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 philosophical thought. In the development of modern philosophy since Descartes and Locke rhetoric has been seen as superfluous to knowledge. Rhetoric has been commonly understood as the speech that plays on the emotions the use of thought and words to persuade, rather than their use as the basis to seek knowledge.

How does the mind generate the principles upon which rational thought is based? Rational thinking exploits the logical power of the word, but logic never enlightens us on the nature of its own starting points. 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 thinker of the Italian humanist tradition, Giambattista Vice (1668–1744), in Vice's understanding of imagination and the sense of human ingenuity contained in the metaphor.

Professor Grassi connects rhetoric with the power of language to bring the starting points for thought into being. This power of speech is at the basis of the philosophical and rational search for truth.