 With respect to the justification question, arguments have been made in both directions. Scientists are often inclined to justify simplicity principles on broadly inductive grounds. According to this argument, scientists select new hypotheses based partly on criteria that have been generated inductively from previous cases of theory choice. Choosing the most parsimonious of the acceptable alternative hypotheses has tended to work in the past. Hence scientists continue to use this as a rule of thumb, and are justified in so doing on inductive grounds. One might try to bolster this point of view by considering a counterfactual world in which all the fundamental constituents of the universe exist in pairs. In such a ‘pairwise’ world, scientists might well prefer pairwise hypotheses in general to their more parsimonious rivals. This line of argument has a couple of significant weaknesses. Firstly, one might legitimately wonder just how successful the choice of parsimonious hypotheses has been; examples from chemistry spring to mind, such as oxygen molecules containing two atoms rather than one. Secondly, and more importantly, there remains the issue of explaining why the preference for parsimonious hypotheses in science has been as successful as it has been.