Dr Danfield explains in his lecture that there's a psychological reason behind every crime and once you have that reason you have the motive and once you have the motive the finding and apprehension of the criminal is merely a matter of police routine. As for murder, he explains, that the greater percentage result from the psychological reactions to frustrated ambitions,  jelousy,and hate. All of this is proved to be correct when Dr Danfield investigates the suspected murder when a vaudevillian actor named Tiny is found dead in his gas filled room.