Question: I've been looking for a general word to define an idea that seems true, but simply isn't.

A simple example: "The Earth is flat." If all you know is the world outside your window, and you've never done any basic experiments, read books, or seen photos to the contrary, this seems like a reasonable explanation. But it doesn't take too much more examination to find out that it just isn't so.

The key point here is that it's something *a reasonable person could reasonably believe*.

The closest I've come to what I'm looking for is "verisimilitude." I looked at [this answer](https://english.stackexchange.com/questions/415707/something-that-sounds-good-but-has-no-true-meaning), but it seems to me that words like "spurious" or "nonsense" seem to imply some ill-intent to deceive, which is not what I'm after. (Or maybe it's more a concept than a word, and that's ok too, if anyone has thoughts on that.)

Answer: 
**Seemingly** works well for the given context.

> 
> appearing to be something, especially when this is not true:
> 
> 
> 

*He remains confident and seemingly untroubled by his recent problems.*

[Cambridge]

You could this say: *The earth is **seemingly** flat.* (which it isn't in reality.)
I would use *ostensibly*.

OED : as appears or is stated to be true, though not necessarily so