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I frequently hear people quoting widely-used idioms or proverbs as if they are fact, simply because they are used frequently by many people. For example, "An apple a day keeps the doctor away."

Can these be called a "truism" even if not true or must they be actually true to be called a "truism"?

Are there more precise terms for the two meanings, "widely believed ideas, which really aren't true" and "widely believed ideas, which really are true"?

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Can you give an example of a truism and check if it is true or not? – Mitch Apr 10 '13 at 11:46
Looking at a dictionary definition will provide the answer you're looking for. There's a caveat, however. What seems self-evidently true isn't true 100% of the time: sometimes it's patently false but unnoticed, like the flatness of Earth & the sun's revolved around the Earth. Some "truisms" are false because they're true only to those who believe in them. Metaphysics and truth are not related. – user21497 Apr 10 '13 at 11:46
Are you sure you are not confusing truism for factoid? It seems like it is a mistaken assumption. – Kris Apr 11 '13 at 14:45
Can these be called a "truism" even if not true or must they be actually true to be called a "truism"? would be off-topic. – Kris Apr 11 '13 at 14:54
up vote 5 down vote accepted

Truisms need only have apparent truth to earn their name, but occasionally the seeming certainty of this truth comes solely from the popularity of the saying itself.

This is, generally speaking, the logical fallacy of argumentum ad populum.

From the Wikipedia article:

In argumentation theory, an argumentum ad populum (Latin for "appeal to the people") is a fallacious argument that concludes a proposition to be true because many or most people believe it. In other words, the basic idea of the argument is: "If many believe so, it is so."

"Widely believed ideas, which really aren't true" is exactly the sense of "apocryphal" which, according to ODO, means: "of doubtful authenticity, although widely circulated as being true."

Lastly, I would refer to "widely believed ideas, which really are true" as "common knowledge."

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To summarize 'no, a truism does not need to be true.' – Mitch Apr 10 '13 at 14:45
@Mitch Should I start my answer with that? Should these questions not receive answers? Should I have formed my answer as a comment? I'm new here and I'm not sure what I should take from your comment if it is, as I assume, constructive and directed at me. – Tyler James Young Apr 10 '13 at 22:18
Your answer is obviously right if you understand the definition, but since the OP is asking, then the implications of the definition (and separately your answer) are probably not apparent. Also, it is a convenience for the quick reader to just say the boring answer along with the explanation. – Mitch Apr 10 '13 at 23:52
@Mitch I've edited my post to begin with a summary introduction of the position it takes. As for deciding what someone is capable of understanding, I'd rather overestimate every time—especially here, where we are meant to "build libraries of high-quality questions and answers." – Tyler James Young Apr 11 '13 at 11:19

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