A more thorough explanation:

The substantial-capacity test is a legal concept used in criminal law to determine if a person is criminally responsible for their actions. According to the Model Penal Code, a person is not responsible for a crime if they lack substantial capacity due to a mental disease or defect to either appreciate the criminality of their conduct or conform their conduct to the law.

This test takes into account both cognitive and volitional weaknesses and was previously used by federal courts and many states. However, since 1984, many jurisdictions have narrowed the insanity defense and adopted a new test resembling the McNaghten rules.

John is charged with murder, but his defense argues that he was not responsible for his actions due to a mental illness. The substantial-capacity test is used to determine if John lacked the capacity to understand the criminality of his actions or conform his behavior to the law.

If the court finds that John lacked substantial capacity, he may be acquitted of the crime or found not guilty by reason of insanity, which could lead to his commitment to a mental institution.