It is very difficult to put it in a phrase, but it has been put in this way: that there must be aberration or weakness of mind; that there must be some form of mental unsoundness; that there must be a state of mind which is bordering on, though not amounting to, insanity; that there must be a mind so affected that responsibility is diminished from full responsibility to partial responsibility. In other words, the prisoner in question must be only partially accountable for his actions. And I think one can see running through the cases that there is implied ... that there must be some form of mental disease.