G. Frege did not consider existence to be a property of things, but rather a property of concepts. The statement that ‘people exist‘ would then mean that the concept ‘person’ is (with regard to its extension) not void. Existence is, on this conception, a predicate of the second order. A similar theory is to be found in B. Russell. In a language of the first order, existence is expressed by an independent predicate, but it can be implied by the use of an existential quantifier. W. V. O. Quine taught that “to be means to be the value of a variable” to which it is bound by an existential quantifier. Carnap reacted, however, by saying that existence understood in this way just means the possibility of being proven in the framework of a given system: “to be means to be the element of a system”. Carnap thus showed that the existential quantifier indicates only instantiation in a universe of discourse, not real existence or being in the “metaphysical” sense of the word. If we should like to introduce the missing predicate of existence we must meet Frege’s rightful demand that the predicate be classified with the subject to which it is attributed as a certain class of thing, and thus that it be distinguished from other things which do not belong to the given class. After all, the attribution of predicates which belong to all things is uninformative (e.g. the attribution of self-identity). Now: from what do we distinguish things by attributing to them existence? From fictions, as the followers of A. Meinong suggested? Fictional things are always conceived incompletely: we can untrivially deny existence not to fictions themselves, but to the corresponding complete things which are possible. The universe of existential reflection has two sub-groups: the class of real things, and potential or merely possible things. Actual entities are in actual being, potential entities are in potential being. To have “potential being” means not to have, and at the same time to be able to have, actual being. By attributing, or denying, existence we distribute things into the two sub-groups mentioned. The universe of non-trivial existential reflection is made of things which can be considered as at least possible: this means abstractable subjects, that is such which can be abstracted from whatever being (actual and potential).
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