The key idea is that criminal liability usually depends on two elements: the act and the mind. Actus reus refers to the prohibited conduct—an actual act or a failure to act when there is a legal duty. It isn’t enough to merely think about wrongdoing; there must be a voluntary act or a legally recognized omission. Mens rea is the mental state behind the act—intent, knowledge, recklessness, or negligence—amounting to fault at the level the law requires for the offense. In most crimes, both elements must be present and proven beyond a reasonable doubt. The person must have committed the act and done so with the requisite mental state for that offense. Different crimes demand different levels of mens rea; some require intent, others knowledge, recklessness, or negligence. Strict liability offenses change the balance: they impose liability for the act or omission without requiring proof of mens rea. In those cases, the defendant can be liable even if they lacked fault or awareness, typically for regulatory or public-safety purposes. The emphasis shifts to preventing the prohibited conduct rather than punishing moral blameworthiness. So the statement that correctly captures the idea defines mens rea as the mental state, actus reus as the unlawful act or omission, and notes that both are normally required, with strict liability being an exception.