Origin and Nature of Pleasure and Pain

"Nearly all the greatest thinkers from the beginning of philosophy have grappled with the subject, yet we are inclined to believe that, from the first, no subject has been more profoundly misunderstood. Whatever the standpoint, whether philosophical or physiological, upon one point only, perhaps, has there always been substantially universal agreement; namely, that pleasure and pain are in some way direct and complementary expressions of the general welfare of the individual.
From Plato and Aristotle down through Descartes, Leibnitz, Hobbes, the idea, at base, has ever been the same: The experience, the judgment, the attainment of a perfect or imperfect life; the perfect or imperfect exercise of a faculty; the furtherance or hindrance of some activity; the rise or fall of some vital function, force, or energy. Everywhere pleasure and pain have been looked upon as complementary terms of a single phenomenon, and as the very essence of expression of the rise and fall of our inmost existence..."