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The paper argues, that a direct formalization of the way common sense thinks about the numerical identity of enduring entities, requires that traditional predicate logic is developed. If everyday language mirrors the world, then persons, organisms, organs, cells, and ordinary material things can lose some parts but nonetheless remain numerically exactly the same entity. In order to formalize this view, two new logical operators are introduced; and they bring with them some non-standard syntax. One of the operators is called ‘the instantiation operator’; it is needed because the existential quantifier and its traditional relatives cannot do the job required. The other operator is called ‘the form-on-matter operator’, and it allows an individual (an instance of a form) to stay the same even though some of its parts (its constituting matter) is taken away from it. Also, a certain kind of predicates, called ‘nature terms’, is needed in order to represent what gives a particular its kind of identity. Both the operators and the nature terms introduced can be used in constructions of formal languages and formal systems, but no such constructions are made in the paper. The paper is structured as a comment on the philosophical problem called ‘the problem of the cats Tibbles and Tib’.

DOI: 10.1007/s11229-007-9199-0

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