Brentano's reform of logic

pp. 41-69

in: Peter Simons, Philosophy and logic in Central Europe from Bolzano to Tarski, Berlin, Springer, 1992

Abstract

This forgotten reform of the logic of terms is based on Brentano's theory of judgement, according to which the basic form of judgement is an affirmation or denial of existence. Brentano uses term—conjunction and —negation, and recognizes several logical relations among concepts (terms). While, like Boole, Brentano abolishes the requirement of existential import, the reform extends beyond this to the rules of inference, which allow syllogisms and other inferences to be elegantly derived. By treating propositions as fictions, following a suggestion of Brentano, and employing Brentano's rules with minor extensions, it is possible to develop a propositional logic within the term logic. The algebra of Brentano's logic, which interestingly mixes intensional and extensional components, is reconstructed. While the algebra of extensions of concepts is of course Boolean, concepts themselves form a quasi—Boolean algebra.