"Mens rea" is Latin for mental state. I learned the term from Professor Bruce Berner at Valparaiso University School of Law in my first-year criminal law class. In the law, he taught, punishment typically depends on an actor's degree of culpability - and culpability in turn depends on an actor's mental state.
Prof. Berner was masterful in explicating the implications of this. The hypotheticals were of this type: Which would be worse?
(1) Planning to kill someone by shooting them and doing so; or
(2) Cleaning your gun while preparing for a hunting trip and handling it so carelessly that it went off and killed someone in the room
Many students would say (1), but Berner pointed out that, in important respects, the second scenario is the scarier one. At least in the first, the person knew what he or she was doing. In the second, the incompetence is so extreme that no one is really safe.
My courses with Prof. Berner were in 1984 and 1985, before mandatory minimum sentences became so prevalent. It would have been interesting to study with him in more recent years, to see how he may have attempted to integrate those into his traditional common law schema.