"Beauty is truth, truth beauty" - explain.

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.