Reichenbach's cubical universe and the problem of the external world

This paper is a sympathetic critique of the argument that Reichenbach develops in Chapter 2 of Experience and Prediction for the thesis that your sense experience justifies your belief in the existence of an external world. After discussing his attack on the positivist theory of meaning, I describe the probability ideas that Reichenbach presents. I argue that Reichenbach begins with an argument grounded in the Law of Likelihood but that he then endorses a different argument that involves prior probabilities. I try to show how this second step in Reichenbach’s approach can be strengthened by using ideas that have been developed recently for understanding causation in terms of the idea of intervention. Chapter 2 of Reichenbach’s Experience and Prediction has two big themes, one negative and one positive. The negative theme is Reichenbach’s attack on the positivist theory of meaning. The positive theme is Reichenbach’s attempt to show why your sense experience justifies your believing that material objects exist independent of experience. The two themes are connected, in that the negative is a necessary prolegomenon to the positive. This is because the positivist theory of meaning entails that solipsism and realism about material objects are equivalent rather than incompatible. If the positivists were right about this, the problem of the external world would be a pseudo-problem – something to dissolve, not solve. My subject here is Reichenbach’s epistemology, but I do have a few comments on what he says about meaning. In Chapter 1, Reichenbach (pp. 30-31) formulates the positivist theory of meaning as having two parts: (1) A sentence has meaning if and only if it is verifiable as true or false. (2) Two sentences have the same meaning if and only if they obtain the same determination as true or false by every possible observation. 1 I thank Martin Barrett, David Chavez, Juan Comesaña, Richard Creath, Clark Glymour, Guven Guzeldere, Daniel Hausman, John Koolage, Flavia Padovani, Alan Richardson, Carolina Sartorio, Friedrich Stadler, Michael Stoelzner, Michael Strevens, Steve Swartzer, and James Woodward for useful comments. I also have benefitted from discussing this paper at the 2008 Reichenbach conference at Bogazici University in Istanbul and at the University of Nebraska, Lincoln. 2 Reichenbach speaks of “propositions” in (1) and of “sentences” in (2); I have made his formulation uniform.