It might appear that an incapacitationist might think mental state is not relevant. If the defendant is dangerous, she should be locked up without regard to her mental state. However, the criminal process and criminal incarceration are a costly business. If we are only interested in confinement, we can use the less costly and less burdensome civil process. If the criminal process is to be relevant to an incapacitationist, it must be because the defendant will continue to be dangerous because she is dangerous.
The notion of blame, however, fits most easily in the retributivist’s theory. To a retributivist, a person is morally culpable, and therefore properly subject to punishment, only if she had a “real choice” in her conduct and knowingly exercised her free will to execute that choice. As Justice Jackson put it in a frequently repeated observation:
The contention that an injury can amount to a crime only when inflicted by intention is no provincial or transient notion. It is as universal and persistent in mature systems of law as belief in freedom of the human will and a consequent ability and duty of the normal individual to choose between good and evil. A relation between some mental element and punishment for a harmful act is almost as instinctive as a child’s “But I didn’t mean to….” Unqualified acceptance of this doctrine by English common law … was indicated by Blackstone’s sweeping statement that to constitute any crime there must first be a “vicious will.” 
No state has fully adopted any one of these goals of punishment as “the” purpose. Indeed, most observers argue that the criminal law should adopt all these purposes, at one time stressing one purpose, at another time another. Where the legislature is silent on the purpose of a particular statute, and where the different philosophies would result in different interpretations, however, a real dilemma arises.
Suppose a statute prohibits “selling drugs,” and Rob sold a white powder that he thought was salt. He would contend that the statute should be interpreted as requiring knowledge of the nature of the item sold. A deterrence theorist might argue that the statute should not be interpreted to require mens rea, because by punishing Rob, others might be deterred from selling white powder unless they assured themselves it was salt. An incapacitationist might similarly argue that the statute should not be interpreted to require knowledge because Rob’s failure to perceive or check the nature of the powder makes him dangerous enough to be imprisoned. A rehabilitationist, on the other hand, would most likely contend that persons who make these mistakes should be trained to be more careful, but not punished as though they knew the powder was heroin; the statute should be required to show knowledge. Finally, a retributivist would adamantly demand that the statute be interpreted to require that a defendant knew it was heroin; a person who sells what he believes to be salt is simply not morally culpable if it turns out that he was wrong.