Time, Real And Imaginary. An Allegory

On the wide level of a mountain's head 
(I knew not where, but 'twas some faery place), 
Their pinions, ostrich-like, for sails outspread, 
Two lovely children run an endless race, 
  A sister and a brother!
  This far outstripp'd the other; 
  Yet ever runs she with reverted face, 
  And looks and listens for the boy behind: 
  For he, alas! is blind! 
O'er rough and smooth with even step he pass'd,
And knows not whether he be first or last.