“Every explanation is after all an hypothesis.”

Causality
Gesammelte Mathematische Werke (1876)

The American Mercury (February 1926)
1920s
Context: By what route do otherwise sane men come to believe such palpable nonsense? How is it possible for a human brain to be divided into two insulated halves, one functioning normally, naturally and even brilliantly, and the other capable only of such ghastly balderdash which issues from the minds of Baptist evangelists? Such balderdash takes various forms, but it is at its worst when it is religious. Why should this be so? What is there in religion that completely flabbergasts the wits of those who believe in it? I see no logical necessity for that flabbergasting. Religion, after all, is nothing but an hypothesis framed to account for what is evidentially unaccounted for. In other fields such hypotheses are common, and yet they do no apparent damage to those who incline to them. But in the religious field they quickly rush the believer to the intellectual Bad Lands. He not only becomes anaesthetic to objective fact; he becomes a violent enemy of objective fact. It annoys and irritates him. He sweeps it away as something somehow evil...

Source: A Neglected Argument for the Reality of God (1908), II
Context: The hypothesis of God is a peculiar one, in that it supposes an infinitely incomprehensible object, although every hypothesis, as such, supposes its object to be truly conceived in the hypothesis. This leaves the hypothesis but one way of understanding itself; namely, as vague yet as true so far as it is definite, and as continually tending to define itself more and more, and without limit. The hypothesis, being thus itself inevitably subject to the law of growth, appears in its vagueness to represent God as so, albeit this is directly contradicted in the hypothesis from its very first phase. But this apparent attribution of growth to God, since it is ineradicable from the hypothesis, cannot, according to the hypothesis, be flatly false. Its implications concerning the Universes will be maintained in the hypothesis, while its implications concerning God will be partly disavowed, and yet held to be less false than their denial would be. Thus the hypothesis will lead to our thinking of features of each Universe as purposed; and this will stand or fall with the hypothesis. Yet a purpose essentially involves growth, and so cannot be attributed to God. Still it will, according to the hypothesis, be less false to speak so than to represent God as purposeless.

Source: Outlander

The Design of Experiments, Edinburgh: Oliver and Boyd, 1935, p. 18
1930s

Conversations with Carl Sagan (2006) http://books.google.ca/books?id=gJ1rDj2nR3EC&pg=PA70&redir_esc=y#v=onepage&q&f=false, edited by Tom Head, p. 70
Context: Those who raise questions about the God hypothesis and the soul hypothesis are by no means all atheists. An atheist is someone who is certain that God does not exist, someone who has compelling evidence against the existence of God. I know of no such compelling evidence. Because God can be relegated to remote times and places and to ultimate causes, we would have to know a great deal more about the universe than we do to be sure that no such God exists. To be certain of the existence of God and to be certain of the nonexistence of God seem to me to be the confident extremes in a subject so riddled with doubt and uncertainty as to inspire very little confidence indeed.

Source: A Long Way Gone: Memoirs of a Boy Soldier

The portion after the second semicolon is widely paraphrased or misquoted. Two examples are "For every complex problem there is an answer that is clear, simple, and wrong" and "There is always an easy solution to every human problem -- neat, plausible, and wrong."
1910s
Source: "The Divine Afflatus" in New York Evening Mail (16 November 1917); later published in Prejudices: Second Series (1920) and A Mencken Chrestomathy (1949)

Source: The Quincunx of Time (1973), Chapter 8, “The Courtship of Posi and Nega” (p. 89)