A defendant states during the commission of the crime he did not understand what he was doing. What could be his defense?

The defense of insanity is applicable in situations where a defendant claims they were unable to understand the nature of their actions at the time a crime was committed due to a mental disorder or impairment. This defense rests on the premise that because the defendant could not comprehend the consequences or wrongfulness of their actions, they should not be held criminally responsible. In instances of mental illness, the legal system may recognize that the individual lacks the capacity to form the requisite intent necessary for a crime.

In contrast, an alibi asserts that the defendant was not present at the crime scene, and self-defense justifies the defendant's actions as necessary to protect themselves from imminent harm. Neither of these could be substantiated if the defendant was indeed present during the crime yet claims a lack of understanding. Hence, insanity is the relevant defense in this context.