A contract made by a mentally incompetent person not so found by prior judicial determination is voidable and will be set aside if the other party knows of his deficiency; or, if the other party is unaware of his deficiency, the contract is unfair to him (Wilson v. The King, 1938 CanLII 30 (SCC),  3 D.L.R. 433 (S.C.C.)). A person is, in law, mentally incompetent when he is by reason of his mental state unable to understand the nature and effect of the contract.
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