Past exam of the mathematics course of the University of Cambridge / 2004 / iii / Paper 36 / 1 / b / Solution

The congruence axiom requires that an affordable alternative indirectly revealed at least as desirable as a chosen alternative is itself chosen. In the notation of part (a),Equivalently, a chosen bundle $x$ and an affordable but unchosen bundle $y$ satisfy $\neg (yRx)$. Since $x{R}_{D}y$ already holds, this means $xPy$. The axiom therefore distinguishes a genuine strict preference for a chosen bundle from indifference between two chosen bundles; it applies to the entire demand correspondence, including budgets with several chosen alternatives.