
Introduction to Logic

Tools for Thought

As with Propositional Logic, we can demonstrate logical entailment in Relational Logic by writing proofs. As with Propositional Logic, it is possible to show that a set of Relational Logic premises logically entails a Relational Logic conclusion if and only if there is a finite proof of the conclusion from the premises. Moreover, it is possible to find such proofs in a finite time.

The Fitch system for Relational Logic is an extension of the Fitch system for Propositional Logic. In addition to the ten logical rules of inference, there are five new rules of inference. In the next section, we introduce two rules of inference for universally quantified sentences. We then introduce two rules of inference for existentially quantified sentences. After that, we describe a new type of rule called Domain Closure. Finally, we illustrate the system with a few examples.

If you are like me, the prospect of going through a discussion of so many rules of inference sounds a little repetitive and boring. However, it is not so bad. Each of the rules has its own quirks and idiosyncrasies, its own personality. In fact, a couple of the rules suffer from a distinct excess of personality. If we are to use the rules correctly, we need to understand these idiosyncrasies.

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