The answer is:
We live.—Knowing that today’s sun may give way to tomorrow’s rain.
The poem "Mutability
," by Percy Bysshe Shelley deals with universal truths of life and how people are in a permanent state of emotional change. In that respect, Bruno's line expands the idea that life is a constant transformation: the great happiness of the present may turn to misery in the future.
Answer:
letter is to word as note is to story
