This essay examines two interlinked efforts in early modern Europe and New England to distinguish legal provisions valid across different societies and time periods from those that were local and transitory and therefore not compulsory in the present. Consider, first, the judicial laws of Moses. A minority of Protestants, whom I will call the “Mosaic legalists,” tried to ascertain which Old Testament judicial ordinances were no longer obligatory because they were particular to the Jewish commonwealth, and which were eternally-valid “appendices” to the natural law and Decalogue. The challenge of differentiating the perpetual from the local also occupied early modern students of the law of nature. Whether one believed that God impressed natural law upon the world or that people deduced natural law from a limited set of first principles such as self-preservation and sociability, one faced the problem of distinguishing immutable natural precepts from rules that arose only to address passing issues in a specific territory. Natural lawyers and Mosaic legalists did not use the same techniques for separating eternal and transient precepts. Each had its own “rules of recognition” (to borrow a helpful modern concept). My essay compares natural lawyers’ and Mosaic legalists’ rules of recognition, their ways of telling immutable from transitory precepts. In this, it goes against the dominant tendency of modern scholarship to approach the Mosaic legalists and natural lawyers separately rather than in tandem. Reading the two in unison highlights how the projects faced common intellectual challenges. In particular, both natural lawyers and Mosaic legalists used stadial theories of historical change and both implied that immutable law should be convenient under present circumstances. Both used temporally- and contextually-sensitive categories to locate an “eternal” law. But these emphases, if pushed too far, threatened the natural lawyers’ and Mosaic legalists’ enterprise, so both groups developed stabilizing conventions. The rules of recognition mattered greatly as controversialists deployed natural and Mosaic law in fights over political oaths and allegiance, usury, sabbatarianism, church governance, and a wide range of other issues. Over and again, early modern actors asked the perennial question: What was God, through scripture and nature, commanding us to do? The Mosaic legalists’ and Protestant natural lawyers’ rules of recognition helped answer this most pressing of questions.