Maybe this is also too obvious, but a check mark for certainty?

How about /=/ for certainty and /≈/ for uncertainty?

Something along these lines, maybe (pun intended)?
=
≈

Edit: late, but glad to see that great minds think alike ;-D

Hmm. Yeah. Good ideas. I guess what I picture in my mind is a solid form, more of a logo like symbol because they need to stand out next to a headline. Example:

X Know: Greenhouse gases are warming the planet.

X Don’t know: How far greenhouse gas levels will rise.

(X = the symbol)

Those mentioned could perhaps work knocked out of a circle or square, but wouldn’t stand out enough as plain text.

Maybe triangles - one pointing up, the other down (the former
being stable and positive, the latter unstable and negative).

hhp

Or arrows:

Or a rightward arrow with a straight stem for certain,
and a two-headed arrow with a wavy stem for uncertain.

hhp

The "therefore" symbol ∴ is associated with that which is proven.

For uncertainty, it seems what you need is a stick figure shrugging with palms up, the universal gesture for "heck if I know".

Unfortunately, these two suggestions clearly do not go together, and I can't think of an appropriate counterpart for either.

Who is the audience for this? Will they pick up on less obvious symbols?

> a stick figure shrugging with palms up...[but]...I can't
> think of an appropriate counterpart

Maybe a "thumbs up" gesture? But perhaps that would also indicate approval, which isn't necessarily what you want.

Thanks guys. The therefore symbol in particular. James: The readers are mostly academic(-minded)s, 30-60 year-olds.

'?' and '!' are pretty unambiguous. Frame them to make 'em pop out:

... oh blast it, I'm bitten by the "Tee-hee I'm ignoring your image" bug that plagues this forum!

I simply drew black rounded rectangles, with the "?" and "!" in white, in Arial Rounded Bold. You'll have to imagine it (or draw them yourself).

How about using some sort of conjunction symbol for certainty (perhaps even the Łukasiewicz logic strong conjunction symbol), and a disjunction for uncertainty? Or a clear symbol for certainty and a blurry one for uncertainty?

This was my idea:

Yes, I figured. Thanks.

Johan: Do you have a visual example? I couldn’t find it.

Hrant: Sindre suggested something along the lines of your triangles. Good call, both of you!

This probably won't work, but:
For certainty you could have a "Q" with
its counter replaced with an "e"* with its
counter replaced with a "D".
http://en.wikipedia.org/wiki/Q.E.D.
Check out the last paragraph of that BTW.

* UC probably can't work.

hhp

[!]

[:]

– ?

[.]

I just found this little guy. His name is U+2368 Functional symbol tilde diaeresis. Looks mighty uncertain, no? And then you could have one with a straight "mouth" for certainty.

On second thought, emoticon-like symbols might really be too childish. But maybe losing the dots and just having a block/dash/minus, and a wiggly tilde-like thing, could work? Or maybe verticals would be better than horizontals, so you'd have a vertical bar and a wiggly vertical. That would actually slightly allude to the formal opposition between exclam and question, without being as literal.

Aside from the question mark to indicate uncertainty, the only symbol I can think of that is specifically associated with the opposite is three dots arranged at the points of an upright equilateral triangle. That is used in mathematics to represent "Quod est demonstratum" - Q. E. D., this has been proved.

John:
http://typophile.com/node/90438#comment-496503
http://typophile.com/node/90438#comment-496607

hhp

Well, it's plane 1, but how about looking through

http://unicode.org/charts/PDF/Unicode-6.0/U60-1F300.pdf

I particularly like "dog dirt," or PILE OF POO (1F4A9)

guys, everyone, and igor too behind the curtain. thanks!

So what did you chose?

Probably not what you want, but:

In symbolic logic, an open square character usually means "It's necessary that", i.e. that what follows it is true no matter what--there is no possible situation in which it would be false. An open diamond (often just a rotated square) means "It's possible that" (in other words, that what follows it is something that is not necessarily false).

In addition, one uses the turnstile (a vertical line with a short horizontal line coming out of the middle of its right side) and double-turnstile (same thing, but with two horizontal lines very close to each other). Turnstile and double-turnstile both mean, roughly, "It is provable that" as long as there is nothing immediately to their left. So the turnstiles mean almost the same thing as "It's necessary that" in many contexts. Sometimes they have slightly different meanings, but I doubt the distinction would matter for your purpose. (If there's something to the left of the turnstile, it means, roughly,that what follows is provable from or is justified by what precedes the turnstile.)

You can probably find examples of these in any font designed for typesetting math, e.g. in a default LaTeX setup.

I actually ended up with just bullets because of the time-frame and because neither of your suggestions worked as good as how I envisioned it. I wanted something very subtle and strong at the same time (oxymoron?), something only a few of the smartest readers would pick up on it, but still "graphical" enough to work as a lead-in for the rest. Thanks anyway!

I love these scarce moments when Typophile is all about obscure knowledge and true typohilia again, as opposed to the ID-my-font-or-do-my-homework direction it’s usually going in nowadays.