 Used without restriction, such rules are obviously unsound: they would allow one to reason from the premise \(\exists x(A(x))\) to the conclusion \(\forall x(A(x))\) in two steps! The rules are therefore hedged with restrictions on the particular instantial terms to be used. For a start, one cannot use the same instantial term in two distinct quantifier instances in the same derivation, but by itself this is not sufficient. Precise formulation of restrictions that will yield a sound and complete system of rules is not trivial: Copi’s textbook ((1954)) went through several editions with different incorrect systems, and Quine (1950a) changed the wording of the restriction between editions of his textbook (though both versions give correct systems). Several articles were published, both in specialist logic journals and in general philosophical ones, noting errors and offering analyses of the situation.