Answer:
The ability to appreciate the criminality of his act
Step-by-step explanation:
For an insanity defense to succeed, he defendant must proof the lack "mens rea", which is Latin for a "guilty mind", and refers to the intention of commiting a crime, necessarily recognizing it as such. Insanity means that a defendant is unable to recognize a crime as such and is unable to behave according to society's norms.
In the United States, the Model Penal Code is often used to determine insanity, and is based on the the same principle. It requires a diagnosis of mental illness and requires the defense to proof that at the time of the incident the defendant was unable to recognize his act as a crime or was unable to behave according to the law due to a lack of control related to mental illness.
If these conditions were met in John's case a judge would most likely have dismissed the criminal charges and declared him not guilty by reason of insanity.