Infallible describes something or someone that is incapable of making errors, failing, or being wrong. The word carries absolute force: not "rarely wrong" or "almost never wrong," but incapable of it altogether. That absoluteness is exactly what makes infallible useful in some contexts and overblown in others.
Core definition of infallible
Infallible is an adjective. It applies to a person, a method, a system, or an argument that cannot fail or be mistaken. The word functions as a strong claim: once you call something infallible, you're leaving no room for error at all.
The noun form is infallibility. The adverb is infallibly, meaning without fail, every single time.
Origin and etymology
Infallible comes from the Medieval Latin infallibilis, built from the prefix in- (not) and fallibilis (liable to err), which in turn comes from the Latin fallere (to deceive or disappoint). English adopted the word in the 15th century, initially in theological writing. Its most famous formal application is the doctrine of papal infallibility, defined by the First Vatican Council in 1870, which holds that the Pope cannot err when speaking on matters of faith and morals from his official seat.
How to use infallible in a sentence
Infallible works as a standard adjective before a noun or after a linking verb. A few clear examples:
- "No system is infallible, and this one is no exception."
- "The test proved an infallible method for detecting the compound."
- "She trusted his instincts, but she didn't consider them infallible."
- "History shows that no political leader is infallible."
Notice that infallible often appears in the negative. Saying something is not infallible is a common way to introduce a caveat or temper an expectation. That reflects the word's absoluteness: because it claims perfection, writers reach for it most often to deny that perfection exists.
Infallible vs fallible
Fallible is the direct antonym of infallible. Fallible describes someone or something that can make mistakes. Human memory, for instance, is famously fallible. A court system is designed to reach correct verdicts, but it remains fallible. The contrast matters in practice: calling something fallible is a realistic concession, while calling something infallible is an extraordinary claim that demands strong evidence.
Don't confuse either word with eminent, which describes outstanding reputation or distinction, not accuracy or reliability. The words share a formal register but cover completely different ground.
Infallible vs unerring
Unerring is the closest near-synonym. It means consistently accurate, never missing. The difference is subtle but real. Unerring implies a track record: something that has always hit its mark. Infallible implies an inherent incapacity to fail, whether or not it has been tested. An unerring shot shows consistent accuracy in practice. An infallible method is one that cannot miss by its very nature. Writers use the words almost interchangeably, but infallible carries stronger, more absolute weight.
Common contexts where infallible appears
Theology was the word's original home, but infallible now turns up across several domains.
In science and medicine, infallible is used carefully, often with negation: "no diagnostic test is infallible." This acknowledges that even reliable tools carry some margin for error. In law, infallibility is similarly denied: no verdict process, no forensic method is infallible. In everyday speech, infallible shows up in both sincere and ironic senses. Someone might call a friend's restaurant recommendations infallible as a genuine compliment, or might say "the infallible sat-nav sent me into a lake" as a sardonic joke.
The ironic use works because infallible sets such a high bar. When something fails spectacularly, calling it infallible first makes the failure funnier or sharper. This is the same mechanism that makes obstinate effective as a slightly comic descriptor for stubborn systems or people that refuse to bend even when they're clearly wrong.
Words often confused with infallible
Infallible is sometimes misused in place of reliable, trustworthy, or accurate. Those words carry degrees; infallible does not. Calling a weather forecast infallible is almost certainly wrong. Calling it reliable is fair. The distinction keeps your writing precise. If a source or method can fail even once under the right conditions, infallible is the wrong word.
Inerrant is another near-synonym, used primarily in religious contexts (biblical inerrancy) to mean free from error. It's narrower than infallible in everyday usage and sounds more technical outside theology.
Pronunciation
Infallible is pronounced in-FAL-uh-bul. The stress falls on the second syllable. Four syllables total. The adjective infallibility extends to seven: in-FAL-uh-BIL-uh-tee, with secondary stress on the fourth syllable.
Quick reference
Part of speech: Adjective
Noun form: Infallibility
Adverb form: Infallibly
Antonym: Fallible
Near-synonyms: Unerring, inerrant, unfailing, certain
Origin: Medieval Latin infallibilis, 15th century