Fallacious
Fallacious adjective - Not using or following good reasoning.
Usage example: it's fallacious to say that something must exist because science hasn't proven its nonexistence
Reasonable is an antonym for fallacious.
Reasonable
Reasonable adjective - According to the rules of logic.
Usage example: his answer is perfectly reasonable
Fallacious is an antonym for reasonable.
Both words in one sentence
- It's a case of the Sharpshooter's Fallacy to claim that it must be Nero, but it is not fallacious to say there are reasonable arguments to believe the Number of the Beast probably references Nero.
Source: Sharpshooter Fallacy - However, one must keep in mind that—depending on the surrounding circumstances—a deductively fallacious argument may still, nonetheless, be a reasonable and (inductively) logical argument that has decent prospects of being true despite the deductive logic being invalid.
Source: Logical Fallacies
Cite this Source
Reasonable and Fallacious. (2016). Retrieved 2026, August 13, from https://thesaurus.plus/related/fallacious/reasonable
Fallacious & Reasonable. N.p., 2016. Web. 13 Aug. 2026. <https://thesaurus.plus/related/fallacious/reasonable>.
Reasonable or Fallacious. 2016. Accessed August 13, 2026. https://thesaurus.plus/related/fallacious/reasonable.
Google Ngram Viewer shows how "fallacious" and "reasonable" have occurred on timeline