Abstract: The seal of the a priori is imprinted on the reception of Kant's philosophy. Piaget's epistemological argumentation seems to ascribe knowledge a more fruitful constructiveness than Kant, seeing the a priori as rooted in unvarying reason. Yet, it seems, he failed to recognize the complexity of Kant's theory, which does not always follow a quid iuris line. Moments of experience, analysis and self-observation played more than a marginal role in his discovery of the a priori. Indeed, Kant himself raises the question of ontogenetic category assimilation in a review which pre-empts Piaget, borrowing the category of `original acquisition' from the doctrine of the laws of natural right. And although Kant should not be elevated to the harbinger of the knowledge on development issues delivered thus far by the history of science and experiments, he did recognize the temporal reference of their categories in principle without resolving their validity in psychogenetic terms. Key Words: a priori • categories • genetic epistemology • Geneva School • neo-Kantianism • original acquisition • Jean Piaget • psychogenesis • self-observation