[Published with "Alastor", 1816.]
We are as clouds that veil the midnight moon;
How restlessly they speed, and gleam, and quiver,
Streaking the darkness radiantly!—yet soon
Night closes round, and they are lost for ever:
Or like forgotten lyres, whose dissonant strings
Give various response to each varying blast,
To whose frail frame no second motion brings
One mood or modulation like the last.
We rest.—A dream has power to poison sleep;
We rise.—One wandering thought pollutes the day;
We feel, conceive or reason, laugh or weep;
Embrace fond woe, or cast our cares away:
It is the same!—For, be it joy or sorrow,
The path of its departure still is free:
These three lines start with subject and verb, like echoes, a pattern known as  syntactic parallels . The repetition gives emphasis to the idea of man’s inability to grasp life positively. 
 Shelley juxtaposes opposites — dream and poison, laugh and weep —and creates a sense of uncertainty and floundering.
Mutability