Oh, boy- Is this one tough! You have identified one of the most debated and analyzed lines in all of poetry, and probably of written thought. If anyone claims to know what these lines mean, I think they are not being fully transparent. The fact of the matter is that Keats' closing line is open to wide interpretation. I think that he would have wanted it this way, as it enhances the feel of negative capability in the poem. This condition stresses that there is an aspect of unknown in human consciousness, a significant part or experience in what it means to be human that will not be entirely grasped and this is something that is not only present, but also good. It is this condition that applies to the poem and the closing lines. The vision on the urn is static, not changing, and one where the lines make sense. The condition featured in art is that "beauty is truth, truth beauty," because art is a snapshot, a moment in time. Within art, this is understood. It is when we step outside of art, when the human consciousness is fluid and dynamic, and when situations arise that are complex- at these moments, it is difficult to fully understand the meaning of the line and how truth and beauty are to be defined. This is where challenge arises, and does so in examining the lines of beauty and truth. In the end, Keats might be saying that the reason why one should be devoted to creating art is that it is the only realm where "beauty is truth, truth beauty" is evident and where one might even have a chance of grasping such concepts, if only for a fleeting instant.