The reach of law extends only to those able to comprehend its terms and abide by its prescriptions. By this very logic, being human is not enough, for some human beings are so lacking in comprehension, so possessed by the ruling power of another, so destitute of personal powers of self-control as to be indistinguishable from infants, wild beasts or even mere tools. Ancient Greek and Roman law fully respected this, and thus provided legal defences based on claims of diminished capacity. Far from making a mockery of justice, therefore, as is sometimes claimed, the insanity defense is implicit in the very concept of justice. As early as Draco's homicide law one finds distinctions made between intentional and non-responsible homicides (Gagarin, 1981). Mitigation based on mental states, then, is as old as Western juridical thought itself.