In some ways, it could be argued that Emerson speaks in the abstract in Nature and Thoreau applies those abstractions in Walden. Find as many examples of this concept as you can. In other words, locate an abstract concept in Emerson and then explain why something Thoreau discusses is actually an application of that abstraction. There are dozens of these, but at the minimum you need to find five.
Article: Introduction to Ralph Waldo Emerson [B 211-214]
Nature [B 214-231 … stop before “Idealism”]
Henry David Thoreau [B 961-964]