System 1 or Effortful System is the system dealing with complex thinking. It can follow rules, compare objects on several attributes, and make deliberate choices between options[1].
It is part of two systems of decision making, the first is the automatic-system(system 1) which decide fast by intuition and the second is the ECS which produce reasoning.The term system 1 and system 2 was given by Stanovich and West[2]