  After participating in the pantheism controversy, Rehberg made his mark in other philosophical controversies of his day.  In the early 1790s he wrote several reviews for the Allgemeine Literatur Zeitung of Karl Leonhard Reinhold's Elementarphilosophie, which had been at the centre stage of the new debates surrounding Kant's critical  philosophy.[27]  Reinhold's Elementarphilosophie was a radical form of foundationalism, whose basic aim was to derive all the results of Kant's philosophy from a single self-evident first principle.  The crucial question for Reinhold's contemporaries concerned the very possibility of such foundationalism.  Is it possible to found all knowledge upon a single first principle?  Given Rehberg's skepticism, it should not be surprising that he took a negative stand on this basic question.  In his January 1791 review of Reinhold's Beyträge zur Berichtigung bisheriger Missverständnisse der Philosophen, the chief exposition of the Elementarphilosophie, Rehberg sketched his misgivings about foundationalism.  It is striking that his chief objections are strictly Kantian, coming straight out of the Kritik der reinen Vernunft itself.  Rehberg implies that Reinhold is guilty of ignoring Kant's own strictures about the limits of knowledge; it is as if he were insinuating that Reinhold would better have heeded Kant's advice to limit himself to popularizing the critical philosophy.  The problem with Reinhold's philosophy, Rehberg argues, is that it begins with definitions, ignoring Kant's warnings that definitions should end rather than begin philosophy.  Reinhold neglected Kant's teaching that there is a fundamental difference in kind between the methods of mathematics and those of philosophy. While mathematical definitions can construct their objects in intuition, philosophical definitions have no such advantages.  What we derive from a definition depends on how we interpret it, on what we read into it in the first place; and there are all kinds of interpretations of the same definition, so no basis of unanimity should be expected simply by beginning with a definition, no matter how simple and unproblematic.  The chief theme of Rehberg's review—and indeed the leitmotif of all his political thought— is contained in his remark that “a definition need not contain a criterion of that which falls under it” (202). Rehberg develops this remark so that it becomes an objection against any attempt to apply general principles of philosophy to the practical world (205-6).  To apply a general first principle, it is necessary to have a criterion to show the subordinate principles to which it applies; but there are no such criteria, because how we apply a principle is a matter of judgment, which, as Kant demonstrated, has no  criteria.[28]  As one might suspect, Rehberg's argument here is politically motivated, because he singles out Reinhold as one of those philosophers who think that his first principles are guidelines for the social and political world (204).  The general conclusion of Rehberg's reasoning could not be more expressly political: “...the whole system of education and culture of man that should be based on scientific insight is false in theory and impossible in execution.” (205).