Proper execution of a legal instrument requires that the person signing have sufficient mental "capacity" to understand the implications of the document. While most people speak of legal "capacity" or "competence" as a rigid black line--either the person has it or doesn't--in fact it can be quite variable depending on the person's abilities and the function for which capacity is required.
One side of the capacity equation involves the client's abilities, which may change from day to day (or even during the day), depending on the course of the illness, fatigue and the effects of medication. On the other side, greater understanding is required for some legal activities than for others. For instance, the capacity required for entering into a contract is higher than that required to execute a will.