A sane person doesn't have any screws loose — in other words, they're free of mental illness and in a reasonable state of mind.
You probably know that the word insane means crazy. Well, the opposite of insane is sane — or not crazy. A sane person is of sound mind and is mentally healthy. Sane people have good judgment, are reasonable, and can tell the difference between what's real and imagined. Whenever someone commits a crime or does something totally outlandish, people usually wonder if they're sane or not.