The most enduring of his written opinions is the one he authored in "Mutual Life Insurance Co. of New York v. Hillmon" (1892), which held that a declarant's out-of-court statement of his intention to do something or go somewhere in the future is admissible under the "state-of-mind" hearsay exception. "The letters in question were competent, not as narratives of facts communicated to by others, nor yet as proof that he actually went away from Wichita, but as evidence that, shortly before the time when other evidence tended to show that he went away, he had the intention of going, and of going with Hillmon, which made it more probable both that he did go and that he went with Hillmon, than if there had been no proof of such intention." This holding was subsequently codified in Rule 803(3) of the "Federal Rules of Evidence", as well as the evidence laws in most states.