Insanity prevents marriage from being contracted, because consent is needed.

Explanation

(English-origin maxim.) A marriage contracted by a person of unsound mind is void ab initio, because the essential element of consent is absent — cite this to annul a marriage where one party lacked mental capacity at the time of the ceremony.

Where this applies

• A family seeks to annul a marriage after discovering the groom was in the grip of a severe psychotic episode during the ceremony and could not understand the vows he was reciting.
• A woman marries a man who, unknown to her, had been declared legally insane the month before; the court annuls the marriage because his insanity destroyed the capacity to consent.
• A guardian petitions to void her ward's marriage, showing that the ward suffers from profound intellectual disability and never understood what marriage means, let alone agreed to it.