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
Sound is an antonym for fallacious.
Sound
Sound adjective - According to the rules of logic.
Usage example: sound reasoning alone should tell you that the result is invalid
Both words in one sentence
- It may sound like a rational thing to do since by definition a fallacious argument makes no sense, and this rule may seem like a mindscrewy special case, but...
Source: Fallacy Fallacy
Cite this Source
Sound and Fallacious. (2016). Retrieved 2026, July 21, from https://thesaurus.plus/related/fallacious/sound
Fallacious & Sound. N.p., 2016. Web. 21 Jul. 2026. <https://thesaurus.plus/related/fallacious/sound>.
Sound or Fallacious. 2016. Accessed July 21, 2026. https://thesaurus.plus/related/fallacious/sound.
Google Ngram Viewer shows how "fallacious" and "sound" have occurred on timeline