ABSTRACT
You are already familiar with the fact that Substitution of Equivalents is a validity-preserving operation. That is to say, if we replace a well-formed part of some valid wff by a wff equivalent to the part, the resulting wff is also valid. You will recall that this replacement need not be carried out uniformly: that is, if a wff X occurs more than once as a part of a wff Y, we may replace any of the occurrences of X by some equivalent wff without replacing the others.
