What is the burden of proving that a party to a will has a valid testamentary capacity?

In Vout v. Hay, 1995 CanLII 105 (SCC), [1995] 2 S.C.R. 876, the court heard an appeal in which it was alleged there were “suspicious circumstances” surrounding the execution of a will. At para. 26, Sopinka J. for the court observed: ...Although the propounder of the will has the legal burden with respect to due execution, knowledge and approval, and testamentary capacity, the propounder, is aided by a rebuttable presumption. Upon proof that the will was duly executed with the requisite formalities, after having been read over to or by a testator who appeared to understand it, it will generally be presumed that the testator knew and approved of the contents and had the necessary testamentary capacity.