Consider two engines, M and L , which are irreversible and reversible respectively. We construct the machine shown in the right figure, with M driving L as a heat pump. Then if M is more efficient than L , the machine will violate the second law of thermodynamics. Since a Carnot heat engine is a reversible heat engine, and all reversible heat engines operate with the same efficiency between the same reservoirs, we have the first part of Carnot's theorem: