What is the test for a person to be able to enter into a contract?

Under California law, persons of unsound mind are not capable of contracting. (Civ. Code, 1556.) Whether a person has an "unsound mind" is related to the concept of consent because the parties' consent to the contract must be free. (Civ. Code, 1565.) The consent essential to a contract's existence cannot be given freely by a person unless he or she is "endowed with such a degree of reason and judgment as will enable him or her to comprehend the subject of negotiation." (Jacks v. Estee (1903) 139 Cal. 507, 512-513.)