Me. Rev. Stat. tit. 38, § 703

The owner or occupant of such mill or canal may answer that the plaintiff has no right, title or estate in the lands alleged to be injured; or that the owner or occupant has a right to maintain such dam, and flow the lands, or divert the water for an agreed price, or without any compensation; or any other matter that may show that the plaintiff cannot maintain the action; but the owner or occupant may not answer that the land described is not injured by such dam or canal.