Capacity relates to soundness of mind or a mind that has full and intelligent knowledge of an act engaged in.

Explanation

(English-origin maxim.) When challenging the validity of a contract, deed, or will, invoke this maxim to argue that the party lacked the mental competence to understand the nature and consequences of the act.

Where this applies

• An elderly man signs over his house to a caretaker, but his children argue he had advanced dementia and lacked the mental capacity to understand what he was doing.
• A party to a contract claims it is void because she was heavily medicated at the time of signing and had no intelligent knowledge of the terms she agreed to.
• A court invalidates a will after medical testimony shows the testator could not comprehend the nature or extent of his property when he executed it.