During the 1760s Kant became increasingly critical of Leibnizianism. According to one of his students, Kant was then attacking Leibniz, Wolff, and Baumgarten, was a declared follower of Newton, and expressed great admiration for the moral philosophy of the Romanticist philosopher Jean-Jacques Rousseau.

His principal work of this period was Untersuchung über die Deutlichkeit der Grundsätze der natürlichen Theologie und der Moral (1764; “An Inquiry into the Distinctness of the Fundamental Principles of Natural Theology and Morals”). In this work he attacked the claim of Leibnizian philosophy that philosophy should model itself on mathematics and aim at constructing a chain of demonstrated truths based on self-evident premises. Kant argued that mathematics proceeds from definitions that are arbitrary, by means of operations that are clearly and sharply defined, upon concepts that can be exhibited in concrete form. In contrast with this method, he argued that philosophy must begin with concepts that are already given, “though confusedly or insufficiently determined,” so that philosophers cannot begin with definitions without thereby shutting themselves up within a circle of words. Philosophy cannot, like mathematics, proceed synthetically; it must analyze and clarify. The importance of the moral order, which he had learned from Rousseau, reinforced the conviction received from his study of Newton that a synthetic philosophy is empty and false.

Besides attacking the methods of the Leibnizians, he also began criticizing their leading ideas. In an essay, “Versuch, den Begriff der negativen Grössen in die Weltweisheit einzuführen” (1763; “An Attempt to Introduce the Conception of Negative Quantities into Philosophy”), he argued that physical opposition as encountered in things cannot be reduced to logical contradiction, in which the same predicate is both affirmed and denied, and, hence, that it is pointless to reduce causality to the logical relation of antecedent and consequent. In an essay of the same year, “Der einzig mögliche Beweisgrund zu einer Demonstration des Daseyns Gottes” (“Enquiry into the Proofs for the Existence of God”), he sharply criticized the Leibnizian concept of Being by charging that the so-called ontological argument, which would prove the existence of God by logic alone, is fallacious because it confuses existential with attributive statements: existence, he declared, is not a predicate of attribution. Moreover, with regard to the nature of space, Kant sided with Newton in his confrontation with Leibniz. Leibniz’s view, that space is “an order of co-existences” and that spatial differences can be stated in conceptual terms, he concluded to be untenable.

Some indication of a possible alternative of Kant’s own to the Leibnizian position can be gathered from his curious Träume eines Geistersehers erläutert durch Träume der Metaphysik (1766; Dreams of a Spirit-Seer, Illustrated by Dreams of Metaphysics). This work is an examination of the whole notion of a world of spirits, in the context of an inquiry into the spiritualist claims of Emanuel Swedenborg, a scientist and biblical scholar. Kant’s position at first seems to have been completely skeptical, and the influence of the Scottish skeptical philosopher David Hume is more apparent here than in any previous work; it was Hume, he later claimed, who first awoke him from his “dogmatic slumber.” Yet Kant was not so much arguing that the notion of a world of spirits is illusory as insisting that humans have no insight into the nature of such a world, a conclusion that has devastating implications for metaphysics as the Leibnizians conceived it. Metaphysicians can dream as well as spiritualists, but this is not to say that their dreams are necessarily empty; there are already hints that moral experience can give content to the ideal of an “intelligible world.” Rousseau thus here acted upon Kant as a counterinfluence to Hume.