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).