In the United States, a witness in court cannot speak of things they heard others say – like “I heard Johnny say he robbed that store." It is termed "hearsay” and is not allowed as evidence. Hearsay includes statements other people wrote, and the witness read – like “I read his diary, and in his diary, Johnny wrote about robbing that store.”
There are a number of exceptions to the hearsay rule. One of the more colorful is dying declarations. “A dying declaration is defined as a statement made by a declarant, who is now unavailable, who made the statement under a belief of certain or impending death, and the statement concerns the causes or circumstances of impending death” according to Cornell Law School. Note that if the dying declaration is about anything other than why the person is dying, it doesn’t count as a dying declaration. And it isn’t admissible in an American court.
