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There are certain cases in which one is tempted to use expressions of the form “ a = a ” or “ p ⊃ p ”. As, for instance, when one would speak of the archetype Proposition, Thing, etc. So Russell in the Principles of Mathematics has rendered the nonsense “ p is a proposition” in symbols by “ p ⊃ p ” and has put it as hypothesis before certain propositions to show that their places for arguments could only be occupied by propositions.