from
Bouvier's Law Dictionary, Revised 6th Ed (1856)
SOUND MIND. That state of a man's mind which is adequate to reason and comes
to a judgment upon ordinary subjects, like other rational men.
2. The law presumes that every person who has acquired his full age is
of sound mind, and consequently competent to make contracts and perform all
his civil duties; and he who asserts to the contrary must prove the
affirmation of his position by explicit evidence, and not by conjectural
proof. 2 Hagg Eccl. R. 434; 3 Addams' R. 86; 8 Watts, R. 66; Ray, Med. Jur.
Sec. 92; 3 Curt. Eccl. R. 671. Vide Unsound mind.