That means that we need truth-conditions for the constant elements in each formula, the elements which are unchanged through all its various 'interpretations'. So soundness depends on truth-conditions of constants. This is something that has come to consciousness in twentieth-century logical theory, but was implicit all along. Besides soundness, logical theory is concerned with other properties of logical systems, among them completeness, which is the ability of a system to generate everything that is, according to a given set of truth-conditions, valid or a consequence. Evaluation goes by the rules of the logical system, provided they are sound, and is sometimes wholly mechanical.
Man is a species.
Socrates is a man. So Socrates is species. The twentieth century has seen a strong revival of interest in these pitfalls, whose existence is a large part of the reason why in the first half of the century logic seemed to analytic philosophers to lie at the centre of their subject. Here are a few more examples. The President of New York is or is not black.