In his famous book on Plato's theory of ideas (1903) Paul Natorp developed a very influential interpretation of the Platonic philosophy. Together with Hermann Cohen's essay about Plato's theory of knowledge (1878), the detailed analysis of Platonic dialogues set forth by Natorp represents a milestone within the tradition of Marburg Neokantianism, both from a systematic and from a historical point of view. Ernst Cassirer was well acquainted with this Neokantian interpretation of Plato, and from the start of his philosophical career paid great attention to Plato's influence on the origins of modern science and philosophical thought. According to the author of this paper, further developments of Cassirer's reading of the Platonic philosophy show nevertheless his increasing differentiation from Cohen and Natorp. When Cassirer elaborated the "Philosophy of Symbolic Forms" (1923-1929), he was no more satisfied with the Marburg epistemological interpretation of Plato as well as with the ontological rethinking of Plato's thought put forth by Natorp in his late work. So a new image of the Platonic philosophy emerges from Cassirer's philosophy of culture and symbolical mental activities; and this image involves at the same time a reformulation of the Neokantian tradition.