Robert Frost's notebooks have been published. A quote from Ormsby's review at New Criterion:
As the Notebooks show, Frost was a sophisticated reader of philosophy; he knew the Greeks from Thales on and had read Hegel, Schopenhauer, William James, and Bergson (a particular favorite), among others. Here he isn’t making a case for some relativistic notion of truth (indeed, he isn’t making a case at all but having what he liked to call “a think”). Instead, he is winkling out the capacity of poetry to capture, in a single dramatic image or synechdoche, a “confluence” of colliding truths. The object of a poem wasn’t to concoct some slick “unity of opposites;” it was to find those words—“words that have been mouthed like a common tin cup,” as he put it—through which multiple oppositions could flash forth from a single point of compression. An early jotting states, “I thought I was an {acromatic} lense. I’m afraid I am a prism” (his brackets and spelling). Metaphor was one obvious “prismatic” device but for Frost’s purposes, the aphorism, the proverb, and, especially, the figure of synechdoche, offered higher powers of refraction.