Background: Lack of precise definitions for terms such as volition and discernment from that point of view of law and psychology; the need to conform the definitions with mental disorders; disagreement among the experts to determine the penal responsibility of the mental patients according to section 149 of the Islamic penal code of Iran, were the main causes of carrying the present study out.

Methods: This study used focus groups technique in five meetings participated by psychiatrists, lawyers and judges. In each meeting the text of the section 149 of the Islamic penal code of Iran was given to audience and they were asked to define the terms of will and discernment that are mentioned in this section.

Findings & Conclusion: Applied definitions of volition and discernment are achieved by summarizing the comments presented by judges, psychiatrists and lawyers, including: will "a kind of mental ability coupled with intelligence that is the direct source of behavior and in conjunction with power leads to free choice versus compulsion and coercion ". If the agent does not have the power, the act does not occur and power without will leads to an act that is not regarded as volitional and authority cannot be regarded as action. The power of discernment may also be described as "the ability to understand the nature of the actions and understand and predict the consequences for themselves and others and recognize the difference between permissible and forbidden".