 All empirical claims are, according to Reichenbach, probabilistic judgments based on relative frequencies in a reference class, or reached by induction. To the extent that reference classes rely on kinds, Reichenbach resorts to psychology: primarily, things are sorted into kinds by the immediate perception of similarity, or by similarity in memory. Less primitively, theory and convention guide the determination of kinds. In practice he recommends the choice of the narrowest reference class for which there are adequate statistics, a recommendation that is of no help (why, or why not, should we throw various astrological theories into the reference class for assessing the prior probability of general relativity?). 