Charles Darwin was shy and meticulous, a wealthy landowner with close friends among the Anglican clergy. He had a gentle, unassuming manner, a strong need for privacy, and an extraordinary commitment to intellectual honesty. As an undergraduate at Cambridge, he had studied halfheartedly toward becoming a clergyman himself, before he discovered his real vocation as a scientist. Later, having established a good but conventional reputation in natural history, he spent 22 years secretly gathering evidence and pondering arguments—both for and against his theory—because he didn’t want to flame out in a burst of unpersuasive notoriety. He may have delayed, too, because of his anxiety about announcing a theory that seemed to challenge conventional religious beliefs—in particular, the Christian beliefs of his wife, Emma. Darwin himself quietly renounced Christianity during his middle age, and later described himself as an agnostic. He continued to believe in a distant, impersonal deity of some sort, a greater entity that had set the universe and its laws into motion, but not in a personal God who had chosen humanity as a specially favored species. Darwin avoided flaunting his lack of religious faith, at least partly in deference to Emma. And she prayed for his soul.
If it is to be established that there is a God, then we have to have good grounds for believing that this is indeed so. Until and unless some such grounds are produced we have literally no reason at all for believing; and in that situation the only reasonable posture must be that of either the negative atheist or the agnostic. So the onus of proof has to rest on the proposition [of theism].
"Agnosticism and Christianity" in Essays Upon Some Controverted Questions (1892), pp. 350-1, 351-2.
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The people who call themselves "Agnostics" have been charged with doing so because they have not the courage to declare themselves "Infidels." It has been insinuated that they have adopted a new name in order to escape the unpleasantness which attaches to their proper denomination. To this wholly erroneous imputation, I have replied by showing that the term "Agnostic" did, as a matter of fact, arise in a manner which negatives it; and my statement has not been, and can not be, refuted. 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. That which Agnostics deny and repudiate, as immoral, is the contrary doctrine, that there are propositions which men ought to believe, without logically satisfactory evidence; and that reprobation ought to attach to the profession of disbelief in such inadequately supported propositions. The justification of the Agnostic principle lies in the success which follows upon its application, whether in the field of natural, or in that of civil, history; and in the fact that, so far as these topics are concerned, no sane man thinks of denying its validity.
Agnosticism is a theory of the Unknowable which assumes its most definite form in the denial of the possibility of any knowledge of God. 1st kind: connected with theory that we know only the phenomenal and a logical deduction from it. 2d kind: held by those who do not hold the phenomenal theory of knowledge but rest their deduction that the Infinite and the Absolute are unknowable on the limitation of human intelligence, maintaining that the infinite transcends the limits of our knowledge, and must on that account remain unknown, while the existence of the infinite God must be a matter of belief.