Tonelli’s life was all spent in the attempt to achieve a complete knowledge of the development of Kant’s philosophy in the context of the German Enlightenment. Beginning with his first monograph, Kant, dall’estetica metafisica all’estetica psicoempirica: Studi sulla genesi del criticismo (1754-1771) e sulle sue fonti Tonelli poses an exegetical dilemma between Kant als Erkenntnistheoretiker and Kant als Metaphysiker making it clear that the analysis of the mutual relationships among logical, epistemological, metaphysical, moral and aesthetic notions leads to the reformulation of a number of interpretations of Kant’s works.
He emphasizes Kant’s juvenile interests for aesthetics as containing in nuce all topics developed later in the Kritik der Urteilskraft. He does fundamental spade-work on the cultural background of the Beobachtungen, with special attention on the philosophy of moral-sense. He deals with the Bemerkungen on behalf of their connections with moral topics and dedicates the conclusion to the connections among aesthetics and logic in Alexander G. Baumgarten, Johann H. Lambert and Christian A. Crusius.
His research yields remarkable results for history of ideas and particularly for the history of aesthetic theories. Tonelli also demonstrates that maintaining unchanged its object, aesthetics changes deeply its conceptual and terminological apparatus. The Elementi metodologici e metafisici in Kant dal 1745 al 1768: Saggio di sociologia della conoscenza contain a complete presentation of Kant’s development from the beginnings to 1768 with reference metaphysics and methodology, which leads Tonelli to the revaluation of the role, up to then unknown in connection with Kant, played by Aristotle.
To the logical and metaphysical tradition of Aristotle and Aristotelianism and to its influence on Kant, Tonelli dedicated articles on the history of specific ideas: “Der historische Ursprung der kantischen Termini Analytik und Dialektik,” “Das Wideraufleben der deusch-aristotelischen Terminologie bei Kant während der Enstehung der Kritik der reinen Vernunft,” “Analysis and Synthesis in Eighteenth-Century Philosophy Prior to Kant,” and “Critique and Related Terms Prior to Kant: A Historical Survey.” These studies prelude to Tonelli’s masterpiece, Kant’s Critique of Reason within the Tradition of Modern Logic, which first published as a paper and posthumously as a book. On the basis of the evidence he had gathered, Tonelli suggests that the Kritik der reinen Vernunft be not a treatise on the theory of knowledge (the reading of most interpreters), and not even a treatise on metaphysics, but a treatise on the methodology of metaphysics, which is equivalent to say a treatise on the transcendental logic as the first of all particular logics.
Tonelli’s expertise was not limited to Kant. he also wrote on a number of philosophers of the Aufklärung such as Christian Thomasius, Johann F. Budde, Martin Knutzen, Alexander G. Baumgarten, Johann G. Sulzer, Johann H. Lambert, Moses Mendelssohn. He was interested on Maupertis, to whom he dedicated an influential book “Le pensée philosophique de the Maupertuis: Son milieu et ses sources.” Deeply aware of Christian A. Crusius’s role for Kant’s development, Tonelli planned the republication of his complete works. Alongside his philosophical studies, Tonelli wrote on literature and poetry of authors such as Albrecht von Haller and Heinrich Heine.
Tonelli often repeated that an historical investigation is truly worthy of such name, if and only if it changes both the interpretation of a determinate subject and the standard of rigour that are deemed opportune for writing future historical surveys. Tonelli’s works surely fulfilled both these objectives. In April 1974, the Frankfurter Allgemeine Zeitung praised his lecture as the “high point” of the First International Kant Congress in Mainz. Tonelli revolutionized the approach to Kantian studies by making it clear a thorough investigation of an author’s background is needed in order to understand his work, and he gave a new impulse to the re-evaluation of the German Aufklärung.