Mens Rea..

"The presence of a "vicious will" or mens rea (Morissette v. United States,  342 U.S. 246, 251 ) was long a requirement of criminal responsibility. But the list of exceptions grew, especially in the expanding regulatory area involving activities affecting public health, safety, and welfare. Id., at 254. The statutory offense of embezzlement, borrowed from the common law where scienter was historically required, was in a different category. 13  Id., at 260-261.
"[W]here Congress borrows terms of art in which are accumulated the legal tradition and meaning  [401 U.S. 601, 608] of centuries of practice, it presumably knows and adopts the cluster of ideas that were attached to each borrowed word in the body of learning from which it was taken and the meaning its use will convey to the judicial mind unless otherwise instructed." Id., at 263.
At the other extreme is Lambert v. California,  355 U.S. 225 , in which a municipal code made it a crime to remain in Los Angeles for more than five days without registering if a person had been convicted of a felony. Being in Los Angeles is not per se blameworthy. The mere failure to register, we held, was quite "unlike the commission of acts, or the failure to act under circumstances that should alert the doer to the consequences of his deed." Id., at 228. The fact that the ordinance was a convenient law enforcement technique did not save it.
"Where a person did not know of the duty to register and where there was no proof of the probability of such knowledge, he may not be convicted consistently with due process. Were it otherwise, the evil would be as great as it is when the law is written in print too fine to read or in a language foreign to the community." Id., at 229-230.  United States v. Freed, 401 U.S. 601 (1971)