A person is said to be of sound mind for the purpose of making a contract, if, at the time when he makes it, he is capable of understanding it and of forming a rational judgment as to its effect upon his interests.

A person who is usually of unsound mind, but occasionally of sound mind, may make a contract when he is of sound mind.

A person who is usually of sound mind, but occasionally of unsound mind, may not make a contract when he is of unsound mind.

Illustrations

(a)A patient in a lunatic asylum, who is at intervals of sound mind, may contract during those intervals.

(b)A sane man, who is delirious from fever or who is so drunk that he cannot understand the terms of a contract, or form a rational judgment as to its effect on his interests, cannot contract whilst such delirium or drunkenness lasts.