What is the test for having a testamentary capacity?

The parties do not disagree about the law that applies with respect to testamentary capacity. They cite Banks v. Goodfellow (1870) L.R. 5 Q.B. 549 at p. 565: ... It is essential to the exercise of such a power that a testator shall understand the nature of the act and its effects; shall understand the extent of the property of which he is disposing; shall be able to comprehend and appreciate the claims to which he ought to give effect; and, with a view to the latter object, that no disorder of the mind shall poison his affections, pervert his sense of right, or prevent the exercise of his natural faculties - that no insane delusion shall influence his will in disposing of his property and bring about a disposal of it which, if the mind had been sound, would not have been made.