A Critical Study of Western Rationalists’ Theories of Natural Law in the Modern Period (Montesquieu, Rousseau, and Kant)

Among the various interpretations of natural law, the most favorite of them states that it refers to the orders of practical intellect regarding Man’s voluntary behaviors in all places and at all times that lead to permanent happiness if obeyed by human beings. The theory of natural law in the modern period has received two opposing empirical and rationalist interpretations. By reason, modern rationalism means calculating reason, which is viewed as a tool for attaining material and immaterial (moral) wishes. Montesquieu, Rousseau, and Kant were three rationalist philosophers of the Age of Enlightenment who discussed and theorized about natural law. In this paper, after a brief account of their theories on natural law, the author evaluates them one by one. In the first section, the author argues that Montesquieu, by posing a self-made myth, states that following natural desire leads Man to happiness. This act of following in his view implies natural law. The most important criticism of Montesquieu’s theory is that he has confused the natural law with the law of nature. In the second section, the author argues that, unlike Montesquieu, Rousseau believes that natural law is not based on the reason but, rather, on human instincts and feelings. The basic problem of this theory is his material approach to human nature, which lowers Man to the level of animals. Finally, the third section presents a critical investigation of Kant’s natural law. In his view, natural law is different from the law of nature. Kant believes that natural law enjoys two characteristics: universality and intrinsicness. However, he has not referred to any of the applications of natural law and has failed in providing a complete explanation of this theory. This failure is rooted in the epistemological system of Kant’s philosophy, based on which the practical wisdom and its orders (or the same natural law) must be deemed unfounded and unreliable. Kant maintains that the issues related to immaterial and even material substances are polemic rather than demonstrative in nature. Similar to other critical studies, the present study was conducted following a mixed narrative-intellectual method. Accordingly, the views of the three rationalist philosophers of the modern period are initially explained and then examined and evaluated based on rational arguments and reasoning.