The insanity defense refers to that branch of the concept of insanity that defines the extent to which men accused of crimes may be relieved of criminal responsibility. (Wrobleski, Henry M. and Hess, Karen M. 2000) A determination of mental illness rests solely on the jury who uses information drawn from the testimony of "expert" witnesses, usually professionals in the field of psychology. The result of such a determination places an individual accordingly, be it placement in a mental facility, incarceration, or outright release. .
There have been known cases where a defendant may have pled temporary insanity. Temporary insanity is a defense used in criminal prosecution in which the state of mind during the crime negates his or hers criminal responsibility. (Siegel, Larry J. 2001) Much like the insanity defense, temporary insanity must be proven; only it is more difficult because in many cases a defendant may have been insane at the time of the crime but sane at the present time. Or the defendant may have been insane during the time of the crime and still insane at the present, which therefore ceases all court proceedings until he is fit (sane) to stand trial. .
At the center of the legal use of insanity lies the mens rea. Every crime involves a physical act, or actus Reus, and a mental act, or mens rea, the non-physical cause of behavior (guilty mind). The mens rea is the mental element required for a crime, and if absent excuses the defendant from criminal responsibility and punishment. (Siegel, Larry J. 2001).
Being insane may help prove absence of mens rea, however, many years of research have still not proven for certain that the insane are free from intentions. Mens rea and the intentions of an act are different from one another, although in most cases mens rea and intentions work together. In better words, it means that the defendant's state of mind was, at the time of the crime was committed made it impossible to have the necessary intent to satisfy the legal definition of the crime.