Time, Real and Imaginary

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 outstript 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 passed,
And knows not whether he be first or last.