The crucial idea here is expressed in this paragraph from section 7.2 "Inference and Protologic" on page 141:



"The first type of reasoning can be described as reasoning from an excluded alternative. This is the type of inference that takes a creature from recognition that one of an incompatible pair of states of affairs holds to the recognition that the other does not hold. Here is an example. Imagine a creature that has learned that the lion and the gazelle will not be at the watering-hole at the same time and, moreover, is in a position to see that the gazelle is drinking happily at the watering-hole. The creature can conclude with confidence that the lion is not in the vicinity. This type of reasoning is one of the ways a creature can learn about what is not immediately perceptible. One can see easily, for example, how this sort of inference could be life-preserving for a creature who is just as threatened by the lion as the gazelle is. It is natural to formalize it in the propositional calculus as an instance of disjunctive syllogism (the transition from "A or B" and "not-A" to "B," where A stands for "The gazelle is not at the water-hole" and B for "The lion is not at the water-hole.)"



From an observation of this kind of fact about the world, it is possible to appreciate how humans have come to reason. And this kind of knowledge is precious here.



This analysis nicely complements Robert Hanna's explication of protologic in _Rationality and Logic_(2006).

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