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In the framework of set theory we cannot distinguish between natural and non-natural predicates. To avoid this shortcoming one can use mathematical structures as conceptual spaces such that natural predicates are characterized as structurally ‘nice’ subsets. In this paper topological and related structures are used for this purpose. We shall discuss several examples taken from conceptual spaces of quantum mechanics (‘orthoframes’), and the geometric logic of refutative and affirmable assertions. In particular we deal with the problem of structurally distinguishing between natural colour predicates and Goodmanian predicates like ‘grue’ and ‘bleen’. Moreover the problem of characterizing natural predicates is reformulated in such a way that its connection with the classical problem of geometric conventionalism becomes manifest. This can be used to shed some new light on Goodman's remarks on the relative entrenchment of predicates as a criterion of projectibility.

DOI: 10.1007/BF01064589

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