 Reichenbach extends the analysis to error probabilities for physical measurements. He then argues that since all physical measurements are subject to error, knowledge of natural laws is possible only if errors occur subject to a probability distribution, a proposition that is synthetic but cannot be established empirically. Empirical knowledge thus requires both an a priori principle of causality for individual events and an a priori principle of probability to ensure that individual events can be aggregated into general laws. No explicit interpretation of probability is forwarded, although Reichenbach implies that probability claims are about frequencies of causally independent events—a notion for which probability theorists would later substitute the idea of independent, identically distributed events (see the entry on  probabilistic causation)—  in observed and unobserved collections of cases. 