Final answer:
Henry David Thoreau uses metaphor as a literary device in a passage from Walden, employing the imagery of fishing in a stream and drinking from it to express themes of time, life, and the pursuit of deeper understanding.
Step-by-step explanation:
In the passage from Walden by Henry David Thoreau, the literary device used is a metaphor. Thoreau compares time to a stream where he goes fishing, which allows him to convey deeper philosophical ideas about life and eternity. The metaphor extends when he talks about drinking from the stream and seeing its sandy bottom, indicating the shallowness of temporal existence as compared to the vastness of eternity. Furthermore, the desire to 'drink deeper; fish in the sky, whose bottom is pebbly with stars' represents a yearning for transcendence and deeper understanding of the universe, with the sky as another extended metaphor for unexplored realms of knowledge or experience.