If a criminal defendant is unable to understand the nature of the court proceeding against him, and is unable to assist in the preparation of his own defense, due to some mental disease or defect, then the defendant may be found by the court to be .

Explanation:

A court may declare a defendant legally insane if he/she pass a series of insanity tests. It includes the "M'Naghten Rule" test, the "Irresistible Impulse" Test,The "Durham Rule" and The "Model Penal Code" Test for Legal Insanity.

A court may allow an insanity defense if the defendant can prove to the court that he/she did not understand what they did, failed to understand right from wrong, was unable to control impulse due to mental disease or some other factors which resulted from mental instability.