Mawson, C.O.S., ed. (1870–1938). Roget’s International Thesaurus. 1922.

Class IV. Words Relating to the Intellectual Faculties
Division (I) Formation of Ideas
Section V. Results of Reasoning

485. Unbelief. Doubt.

   NOUN:UNBELIEF, disbelief, misbelief; discredit, miscreance or miscreancy [archaic]; infidelity (irreligion) [See Irreligion]; wrangling, ergotism [rare]; dissent [See Dissent]; change of opinion [See Belief]; retractation [See Tergiversation].
  DOUBT (uncertainty) [See Uncertainty]; skepticism or scepticism, misgiving, demur; distrust, mistrust; misdoubt, suspicion, jealousy, scruple, qualm; onus probandi [L.].
  INCREDIBILITY, incredibleness, incredulity, unbelievability.
  AGNOSTIC, skeptic or sceptic; unbeliever [See Incredulity].
   VERB:DISBELIEVE, discredit; not believe [See Belief]; misbelieve; refuse to admit (dissent) [See Dissent]; refuse to believe (incredulity) [See Incredulity].
  DOUBT; be doubtful (uncertain) [See Uncertainty]; doubt the truth of; be skeptical as to &c. adj.; diffide [obs.], distrust, mistrust; suspect, smoke, scent, smell, smell a rat [colloq.], have -, harbor -, entertain- -doubts, – suspicions; have one’s doubts.
  throw doubt upon, raise a question; bring -, call- in question; question, challenge; dispute; deny [See Negation]; cause -, raise -, start -, suggest -, awake- a -doubt, – suspicion; cavil, wrangle, ergotize [rare].
  DEMUR, stick at, pause, hesitate, shy at, scruple; stop to consider, waver.
  hang in suspense, hang in doubt.
  STAGGER, startle; shake -, stagger- one’s -faith, – belief.
   ADJECTIVE:UNBELIEVING; skeptical or sceptical, incredulous -, skeptical- as to; distrustful of, shy of, suspicious of; doubting &c. v.
  DOUBTFUL (uncertain) [See Uncertainty]; disputable; unworthy -, undeserving- of -belief [See Belief]; questionable; suspect [archaic], suspicious; open to -suspicion, – doubt; staggering, hard to believe, incredible, unbelievable, not to be believed, inconceivable.
  FALLIBLE (uncertain) [See Uncertainty]; undemonstrable; controvertible (untrue) [See Error].
   ADVERB:WITH CAUTION, cum grano salis [L.]; with grains of allowance.
  1. Fronti nulla fides.
  2. Nimium ne crede colori.—Vergil
  3. Timeo Danaos et dona ferentes.—Vergil
  4. Credat Judæus Apella.—Horace
  5. Let those believe who may.
  6. Ad tristem partem strenua est suspicio.—Syrus