This article explores Kant’s critique of Leibniz’s Principium Identitas Indiscernibilium within the amphiboly of the concepts of reflection, a key passage in the Critique of Pure Reason which, however, is often overlooked. Kant fundamentally challenges 18th-century rationalism, which equates thinking and being. He criticizes Leibniz’s praedicatum inest subjecto principle as an abuse of formal logic, particularly in the amphiboly. This reveals why Leibniz and his followers misused logic as a metaphysical organon. By exposing this confusion, Kant discredits Leibniz’s Monadology as a pseudo-ontology. The disagreement between Kant and Leibniz stems from their distinct concepts of truth and cognition, marking a key rupture between rationalism and Kant’s transcendental philosophy.