Belief should be proportioned to the evidence

Atheists, secular humanists, agnostics, and deists commonly affirm that belief should be proportioned to the evidence, that the strength of one's conviction ought to match the strength of the reasons and evidence supporting it, and that claims lacking sufficient support should be doubted or withheld rather than accepted on faith or authority. The four positions apply the principle differently. Atheism itself divides between what philosophers sometimes call a strong or positive form, the affirmative claim that no god exists, and a weaker or negative form, sometimes argued for by philosophers such as Antony Flew under the label a presumption of atheism, which withholds belief in any god for lack of sufficient evidence without claiming to have positively disproved the possibility, a distinction close to, though not identical with, the line agnosticism draws between atheism and theism as a matter of knowledge rather than belief.

Deism represents a historically distinct application of the same evidentialist instinct: rather than concluding that no god exists, Enlightenment deists such as Voltaire and, in his later writings, Thomas Paine in The Age of Reason, argued that reason and the observable order of nature support belief in a creator God who set the universe in motion, while rejecting miracles, revelation, and organized religious authority as unsupported by the same evidential standard, a position that lets deism affirm a Creator while still applying the evidentialist principle strictly against claims of specific scriptural revelation.

This is a shared epistemic commitment across the skeptical and rationalist worldviews, underlying the atheist's conclusion that gods do not exist or should not be believed in, the agnostic's suspension of judgment, the deist's preference for reason over revelation, and the humanist's reliance on science and free inquiry as the proper guide to belief and conduct. It descends from the Enlightenment and the rise of the modern scientific method, and expresses a conviction that intellectual honesty requires following the evidence where it leads rather than assuming a conclusion in advance.

The principle has not gone unchallenged, including from within philosophy rather than only from religious believers. The philosopher William James, responding directly to the strict evidentialism associated with W. K. Clifford, argued in his essay The Will to Believe that certain kinds of questions, live options that are forced and momentous rather than trivial or avoidable, cannot always wait on sufficient evidence before a person must, in practice, decide how to live, and that in such cases a passional or volitional element in belief is unavoidable and not necessarily irrational. Defenders of strict evidentialism have generally responded that James's argument, whatever its merits for practical choices, does not license treating factual claims about what exists as though they could be settled by preference, keeping the underlying dispute over evidentialism's proper scope very much alive.