This work makes a “reduction” of the Critique of practical reason with two objectives: (i) to identify the fundamental arguments of this masterpiece (therefore, we make a synthesis of the fundamental approaches of Kant’s work) and (ii) criticize, from the rosminian perspective, such Kantian arguments. The author uses for this criticism Rosmini’s book entitled Storia comparativa e critica dei sistemi intorno al principe della morale, for this critique where the Roveretan philosopher dedicates several pages to examining Kant’s moral doctrine.