Final answer:
Arnold has committed the tort of libel by writing and sending a defamatory letter about Bert. This is classified as libel because it was a written statement, and not spoken, which would be slander. The legal context for libel and slander cases varies for public figures and private individuals.
Step-by-step explanation:
If Arnold wrote a defamatory letter about Bert, mailed it to Bert, and showed it only to Bert's friend Stan, Arnold has committed the tort of libel. Libel involves publishing false statements in written or fixed form that damage a person's reputation. Unlike slander, which is spoken defamation, libel pertains to written or published statements. Since Arnold's actions involved writing and sending a letter, this falls under the category of libel and not slander.
It's important to note that in defamation cases, public figures need to prove the statement was made with actual malice or reckless disregard for the truth, as established in New York Times v. Sullivan. For private individuals, the standards can vary by state, but the claimant must show that the statement was false and that it caused damage to their reputation.