Compos mentis is a legal term meaning that one has mastery of one’s own mind. This includes both understanding and memory. Compos stands for one who has mastery, control, or power over something, whereas mentis stands for mind or mental competence. A person described in this way is balanced, clearheaded, sane, lucid, normal, right, and stable. Non compos mentis is also a legal term meaning the opposite. In short, such a person is not of the right mind and thus not legally competent. According to Samuel Johnson’s Dictionary of 1755, the latter expression got mangled into “nincompoop” in the Seventeenth Century. Many scholars dispute this connection, but it remains of interest nonetheless. Now, show me anyone who is compos mentis for true. Mastery of one’s own mind is no less than a miracle in the human race. If there is such a person, he or she is enlightened for sure. All the others are nincompoops by definition. As far as legal definitions go, they are for the nincompoops, anyhow.