William Kingdon Clifford, answered from the texts and cited to the page.
The wrong I am pointing at is not a wrong of content but of character. A man who believes a false thing because he examined the evidence and drew an incorrect conclusion has made an error; a man who believes without examining at all has done something worse, because he has corrupted the instrument by which truth is sought. Consider the thief.
If I steal money from a person, there may be no harm done by the mere transfer of possession; he may not feel the loss, or it may even prevent him from using the money badly.1 The harm is not in the transaction but in what I have made of myself. And so with belief: if I let myself believe anything on insufficient evidence, there may be no great harm done by the mere belief; it may be true after all, or I may never have occasion to exhibit it in outward acts.2
But I cannot help doing this great wrong toward Man, that I make myself credulous.3 Credulity, then, is the habit of believing for unworthy reasons — and a habit, once formed, does not stay still. Every time we let ourselves believe for unworthy reasons, we weaken our powers of self-control, of doubting, of judicially and fairly weighing evidence.4
The credulous man does not merely hold bad beliefs; he becomes less capable of holding good ones. And what he becomes, he spreads. Habitual want of care about what I believe leads to habitual want of care in others about the truth of what is told to me.5 Men speak the truth to one another when each reveres the truth in his own mind and in the other's mind; but when I believe things because I want to believe them, and because they are comforting and pleasant, my friend learns to cry Peace to me when there is no peace.6
The credulous man is father to the liar and the cheat.7 The danger to society is not merely that it should believe wrong things, though that is great enough; but that it should become credulous, and lose the habit of testing things and inquiring into them — for then it must sink back into savagery.8
If I steal money from any person, there may be no harm done by the mere transfer of possession; he may not feel the loss, or it may prevent him from using the money badly.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
if I let myself believe anything on insufficient evidence, there may be no great harm done by the mere belief; it may be true after all, or I may never have occasion to exhibit it in outward acts.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
But I cannot help doing this great wrong toward Man, that I make myself credulous.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
Every time we let ourselves believe for unworthy reasons, we weaken our powers of self-control, of doubting, of judicially and fairly weighing evidence.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
Habitual want of care about what I believe leads to habitual want of care in others about the truth of what is told to me.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
but how shall my friend revere the truth in my mind when I myself am careless about it, when I believe things because I want to believe them, and because they are comforting and pleasant? Will he not learn to cry, 'Peace,' to me, when there is no peace?The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
The credulous man is father to the liar and the cheat; he lives in the bosom of this his family, and it is no marvel if he should become even as they are.The Scientific Basis of Morals and Other Essays, incl. The Ethics of Belief
The danger to society is not merely that it should believe wrong things, though that is great enough; but that it should become credulous, and lose the habit of testing things and inquiring into them; for then it must sink back into savagery.Lectures and Essays Vol II, ed. Pollock