T HE D ISAGREEMENT B ETWEEN S CHLICK AND R EICHENBACH ON K ANTIAN P HILOSOPHY AND C ONVENTIONALISM

3. T HE D ISAGREEMENT B ETWEEN S CHLICK AND R EICHENBACH ON K ANTIAN P HILOSOPHY AND C ONVENTIONALISM

This divergence of opinions on the object of knowledge refers back to their well- known disagreement regarding the way in which they rejected Kantian synthetic

a priori judgements. Reichenbach distinguished between two different meanings of the concept of

a priori: 1) a priori in the sense of “‘necessarily true’ or ‘true for all times’”, and thus of absolutely independent of experience; 2) a priori, in the sense of “‘con- stituting the concept of object’”, that is, the principle that forms the object of sci- entific knowledge, more specifically of physics. The theory of relativity shows

the indefensibility of the first notion of a priori, but not of the second 5 . On the contrary, since the years of the first edition of Allgemeine Erkenntnis- lehre, Schlick did not seem willing to concede anything to Kantianism. On this point he criticizes both Cassirer and Reichenbach, and maintains that the essen- tial characteristic of the Kantian synthetic a priori is the connection, in the very same concept, between the apodeictic certainty and the constitutive value. Con- sequently, he does not make a distinction between the two meanings of Kantian synthetic a priori, thereby avoiding its adoption in any of its two senses. The more general and abstract component of scientific knowledge should be assimi- lated not in Kantian constitutive principles (even if devoid of any apodeictic validity) but to very general hypotheses or to conventions as defined by

Poincaré 6 . And if we ask ourselves what ‘convention’ in Poincaré’s sense means, the only sensible answer we can give is that we must be talking about

conventions of a linguistic-definitional nature – conventions the task of which consists in establishing scientific language regarding the formulation of both scientific facts and laws. Poincaré explicitly states this thesis in his pages on geometry and in his polemic against Eduard Le Roy’s radical conventionalism (“nominalism”). 7

The terms of agreement and disagreement between Schlick and Reichenbach in the 1920s are now clear. They both took as their starting point a general inter- pretation of knowledge intended as coordination and the negation of Kant’s theory of the synthetic a priori. Within this very same framework, though, Schlick states that in order to understand cognitive coordination we need admit only the existence of linguistic conventions, in full conformity with his idea that in the cognitive process we have a designation of objects and facts by means of suitable concepts and judgements on the basis of previously fixed conventions. According to Reichenbach cognitive coordination can be understood only if we admit the existence of coordination principles that constitute the objects of knowledge and that, as such, have to be assimilated to Kant’s synthetic a priori principles in all features excepting that of their apodeictic validity.

O N T HE F ORMATION OF L OGICAL E MPIRICISM 13

The terms of their disagreement were thus as profound and strong as the terms of their agreement. Despite this, in the early 1920s both authors gradually developed a more similar perspective that aimed especially at defending – in

contrast to Kant and Cassirer’s neo-Kantianism – ‘the contingent nature of the cognitive synthesis’, in other words, the idea – related to the thesis of the inde-

pendence of perception of reason – that the very same possibility of coordinating concepts to experiences and obtaining a unique coordination (truth) depends on experience. This possibility has been proven by now, but we cannot demonstrate

a priori its necessity 8 . All this is indicated by the numerous letters they ex- changed on this topic between September and December 1920, as well as by the conclusive comment on their discussion made by Reichenbach in “Der gegen- wätige Stand der Relativitätsdiskussion”. 9