What is the test to determine testamentary capacity?

The test to determine testamentary capacity was articulated in Banks v. Goodfellow (1870), L.R. 5 Q.B. 549 (Eng. Q.B.) and remains apposite. To prove that the testator or testatrix had testamentary capacity, the proponents of his or her will have to lead evidence that establishes that the testator or testatrix: a) understood the nature of the act of making a will and its effects; b) understood the extent of the property of which he or she is disposing; c) able to comprehend and appreciate the claims to which he or she ought to give effect; and d) had no disorder of the mind that poisoned his or her affection, perverted his or her sense of right, or prevented the exercise of his or her natural faculties – that no insane delusion influenced his or her will in disposing of his or her property and bringing about a disposal which, if the mind had been sound, would not have been made.