No temper of mind, no habit of life, however pernicious, has ever been treated as a
crime, unless it displayed itself in some definite overt act. The reasons for imposing this great leading restriction upon the sphere of criminal law are obvious. If it were not so restricted it would be utterly intolerable; all mankind would be criminals, and most of their lives would be passed in trying and punishing
each other for offences which could never be proved.
James Fitzjames Stephen, A History ofthe Criminal Law of England (1883)