What important idea is conveyed in the title of henry wadsworth longfellow’s poem “the tide rises, the tide falls”

The existence of opposites

Explanation:

In this title, the author highlights how a single object can go through opposite processes. The tide rising and the tide falling are two opposites aspects of the same natural process. It is likely that the author wanted to highlight the existence of opposites in the world, and how they appear in everyday life and in the natural world.