be compos mentis

To be sane or mentally competent. "Compos mentis" is Latin for "of sound mind." I doubt that Travis was compos mentis when he committed that crime. A: "Have you heard some of the orders he's been giving lately?" B: "I know, he doesn't seem to be compos mentis to me." You could be held legally liable if the judge rules that you were compos mentis at the time of the fire.

be ˌcompos ˈmentis

(from Latin, formal or humorous) having full control of your mind: Are you sure she was fully compos mentis when she said that?