Validity and indexicality in Buridan´s concept of logical consequence

MANUEL DAHLQUIST

Los Angeles

Simposio; Inaugural Pan-American Symposium on the History of Logic: Validity throughout History; 2019

University of California Los Angeles (UCLA)

The medieval logicians of the fourteenth century distinguish different types of consequences. The basic or more general concept, is identified with what we now call the pre-theoretical concept of logical consequence, which usually is expressed as follows: If the premises are true it is impossible for the conclusion to be false.This work deals with the criticisms that Buridan makes to this concept of valid consequence that he has received from tradition. Buridan built inferences whose validity can not question but are not logically valid, showing that the concept of valid consequence, characterized in terms of modality and truth, is incapable of capturing inferences that we would like to consider valid. In other words, it shows that modality and truth are insufficient as conceptual bases of the concept of consequence.At least two important interpretations have been made, both of the causes and of the end of Buridan´s criticisms (Klima, 2004; Dutilh Novaes, 2005). I am going to propose a different one, different from the previous two: I will argue that the consideration of time as a fundamental component of both the logical language and the inferences, made Buridan consider a new type of validity; a type of validity recently called contextual validity (Blackburn and Jorgensen, 2013), with respect to which, classical validity is a proper subset. This kind of validity -contextual validity- is directly linked to temporal indexical propositions and this indexicals captured Buridan´s interest for the same reasons that caught the interest of Kaplan and Kamp in the second half of the 20th century: indexicals are interesting because they give rise to a new species of validity, and Buridan was the first to discover it!