Non compos mentis is a Latin term that refers to a state of being mentally incapacitated or lacking soundness of mind. Antonyms of 'non compos mentis' are words or phrases that convey a state of mental lucidity, soundness, or sanity. Some antonyms of non compos mentis include coherent, clear-headed, reasonable, rational, lucid, and sane. These words indicate a mental state where the individual is fully able to understand and make rational decisions. Being non compos mentis can lead to mental disorders such as dementia and insanity, while being coherent and sane promotes mental health and well-being. Hence, it is essential to recognize signs of mental instability and provide appropriate care and support to individuals experiencing such problems.