Competence is the ability to handle business affairs with ordinary prudence. It is a legal not a medical formula, and in a sense, it must be defined negatively. Every one is assumed to be competent. If you think some one is incompetent, you must prove it. He does not have to prove that he is competent. Therefore, the formula is a criterion of incompetency not competency. To declare a person incompetent, these findings must coexist: 1. He has a mental disorder. 2. This causes bad judgment. 3. Because of the bad judgment, the patient either: (a) Squanders his money or (b) Is easily victimized by designing persons or (c) Hoards his assets to the point of depriving himself or his family of necessities.
Henry Davidson, An Appraisal of Competency, 9 Clev.-Marshall L. Rev. 441 (1960)