Moreover, speaking for myself, and without impugning the right of any other person to use the term in another sense, I further say that agnosticism is not properly described as a “negative” creed, nor indeed as a creed of any kind, except in so far as it expresses absolute faith in the validity of a principle which is as much ethical as intellectual. This principle may be stated in various ways, but they all amount to this: that it is wrong for a man to say that he is certain of the objective truth of any proposition unless he can produce evidence which logically justifies that certainty. This is what agnosticism asserts; and, in my opinion, it is all that is essential to agnosticism.

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

Essay (1889-06), “Agnosticism and Christianity,” The Nineteenth Century magazine, Vol. 25, No. 148
    (Source)

Collected in Henry Wace (ed.), Christianity and Agnosticism, ch. 9 (1889), a collection of Huxley essays and replies to them (several by Wace).) The passage is sometimes miscited as coming from his earlier essay "Agnosticism," from the 1889-02 issue (Vol. 25, No. 144) (Collected).