 A student of Reichenbach (1938, 1949), Salmon had adopted and defended the limiting frequency interpretation of physical probability, where “\(P(G/F) = r\)” means that \(G\) occurs with a limiting frequency \(r\) in a reference class of instances of \(F\), which has to be infinite for limits to exist (Salmon 1967, 1971). On this approach, a property \(H\) is explanatorily relevant to the occurrence of an attribute \(G\) within a reference class \(F\) just in case (SR):