ABSTRACT
Everyone knows that Formal Logic has something to do with argument or reasoning. An inference is a valid deductive inference if and only if the conclusion logically follows from the premiss(es); that is, if and only if the truth of the permiss(es) gives an absolute guarantee of the truth of the conclusion. It can be seen that a valid inference is a sound inference, and that the heart of this notion of validity is that the combination of true premisses and false conclusion is ruled out. An inferential schema is, as it were, a pattern for a set of inferences, and we extend the notion of validity to cover such a schema by requiring that every inference constructed on that pattern should be valid. The classification of propositional schemata is analogous to that of propositions. A propositional schema may be either valid, contingent or inconsistent.
