Wills, Trusts, and Estates
[T]he testator . . . must be capable of knowing and understanding in a general way:
- the nature and extent of his or her property,
- the natural objects of his or her bounty, and
- the disposition that he or she is making of that property, and must also be capable of
- relating these elements to one another and forming an orderly desire regarding the disposition of the property.
Copyright, The American Law Institute
He does not have to actually know these things, only be capable of knowing them.
Making a lifetime gift requires more capacity than making a will, and making a will requires more capacity than getting married.
Someone can be mostly crazy, but as long as his will is made in a moment of lucidity, it will be valid.