The paper is a brief survey of some sequent calculi which do not follow strictly the shape of sequent calculus introduced by Gentzen. We propose the following rough classification of all SC: Systems which are based on some deviations from the ordinary notion of a sequent are called generalised; remaining ones are called ordinary. Among the latter we distinguish three types according to the proportion between the number of primitive sequents and rules. In particular, in one of these types, called (...) Gentzen’s type, we have a subtype of standard SC due to Gentzen. Hence by nonstandard ones we mean all these ordinary SC where other kinds of rules are applied than those admitted in standard Gentzen’s sequent calculi. We describe briefly some of the most interesting or important nonstandard SC belonging to the three abovementioned types. (shrink)

I argue for the thesis (UL) that there are certain logical abilities that any rational creature must have. Opposition to UL comes from naturalized epistemologists who hold that it is a purely empirical question which logical abilities a rational creature has. I provide arguments that any creatures meeting certain conditions—plausible necessary conditions on rationality—must have certain specific logical concepts and be able to use them in certain specific ways. For example, I argue that any creature able to grasp theories must (...) have a concept of conjunction subject to the usual introduction and elimination rules. I also deal with disjunction, conditionality and negation. Finally, I put UL to work in showing how it could be used to define a notion of logical obviousness that would be well suited to certain contexts—e.g. radical translation and epistemic logic—in which a concept of obviousness is often invoked. (shrink)

How are social and institutional circumstances linked to the knowledge that scientists produce? To answer this question it is necessary to take risks: speculative but testable theories must be proposed. It will be my aim to explain and then apply one such theory. This will enable me to propose an hypothesis about the connexion between social processes and the style and content of mathematical knowledge.

This paper deals with Popper's little-known work on deductive logic, published between 1947 and 1949. According to his theory of deductive inference, the meaning of logical signs is determined by certain rules derived from ?inferential definitions? of those signs. Although strong arguments have been presented against Popper's claims (e.g. by Curry, Kleene, Lejewski and McKinsey), his theory can be reconstructed when it is viewed primarily as an attempt to demarcate logical from non-logical constants rather than as a semantic foundation for (...) logic. A criterion of logicality is obtained which is based on conjunction, implication and universal quantification as fundamental logical operations. (shrink)

Karl Popper developed a theory of deductive logic in the late 1940s. In his approach, logic is a metalinguistic theory of deducibility relations that are based on certain purely structural rules. Logical constants are then characterized in terms of deducibility relations. Characterizations of this kind are also called inferential definitions by Popper. In this paper, we expound his theory and elaborate some of his ideas and results that in some cases were only sketched by him. Our focus is on Popper's (...) notion of duality, his theory of modalities, and his treatment of different kinds of negation. This allows us to show how his works on logic anticipate some later developments and discussions in philosophical logic, pertaining to trivializing connectives, the duality of logical constants, dual-intuitionistic logic, the conservativeness of language extensions, the existence of a bi-intuitionistic logic, the non-logicality of minimal negation, and to the problem of logicality in general. (shrink)

Simon Evnine examines various epistemic aspects of what it is to be a person. Persons are defined as finite beings that have beliefs, including second-order beliefs about their own and others' beliefs, and are agents, capable of making long-term plans. It is argued that for any being meeting these conditions, a number of epistemic consequences obtain. First, all such beings must have certain logical concepts and be able to use them in certain ways. Secondly, there are at least two principles (...) governing belief that it is rational for persons to satisfy and are such that nothing can be a person at all unless it satisfies them to a large extent. These principles are that one believe the conjunction of one's beliefs and that one treat one's future beliefs as, by and large, better than one's current beliefs. Thirdly, persons both occupy epistemic points of view on the world and show up within those views. This makes it impossible for them to be completely objective about their own beliefs. Ideals of rationality that require such objectivity, while not necessarily wrong, are intrinsically problematic for persons. This "aspectual dualism" is characteristic of treatments of persons in the Kantian tradition. In sum, these epistemic consequences support a traditional view of the nature of persons, one in opposition to much recent theorizing. (shrink)

Resumo: Pedro Hispano define os sincategoremas como expressões que revelam de que maneira os sujeitos e os predicados estão de fato relacionados nas proposições, contribuindo assim para o estabelecer o que elas significam e fixar as condições de verdade e as formas lógicas correspondentes. Entre as expressões que ele julga serem sincategoremáticas, ‘não’, ‘e’, ‘ou’, ‘se’, ‘todo’ e ‘necessário’ se destacam atualmente como constantes lógicas. Todavia, opondo-se a grande parte dos lógicos contemporâneos para quem tais expressões possuem um significado fixo (...) na medida em que integram as formas lógicas das respectivas proposições, Pedro Hispano admite que seu significado pode ser modificado quando a expressões do mesmo tipo credita a capacidade de atuarem em determinados contextos como categoremas. Além disso, ao defender que as expressões ora em questão explicitam as formas lógicas das proposições mediante a articulação dos categoremas a elas associados, ele também demonstra divergir dos principais critérios de demarcação das constantes lógicas atualmente vigentes que não preveem a atribuição de tal comportamento a expressões desse tipo. Portanto, não resta dúvida de que a teoria de Pedro Hispano sobre os sincategoremas enriquece a nossa compreensão ainda insuficiente da natureza das constantes lógicas e contribui para a resolução do problema da demarcação de tais expressões.: According to Petrus Hispanus, syncategoremata are expressions that determine how subjects and predicates are actually related in propositions. They contribute to establishing what the categoremata mean and to specifying the truth conditions of the corresponding logical forms. Among the expressions Peter of Spain thinks of as syncategorematic, ‘not’, ‘and’, ‘or’, ‘if’, ‘all, and ‘necessary’ are nowadays considered to be logical operators. But unlike the contemporary logicians who argue that these expressions have fixed meanings because they belong to the logical forms of the corresponding propositions, Peter of Spain allows that their meanings can be modified, attributing to them the ability to act in certain contexts as categoremata. In addition, he argues that these expressions make explicit the logical forms through the articulation of corresponding propositions’ categoremata; thus he also diverges from the main contemporary criteria of demarcation of logical constants, which does not allow that these expressions have such behavior. Therefore, there is no doubt that Hispanus’ theory of the syncategoremata enriches our still insufficient understanding of the logical constants and contributes to the resolution of the problem of the demarcation of such expressions. (shrink)