Tenn. Code Ann. § 24-1-202: Transactions with mentally incompetent party

It is not lawful for any party to any action, suit, or proceeding to testify as to any transaction or conversation with, or statement by, any opposite party in interest, if such opposite party is incapacitated or disqualified to testify thereto, by reason of idiocy, lunacy, or insanity, unless called by the opposite side, and then only in the discretion of the court; provided, if a corporation be a party, this disqualification shall extend to its officers of every grade and its directors.