Moreover, the revelation by which God makes himself known to man, does not admit of being limited exclusively to the written word. Nature itself is a book written on both sides, both within and without, in every line of which the finger of God is discernible. It is, as it were, a Holy Writ in visible form and bodily shape—a song of praise on the Creator's omnipotence composed in living imagery. But besides Scripture and nature—those two great witnesses to the greatness and majesty of God—there is in the voice of conscience nothing less than a divine revelation within man. This is the first awakening call to the two other louder and fuller proclamations of revealed truth. And, lastly, in universal history we have set before us a real and manifold application and progressive development of revelation. Here the luminous threads of a divine and higher guidance glimmer through the remarkable events of history. For, not only in the career of whole ages and nations, but also in the lives of individuals, the ruling and benignant hand of Providence is every where visible.
Fourfold, consequently, is the source of revelation, from which man derives his knowledge of the Deity, learns his will, and understands his operation and power — conscience, nature, Holy Writ, and universal history.
Friedrich von Schlegel, The Philosophy of Life, Morrison, tr., p. 65.
Schlegel, of course, is here adding universal history to Scripture, nature, and conscience, which were more commonly recognized. The idea is similar to Vico's notion of civil theology based on ideal eternal history, but it's difficult to pin down any definite historical connection between the two.