The legal terms for this Capacity to make a will. The classic definition was given in a 1870 case of Banks -v- Goodfellow where the test was said to be that the person making the will must: understand the nature of making a will and its effect where a house has to be...

When a person makes a will and the original was last seen in their possession but can't be found when they die, then there is a legal presumption that they have destroyed the original intending to cancel the will. Like other legal presumptions, evidence can be given...

the Supreme Court will not grant probate on a will which was not fully known and approved by the testator. due execution of a will (signing it in front of witnesses) raises a presumption that the testator knew and approved of its contents probate may still be granted...