This book addresses the relationship between the epistemological and ethical thoughts of Plato and Democritus according to the interpretation of the neo-Kantian philosopher Paul Natorp. The intention is initially to focus on the philosophical interests of the Marburg School, particularly on the epistemology and ethics examined by H. Cohen and P. Natorp, while also highlighting the connections to ancient thought. After a careful examination of Natorp's interpretation of Platonism, the volume explores the fundamental pathways of the epistemology, physics, and ethics of Democritus. Notably, there is a substantial section dedicated to the philological-philosophical analysis of the 230 ethical fragments of Democritus, which were analyzed within Natorp's treatise "The Ethics of Democritus." The secondary literature dedicated to this topic was widely divided in the 19th and 20th centuries and has been extensively studied by G. Ibscher. Essentially, this work re-evaluates this original interpretation of Democritus' ethics in close relation to epistemology, also aiming to overcome its marginalization by Hegelian historiography as represented by interpreters such as E. Zeller and W. Jaeger.