What is the origin of the idiom "in one's right mind" and when to use it?

The idiom "in one's right mind" can be traced back to ancient legal and philosophical traditions. In English common law, the concept of being "of sound mind" was crucial in determining a person's legal capacity, particularly in matters such as making contracts or executing a will. The origin of this phrase can be connected to the historical development of legal principles and practices. It is commonly employed in discussions related to sanity, rationality, or mental competence.