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. §92; 3 Curt. Eccl. R. 671. Vide Unsound mind.