In his 2010 religion debate with Tony Blair, roughly a year before his untimely death, Christopher Hitchens decided to address what he said was most "twisted and immoral in the faith mentality, its essential fanaticism, its consideration of the human being as a raw material, and its fantasy of purity." The debate extended the hardline he adopted in his 2007 best-seller "God Is Not Great," that religion -- a "malignant force" -- "poisons everything."

With the New York Times Book Review and others calling the polemic "an all-out attack on all aspects of religion," one might assume Hitchens would've been similarly uncompromising about agnosticism as failing to reach his own level of certainty in "antitheism," his preferred self-description. Yet, of the seven brief references to agnosticism that appear in "God Is Not Great," all are unmistakably supportive. What's more, Hitchens's book did something increasingly rare among atheists and critics of religion: whenever possible, it grouped agnostics with atheists and freethinkers, as allies with shared arguments against monotheism, zealotry and fundamentalism.

In "Putting It Mildly," his opening chapter, Hitchens wrote: "Not all can be agreed on matters of aesthetics, but we secular humanists and atheists and agnostics do not wish to deprive humanity of its wonders or consolations. Not in the least." Among other things, the sentence abounds in inclusiveness: "we secular humanists and atheists and agnostics..."

A later chapter reinforces the point when Hitchens discusses "American freethinkers and agnostics and atheists" with almost ostentatious use of the connective "and."

Uniting agnostics and atheists not only made good political sense -- given the size of their combined populations -- it also underscored Hitchens's firm grasp of history. As Susan Budd put it in her excellent study "Varieties of Unbelief: Atheists and Agnostics in English Society, 1850-1960," "the conversion to atheism" in those years "usually followed two distinct phases: the conversion from Christianity to unbelief or uncertainty ... and the move from unbelief to positive commitment to secularism." Arguably, a similar two-step exists today.

That Hitchens took a glass-half-full approach to agnostics is notable because it's sharply at odds with the line of his compatriot, Richard Dawkins, whose thoughts on agnostics in "The God Delusion" are almost uniformly negative. And while both writers are noted for their scorn, even withering contempt, Hitchens's was directed mainly at zealots and hypocrites. Dawkins, by contrast, targets "faith-heads" and agnostics.

In his section of the book describing "The Poverty of Agnosticism," Dawkins begins not with the arguments of Thomas H. Huxley, Darwin's self-appointed "bulldog," or those of other notable intellectuals such as Bertrand Russell and Robert Ingersoll, America's "Great Agnostic," but with the memory of being "harangued" at school by a "robust Muscular Christian" preacher who "couldn't stand agnostics: namby-pamby, mushy pap, weak-tea, weedy, pallid fence-sitters."

It's an odd way to begin such a key section, not least because Dawkins can't resist adding: "He was partly right, but for wholly the wrong reason." The partial rightness of the insult doesn't come until much-later into the section, with the seemingly watertight distinction between Temporary and Permanent Agnosticism. But the initial paragraph seems compelled to continue "in the same vein," as Dawkins puts it. He turns to Quentin de la Bédoyère, science editor of the Catholic Herald, for whom the Catholic historian Hugh Ross Williamson respected firm religious belief and certain unbelief but "reserved his contempt for the wishy-washy boneless mediocrities who flapped around in the middle."

What are such outrageous statements doing in Dawkins's book, not least when there are so many other ways of addressing religious skepticism and doubt, including on their own terms?

"There is nothing wrong with being agnostic in cases where we lack evidence one way or the other," Dawkins at one point tries to comfort with a pat on the head, shortly before invoking the acronym PAP for what he says is "Permanent Agnosticism in Principle." But far from working with agnostics' already manifold criticisms of religion or looking to shore up their common ground with "freethinkers and atheists," as Hitchens took pains to do, Dawkins can find only fault with Huxley's and others' well-reasoned position, which was: "In matters of the intellect follow your reason as far as it will take you, without regard to any other consideration." At the same time, "do not pretend that conclusions are certain which are not demonstrated or demonstrable."

According to Dawkins, Huxley "seems to have been ignoring the shading of probability" for whether God exists, even though the essay in question, "Agnosticism" (1889), invokes probability as a term and concept no fewer than three times. Still, Dawkins feels sufficiently confident about Huxley's missteps to insist, "The existence of God is a scientific hypothesis like any other." "Either he exists or he doesn't," he writes a fraction earlier. "It's a scientific question."

Huxley also called his hypothesis "scientific." The difference is, his questions didn't end with whether God exists. A fierce defender of Darwin and of his theories of evolution, he also sought to establish whether the "problem of existence" was soluble, a central conundrum for agnosticism (and not a few atheists). For Huxley, at least, that conundrum wasn't soluble. While Darwinism made it possible to ask anew, what are we and how did we come to evolve, the questions fueling the agnosticism of Huxley and others came closer to asking, with metaphysical urgency, why are we?

One can try insisting that probability should be the guiding rationale for answering such questions, urging people to distinguish as much as possible between the natural and the supernatural, as Dawkins joins most agnostics in doing. Yet, by Huxley's rulebook, one would still not close the door on doubt, much less nail it firmly shut. For good reason, Dawkins titles one of his chapters "Why There Almost Certainly Is No God" (my emphasis) and later describes himself, on a scale of religious-to-atheistic conviction, as a "7-leaning 6," where 7 designates a "strong atheist" and 6 represents a "de facto atheist" who thinks "God is very improbable" (just as Huxley did) and "live[s] my life on the assumption that he is not there," but nonetheless treats the issue as one of "very low probability."

For Dawkins, all the same, agnosticism's embrace of a similar unknown points not to its stringency or capaciousness, but to its "poverty." "I am agnostic," he later quips, "to the extent that I am agnostic about fairies at the bottom of the garden." At such moments, the vast, considered history of agnosticism slips into caricature. The dignity and candor of honest disbelief morph into the cop-out of weak tea, weedy, pallid fence-sitting. We are left with playground taunts, where rigorous skeptics are mocked as namby-pamby while unbelievers crown themselves "brights." (Hitchens, of course, winced over such regrettable moves.)

Still, when doubt is acknowledged, even embraced, it sharpens conviction, strengthens resilience and modulates extremism -- among other ways by testing beliefs and asking us to consider in what they really consist. Agnosticism also makes it possible for people to change their minds, a prime element in the rise of secularism and freethought, as I detail in "The Age of Doubt." When a culture is polarized over such matters, however, it tends to shun the possibility of moderation through religious and intellectual doubt. And though in doing so it seems to gain an illusion of certainty, in the frequent flight to extremes it risks losing the considered center. There, where freethought inevitably meets agnosticism and atheism, shades of gray are unavoidable -- and sometimes even welcome.