It has already been suggested that, in the history of the concept of the natural law since the thirteenth century, the rationalist natural law of the seventeenth and eighteenth centuries presents an important turning-point. For a number of reasons, not least because it is closer to our time, the natural law of the Age of Reason has usurped the attention of historians of thought and has tended to obscure their vision of the natural law of the scholastic revival of the sixteenth and seventeenth centuries and, possibly even more so, their vision of the natural law of the High Scholasticism of the thirteenth century. More serious still, the rationalist natural law proved extremely vulnerable to criticism, particularly from the historical schools of the nineteenth century; and the impression is sometimes conveyed that refutations of this natural law amount to disposing of the concept purely and simply. Nothing could be further from the truth. To see that such is the case, we must briefly examine the rationalists' natural law.