Experience and Prediction (1938) was Hans Reichenbach's major epistemological treatise¹ He regarded it as his refutation of logical positivism. The main theme of this book — the foundation for his critique of positivism — is his thoroughgoing probabilism. It is interesting to note that in 1933 Reichenbach published a glowing review of Rudolf Carnap's Der Logische Aufbau der Welt, his only criticism being an inability to see how probability could fit into the picture. In the first chapter of Experience and Prediction he presents his version of the verifiability theory of cognitive meaning. Instead of demanding, as the logical positivists had, the conclusive verifiability of a sentence if it is to qualify as cognitively meaningful, he insists upon probabilistic verifiability. A sentence is cognitively meaningful if and only if it is physically possible to have empirical evidence in terms of which a probability (more accurately, a weight ²) can be assigned to it.³