We could never discover a principle that commands all rational beings with such absolute authority through a method of empirical moral philosophy; we must use the a priori method.
But both in his normative works and in his foundational work, the Groundwork of the Metaphysics of Morals, Kant makes explicit that the supreme moral principle itself must be discovered a priori, through a method of pure moral philosophy (G 4 387 92).
Buridan applied these analytical techniques so successfully in his metaphysics, natural philosophy, and ethics that, for many of his successors, they were identified with the very method of philosophy, understood as a secular practice, i.e., as distinct from theology.
Throughout his subsequent career he remained true to this tradition's rejection of the possibility of synthetic a priori knowledge, and so he saw the method of philosophy to be the analysis of the meaning of key terms, such as 'causality'truth'th'knowledgedge'freedom'om', and so on.
Graeco-Roman antiquity did not speculate about art, or speculated very little; its chief concern was to create a method of artistic instruction, not a "philosophy" but an "empirical science" of art.
In a sense, this is a venerable thesis in some ancient, specifically Platonic philosophy which envisaged the goal and method of philosophy in terms of virtue and the good.
After becoming acquainted with this body of work, Russell abandoned all vestiges of his earlier idealism and adopted the view, which he was to hold for the rest of his life, that analysis rather than synthesis was the surest method of philosophy and that therefore all the grand system building of previous philosophers was misconceived.
In MWO, Lewis (1929, Chp. 1) argued that the proper method of philosophy isn't transcendental but rather reflective.
In this regard he never swayed from Brentano's precept that the true method of philosophy is no different from that of the natural sciences.
One of the theses that Brentano defended in this disputation was: "The true method of philosophy is none other than that of the natural sciences" (Brentano, [ed.] Kraus 1929: 147).
