The testator possesses sound mind for the purposes of making a will if he or she: (1) understands the nature of the act of making a will or codicil thereto, (2) knows the extent and character of the property subject to the will, (3) knows and understands the proposed disposition of that property, and (4) knows the natural objects of his or her bounty (i.e. his or her heirs). Whether the testator was of sound mind is tested (determined) by the state of the testator's mind at the time the will or codicil is executed (written and signed) and varies by state.
A person making a will must be of sound mind. The will maker cannot be mentally ill or handicapped. In addition, if the will maker is terminally ill and heavily sedated, then it is questionable whether such a will could stand up in court.
A requirement for anyone making a legal document, such as a will or healthcare directive. For example, although he can be eccentric or forgetful, a person writing a will must know what he owns, the identities of his family and close friends, and how the will distributes his property. If a person isn't of sound mind, and someone later challenges the validity of the document in a lawsuit, the judge could rule that the document is invalid and has no legal effect. (Such lawsuits are quite rare.)