Nexing Seminar
special issue on the occasion of ENFA1

The Logic of Thinking

Coimbra, 2002, May 18

University of Coimbra
Faculty of Letters

NEXING is a multi-disciplinary project fostering the convergence of methods, results and expertise from Informatics, Applied Logic, Cognitive Psychology and Formal Linguistics with respect to the processing of natural language and negation.

Under the scope of the project activities, the following seminar is going to take place at the University of Coimbra.

Invited Speaker

Pieter Seuren
(Max Planck Institute for Psycholinguistics, Nijmegen)


The Logic of Thinking


Traditional logic derives partly from Aristotle and partly from Stoic philosophy. Aristotle created predicate calculus and syllogistics; the Stoa gave us propositional calculus. After +-1850 it gradually became clear (a) that Aristotelian predicate calculus suffers from a few serious logical defects, and (b) that syllogistics is merely an application of standard set theory (Boolean algebra), and is thus superfluous as a separate branch of logic. Modern (Russellian) logic took over propositional calculus from the Stoics, dropped syllogistics, and replaced Aristotelian predicate calculus with modern predicate calculus. All in all, Aristotle had been redundant.

The problem, however, is that the logically faulty Aristotelian predicate calculus reflects natural semantic intuitions much better than modern predicate calculus, which may be logically correct but is empirically deficient for the semantic analysis of natural language. What actually happened over the past fifty years is that modern formal semantics adopted standard Russellian predicate calculus and decided to rely on the new discipline of pragmatics to polish away the discrepancies with natural intuitions. In order to do so, pragmatics looks for explanations in the functionality and dynamism of human linguistics interaction. No-one thought of looking into logic itself and subjecting it to closer scrutinity. Logic was, and still is, looked upon as one, indivisible and unassailable.

It is the purpose of the present paper to show this to be a myth. A logic is defined by the semantic definitions of the logical constants that occur in it, and it is very well posssible to devise alternative logics by varying on those definitions. It will be shown how some simple modifications deliver a small set of non-standard but sounds logics which preserve most of the advantages of Aristotelian logic. The variety that accounts best for natural intuitions will then count as the best approximation to the 'logic of thinking' and thus to 'the logic of language'. Pragmatics is thus no longer needed as a source of explanation for the phenomena at issue. But one must get used to the idea that the logic of thinking and thus of language almost certainly differs in important respects from the logic wich mathematics is best served.

Date and time

9-10 am, 2002, May 18.


The seminar is going to take place at the Faculty of Letters of the University of Coimbra (room announced at the main entrance).


Attendance of the seminar is free of charge and open to everyone.


ENFA1-Primeiro Encontro Nacional de Filosofia Analítica" is a conference in the area of Analytical Philosophy to be held in Coimbra, 2002, May 17-18. This meeting is meant to be the kick-off initiative of the future Portuguese Society of Analytical Philosophy. To join the celebration of this event, in association with the organizing committee, the  NEXING project is offering this seminar as one of the plenary sessions of ENFA1.


The NEXING project is funded by FCT under the contract FCT/SAPIENS99/34076/99.