Grecian Urn," it can be difficult to understand who is actually speaking and who is being addressed. The speaker could be the poet or the urn itself; the audience could be the urn, or the reader, or even the figures on the urn. The use of quotation marks at the poem's end complicates things even further. In the manuscript copy of the poem, there are no quotation marks at all; the final lines seem to be spoken by the same voice as the rest of the poem. In the printed version, quotation marks are used, suggesting that a voice other than the predominant one of the poem is speaking.
On the face of it, it would seem that the "thou" is the urn, which says the last two lines to "man," or mankind in general. The presence of quotation marks raises the possibility that we are meant to understand the last two lines ironically: they emphasize that this is what the urn says, so it is up to the reader to decide if this is to be taken at face value. On the other hand, if there are no quotation marks, then the last two lines are not a representation of the urn's speech, but a paraphrase of the sorts of things one might gather the urn has to say about art and beauty. In this case, the lines are clearly the speaker's; they may be what the poet thinks about beauty and truth.