A person is said to be of sound mind if at the time when he makes a contract agreement or will, he is capable of understanding it, and of forming a rational judgement, as to its effect upon his interest.

A person might be said of sound mind when that person is sane or when, not insane or when he/she is not mentally retarded.

A person cannot execute a will when he/she is mentally retarded.

A person who is insane can execute a will in a lucid interval (interval in which he/she is of sound mind) when he/she is sane and is capable to understand.

A person who has executed a will whose mind has been impaired temporarily by intoxication or from illness or from any other cause, and he/she does not know what he is doing, is void.