a. Capable of or subject to change or alteration.
b. Prone to frequent change; inconstant: (American Heritage Dictionary; on TheFreeDictionary.com)
In "Mutability," Shelly employs Romanticism's emphasis on the emblematic "language" of nature to symbolize the inconstancy of human nature, life, and experience. His method of doing this is to begin the poem with imagery of the mutable clouds against a "midnight moon" that, sweeping past, are soon enveloped in the darkness of night. He then shifts the emphasis in the third stanza of this lyric poem to the sleeping and waking experience of humans, emphasizing the fragile and mutable nature of experience.
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.
We rest. – A dream has power to poison sleep;
We rise. – One wandering thought pollutes the day;