The verb is derived from Middle Englishdouten(“to be in doubt, feel unsure; to be afraid or worried; to hesitate; to be confused; to have respect or reverence”)[and other forms], from Old Frenchdouter, doter, duter (compare Middle Frenchdoubter), from Latindubitāre(“to hesitate”), the presentactiveinfinitive of dubitō(“to be uncertain, doubt; to hesitate, waver in coming to an opinion; to consider, ponder”); the further etymology is uncertain, but one theory is that dubitō may be derived from dubius(“fluctuating, wavering; doubtful, dubious, uncertain”), from duhibius(“held as two”), from duo(“two”) (ultimately from Proto-Indo-European*dwóh₁(“two”)) + habeō(“to have, hold”) (possibly ultimately from Proto-Indo-European*gʰeh₁bʰ-(“to grab, take”)). Although the Middle English form of the word was spelled without a b, this letter was later introduced through the influence of the Latin words dubitāre and dubitō. However, the English word continued to be pronounced without the b sound.
Ther be but two wayes onely. The one whiche by followyng the affections ledeth to perdicion. The other whyche throughe the mortifyenge of the fleſhe: ledeth to lyfe, why doubteſt thou in thy ſelf: There is no thyrde way.
For they be not termed Eclogues, but Aeglogues, which ſentence this author very well obſerving, vpon good iudgement, though in deede few Goteheards haue to doe herein, neuertheleſſe doubteth not to call them by the vſed and beſt knowne name.
1585 September 9, “How a Man may Ivdge or Discerne of Him Self, vvhether He be a True Christian or Not.[…]”, in A Christian Directorie Gviding Men to Their Salvation. Devided into Three Bookes.[…], [Rouen: s.n.], OCLC28887498, pages 316–317:
And as for that faith, vvhich is vvithout vvorkes, and yet ſeemeth to thes men to be ſufficient for their ſaluation; he proteſteth, that it is ſo vnprofitable, as he doubteth not to ſaye of hymſelf; [...]
For never (I thinke) was there any woman, that with more unremovable determinatiõ gave her selfe to the coũcell of Love, after she had once set before her mind the worthines of your cousin Amphialus; & yet is nether her wisdome doubted of, nor honour blemished.
1594, Richard Hooker, “The Second Booke. Concerning Their First Position who Vrge Reformation in the Church of England: Namely, that Scripture is the Only Rule of All Things which in this Life may be Done by Men.”, in Of the Lawes of Ecclesiastical Politie, Eight Bookes, London: Printed by William Stansbye, published 1622, OCLC1029957510, page 73:
Now it is not required nor can be exacted at our hands, that we ſhould yeeld vnto any thing other aſſent, then ſuch as doth anſwer the euidence which is to be had of that wee aſſent to. For which cauſe euen in matters diuine, concerning ſome things we may lawfully doubt and ſuſpend our iudgement, inclining neyther to one ſide or other, [...]
As to the efficacy of such legislation and taxation a word may be said. No one doubts that it is possible, by the employment of such methods, to make the rich poorer. [...] But the really important question—for all serious-minded inquirers—is whether the employment of these weapons will diminish the poverty or increase the prosperity of the relatively poor.
[B]oth colonisers and colonised lost faith in the colonisers' vision of the future. Europeans doubted whether their aims were attainable; Africans doubted whether they were desirable.
2011, Kent Koppelman, “Diversity and Discrimination: The Argument over Affirmative Action”, in The Great Diversity Debate: Embracing Pluralism in School and Society, New York, N.Y.: Teachers College Press, →ISBN, page 99:
In one study, 60% of Black students believed that their academic abilities were doubted by their White peers, and 60% felt that their White professors doubted them as well.
I walk by the Rule of my Maſter, you walk by the rude working of your fancies. You are counted thieves already, by the Lord of the way; therefore, I doubt you will not be found true men at the end of the way.
Rushing to the door of the church, [Robert the] Bruce met two powerful barons, [Roger de] Kirkpatrick of Closeburn, and James de Lindsay, who eagerly asked him what tidings? "Bad tidings," answered Bruce, "I doubt I have slain [John] Comyn." "Doubtest thou?" said Kirkpatrick, "I make sicker" (i.e. sure.) With these words, he and Lindsay rushed into the church, and dispatched the wounded Comyn.
[H]ow many good Christians are there, who consider themselves the beloved of Christ & the invariable followers of his gospel, who with all his precepts in their mind go to Africa, wrest the mother from the infant—the father from the wife—chain them to the whip & lash, they & their posterity for ever, nay hold this scourge in their own hand & inflict it with all the gout of their abominable appetites, & who do not doubt that they are violating the whole doctrine of the author of their religion—To what absurdities may not the human mind bring itself when this can be thought by them less offensive to God, than eating meat on a friday?—
At last, as they more faintly wrestling lay, / Juan contrived to give an awkward blow, / And then his only garment quite gave way; / He fled, like Joseph, leaving it; but there, / I doubt, all likeness ends between the pair.
I shall never know whether they got at the truth o' the robbery, nor whether Mr Paston could ha' given me any light about the drawing o' the lots. It's dark to me, Mrs Winthrop, that is; I doubt it'll be dark to the last.
In archaic usage, the phrase after "doubt" is what the doubter worries may be the case; in modern usage, that phrase is what the doubter worries may not be the case. Thus the archaic "I doubt he may be lying" is equivalent to the modern "I doubt he is telling the truth."
In Scotland the archaic usage is still current but with a meaning boadened beyond worry: to doubt something is to consider it likely, so examples include not just "I doubt he's lying," but also "I doubt we'll arrive before dark."
She was very pretty: exceedingly pretty. With a dimpled, surprised-looking, capital face; a ripe little mouth, that seemed made to be kissed—as no doubt it was; all kinds of good little dots about her chin, that melted into one another when she laughed; and the sunniest pair of eyes you ever saw in any little creature's head.
After all, the search for such assurances will itself require us to marshall our cognitive resources. It will itself involve the use of methods about which we can sensibly have doubts, doubts that cannot be addressed without begging the question.