35This implements the *indiscernability of identicals* principle, if two terms are equal then they have the same properties. In other words, if ``x=y``, then we can substitute ``y`` for ``x`` in any expression. In our proofs we can express this as:
35This implements the *indiscernability of identicals* principle, if two terms are equal then they have the same properties. In other words, if ``x=y``, then we can substitute ``y`` for ``x`` in any expression. In our proofs we can express this as: