The language of FOL is sometimes criticized as being an overly “static” representation formalism. Sentences of FOL are either true or false in an interpretation and stay that way. Unlike procedural representations or production systems, there is seemingly nothing in FOL corresponding to any sort of change.

In fact, there are two sorts of changes that we might want to consider. First, there is the idea of changing what is believed about the world. Although we will not dwell on it here, this is a very important aspect of real-world reasoning. Suppose α is a sentence saying that birds are the descendants of dinosaurs. At some point you might come to believe that α is true, perhaps by being told directly. If you had no beliefs ...

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