Agnosticism, in fact, is not a creed, but a method, the essence of which lies in the rigorous application of a single principle. That principle is of great antiquity; it is as old as Socrates; as old as the writer who said, “Try all things, hold fast by that which is good”; it is the foundation of the Reformation, which simply illustrated the axiom that every man should be able to give a reason for the faith that is in him, it is the great principle of Descartes; it is the fundamental axiom of modern science. Positively the principle may be expressed: In matters of the intellect, follow your reason as far as it will take you, without regard to any other consideration. And negatively: In matters of the intellect, do not pretend that conclusions are certain which are not demonstrated or demonstrable. That I take to be the agnostic faith, which if a man keep whole and undefiled, he shall not be ashamed to look the universe in the face, whatever the future may have in store for him.

T. H. Huxley (1825–1895) English biologist [Thomas Henry Huxley]

Essay (1889-02), “Agnosticism,” The Nineteenth Century, Vol. 25, No. 144
    (Source)

Collected in Henry Wace (ed.), Christianity and Agnosticism, ch. 2 (1889).