 Finally, it is worth noticing that Nagel introduces a distinction between arbitrary and non-arbitrary reductions: These criteria are “non-formal” (1961: chap. 11, Sec. III). The first criterion Nagel mentions is that the premises of a reduction—the bridge laws and the reducing theory—should be well established rather than arbitrarily chosen (1961: 358). Second, Nagel alludes to the fact that the reducing theory should be better established than the reduced one. The third criterion states that reduction is concerned with unification. The fourth criterion states that the reducing theory corrects and augments the reduced theory.