When a person makes a Will, or changes an existing one, they have to understand what they are doing for it to be valid. This is the essence of testamentary capacity. For a century and a half, the benchmark test for having the mental capacity to make a valid Will, in common law jurisdictions such as NSW, has been based on the nineteenth-century English case Banks v Goodfellow (1870). Banks concerned an individual who suffered delusions but whose Will was held to be valid because the delusions did not interfere with his ability to comprehend what he was doing when he made his Will.