There is no rule requiring an attesting witness to have ‘mental capacity’ as such. (Section 14 of the Wills Act 1837 refers to a witness not having to be ‘competent’, but that is thought to refer to the old rules of evidence governing competence to be a witness, rather than mental capacity.)
Mental capacity is time and transaction specific. Indeed, a minor (child) can act as an attesting witness: Wilson v Beddard (1841) 12 Sim 28 (not reported by LexisNexis®). However, an attesting witness must be able to carry out the essential function of witnessing – being both physically and mentally ‘present’—which implies certain minimum attributes: they must be able to ‘see and be conscious of the act done, and to be able to prove it by their own evidence’: Hudson v Parker (1844) 1 Rob Ecc 14 (not reported by LexisNexis®). That case noted that a witness who was asleep or intoxicated or ‘of unsound mind’ might not be ‘present’ in that sense. So a blind person cannot act as an attesting witness: Re Gibson. So too someone who lacks the mental capacity to observe and take heed of what is going on, or to give evidence about it later, cannot be a witness. Someone in a coma, or delirious or delusional, would presumably be unable to act as a witness.
These requirements must be fulfilled at the time the Will is made, but of course the Will is not invalidated by an attesting witness subsequently becoming blind, or in capable of giving evidence. See: V. Witness: Williams on Wills [12.17], especially footnote 2.
It might therefore be argued that someone with so severe a short-term memory impairment that they could not say shortly afterward what they had seen, may not be able to validly witness a Will. But we are not aware of any specific legal authority requiring someone to have full testamentary capacity themselves, or even capacity to manage their own financial affairs, in order to act as an attesting witness.
The short answer to the question is therefore: it depends on how far and in what sense the attesting witness ‘lacked mental capacity’ at the time of the Will being executed.
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