The Principles of Philosophy: A Defense of Reason presents an absolute and universal defense of the rational moral law, also called the principle of justice: don't do to others what you don't want them to do to you . It provides an intellectual platform to advocate equal human rights for all human beings. In addition, the book presents a strong and uncompromising criticism of skepticism and relativism . It analyzes several modern currents of thought that, even if implicitly and unintentionally, lead to skepticism or relativism. Specifically, the book includes a detailed criticism of Descartes, Hume, Schopenhauer, Marx, James, Nietzsche, Freud, Russell, and Heidegger. Contrary to what skeptical and relativistic doctrines imply, truth is absolute and human reason can know at least some truths, including the truth of the principle of justice. The book presents an academic and thorough investigation of the problem but in clear and simple language. The main goal of the book is intellectual systematization .

Moreover, it discusses the views of both Eastern and Western thinkers, from ancient times to the postmodern era, regarding a significant philosophical issue.