I'm thinking of a major league baseball game that was played yesterday. Tell me, did the home team win?
Or rather, don't tell me, because you don't know.
By my count, there were fifteen major league baseball games played yesterday, eleven of which were won by the home team. The odds are, then, that the game I have in mind was won by the home team. But suppose you know I'm a Phillies fan (to the extent I'm a baseball fan at all), and you also know the Phillies lost at home yesterday. And I'll add that I did not choose the game I have in mind purely randomly.
Now do you know?
You may have enough knowledge to place a rational bet. You may have a strong suspicion about what I'm up to which implies what the answer is. But no one else in the world knows whether the home team won the game I have in mind.
Any answer you give is underdetermined. Given what you know, there is another possible answer contrary to your own. You simply do not have enough information to determine, with certainty, the correct answer. The best you can do is guess.
Now, if you did guess, I could tell you whether your guess was correct, and if it was, you could say, "I knew it!" And we'd both have a good laugh, since of course the whole point is that you don't know it.
Another thing you don't know is what it is I've read in the last twenty-four hours that prompted me to write this post. You don't know the answer to the question,
What have I recently read that was a perfect illustration of an underdetermined assertion, made in a situation where all the available information, while consistent with the assertion, is also consistent with a contradictory assertion?
People generally like their situations resolved. When a situation is consistent with something they want to be true, it's often game over: if what they want to be true could be true, then it is true, and pointing out that what they want to be true could also be false is at best bullheaded contrariness.
Situations that only admit of underdetermined explanation or answers are everywhere. (Some say it's true of every situation.) We have to do a lot of guessing to get through the day.
What we don't have to do, though, is to confuse what we guess with what we know.