In One's Right Mind Meaning

(idiomatic) Thinking clearly, sensibly, and reasonably.

Example: 1849, Charlotte Bronte, Shirley, ch. 1:
  â€œIs Mike Hartley in his right mind, do you think, sir?” inquired Sweeting simply.
  â€œCan't tell, Davy. He may be crazed, or he may be only crafty.”
1873, Mark Twain, The Gilded Age, ch. 56:
  If you find she was not in her right mind, that she was the victim of insanity, hereditary or momentary, as it has been explained, your verdict will take that into account.
1905, H. G. Wells, "The Empire of the Ants":
  But the poor fellow was—what is it?—demented. He was not in his right mind.
2010 Oct. 31, Alice Park, "New Research on Understanding Alzheimer's," Time:
  Who in his right mind would want to know he had a disease that would inevitably rob him of that mind?