We have seen from our work on Horn clauses and procedural systems in previous chapters that the concept of an if-then conditional or rule—if P is true then Q is true—is central to knowledge representation. Whereas the semantics of the logical formula (P image Q) is simple and clear, it suggests that a rule of this sort is no more than a form of disjunction: Either P is false or Q is true. However, as we saw in Chapter 6, from a reasoning point of view we can look at these rules in different ways. In particular, a rule can be understood procedurally as either

moving from assertions of P to assertions of Q, or

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