This paper aims at presenting a reconstruction of the main theses of Lambert’s thought and their role in the establishment of Kant’s theoretical philosophy. In order to do so, the paper is divided into three sections. Initially, a consideration of Lambert’s attempt to assure illusion within the domain of phenomenology and to institute the domain of metaphysical truth is carried out. Secondly, Kant’s first step away from Lambert’s proposal, which is presented in the Inaugural Dissertation’s thesis that the sensible and (...) intellectual domains of knowledge are grounded upon two distinct and intransmutable faculties, is taken into account. Finally, Kant’s definitive step away from Lambert’s proposal, i.e., the justification of appearances in the Duisburg Nachlaβ in an objective and non-illusory domain, due to the determination of the faculty of understanding, is pointed out. (shrink)