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- The distinction between logical and extra-logical vocabulary should be explained. In a first approach, a notion is called 'logical' if it applies to sentences only. E.g. the connective 'x and y' joins two sentences x, y and hence is logical vocabulary, while 'x is human' applies to a real-world object x and hence is extra-logical. However, a quantifier like 'each x satisfies y' is usually considered as logical vocabulary although it applies to a real-world object x and a sentence y. (en)
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