As we saw in Chapter 2, criminal law is distinguished from all other fields of law because of the sanctions it can impose: loss of liberty and moral stigmatization. We regularly incarcerate, or otherwise deprive of freedom, persons who are not morally blameworthy---the mentally ill, the addicted, the fatally contagious, and so on. However, only criminal punishment declares that defendants are to blame for their acts; the essence of the judgment is not that they should be incarcerated for our sakes, but that they deserve punishment because they have chosen freely to violate the criminal law. Such a free choice appears to require that they knew what they were doing, and were aware, or at least risked, that it was morally blameworthy. For centuries, the law has captured this notion of free will and knowledge by looking for mens rea---Latin for “guilty mind.” This chapter is concerned with the basic definitions of mens rea.