 Suppose that there are ten different kinds of fundamental particles. So, there are fifty-five possible kinds of two-particle interactions. Suppose that fifty-four of these kinds have been studied and fifty-four laws have been discovered. The interaction of X and Y particles have not been studied because conditions are such that they never will interact. Nevertheless, it seems that it might be a law that, when X particles and Y particles interact, P occurs. Similarly it might be a law that when X and Y particles interact, Q occurs. There seems to be nothing about the local matters of particular fact in this world that fixes which of these generalizations is a law (Tooley 1977, 669).