By going back to the Italian humanist tradition and aspects of earlier Greek and Latin thought, Ernesto Grassi develops a conception of rhetoric as the basis of philosophy. Grassi explores the sense in which the first principles of rational thought come from the metaphorical power of the word. He finds the basis for his conception in the last great thinker of the Italian humanist tradition, Giambattista Vico (1668-1744). He concentrates on Vico's understanding of imagination and the sense of human ingenuity contained in metaphor. For Grassi, rhetorical activity is the essence and inner life of thought when connected to the metaphorical power of the word.

Originally published in English in 1980, Rhetoric as Philosophy has been out of print for some time. In his foreword to this reprint edition, Burke scholar Timothy W. Crusius rues the lack of concentrated attention to Grassi because "what he had to say about rhetoric is at least as significant as, for example, what Kenneth Burke taught us."