 To see what can go wrong, let us consider the (open) formula \(\forall x(Rxy\land Rxr)\), where \(R\) is a two-place predicate, \(r\)  is a name constant abbreviating “Russell” and \(\land\) is conjunction.  \(\forall x(Rxy\land Rxr)\) contains a free occurrence of \(y\) (that is, \(y\) is a free variable of the formula), however, \(y\) is not free for a substitution of a term that contains a free occurrence of \(x\), for instance, \(x\) itself. More formally, the occurrence of \(y\) in the second argument place of \(R\) in  \(\forall x(Rxy\land Rxr)\) is not bound by a quantifier (the only quantifier) of the formula, whereas \(\forall x(Rxx\land Rxr)\) is a closed formula, that is, it contains no free occurrences of variables.  Informally, the following natural language sentences could be thought of as interpretations of the previous formulas. “Everybody reads him and Russell,” (where ‘him’ is deictic, or perhaps, anaphoric) and “Everybody reads himself and Russell.” Obviously, the meanings of the two sentences are vastly different, even if we assume that everybody pens something. As a contrast, \(\forall x(Rxw\land Rxr)\) exhibits an unproblematic substitution of the name constant \(w\) for the free occurrence of \(y\). (The latter formula, perhaps, formalizes the sentence “Everybody reads Ludwig Wittgenstein and Russell.”) These examples are meant to demonstrate the more complex part of the problem Moses Schönfinkel set out to solve, and for what he invented  CL.[1]