There are certain cases where the phrases "I guess" and "I'm guessing" are both legal to use. For example:
Q: Why do birds fly south for the winter? A: I guess it's because it's warmer down south.
Q: Why do birds fly south for the winter? A: I'm guessing it's because it's warmer down south.
Q: Why shouldn't you press this button during flight? A: I guess it would make the airplane hard to control.
Q: Why shouldn't you press this button during flight? A: I'm guessing it would make the airplane hard to control.
In these cases, is there a difference between "I guess" and "I'm guessing"—some subtle nuance that one phrase has but the other phrase doesn't have? Or are the two phrases exactly the same?
The impression I've gotten as a native speaker is that "I guess" doesn't literally mean "I guess"; it means something more like "I conclude". In particular, the phrase "I guess" seems to indicate that I am satisfied with my own conclusion and I am no longer looking for information. The phrase "I'm guessing", on the other hand, indicates that I'm uncertain about what I'm saying, and I am interested in more information.
So in my mind, the four answers above are probably saying:
I guess it's because it's warmer down south. (I thought about it just now, and my conclusion is that it's probably because it's warmer down south. I'm not sure about this, but I'm not going to try to gather more information.)
I'm guessing it's because it's warmer down south. (My best guess is that it's because it's warmer down south. Do you have more information?)
I guess it would make the airplane hard to control. (I thought about it just now, and I concluded that it would make the airplane hard to control. I'm pretty confident in my conclusion, but I'm not sure that that's the actual reason why you shouldn't press that button.)
I'm guessing it would make the airplane hard to control. (My best guess is that it's because it would make the airplane hard to control. Is that right—would it actually make the airplane hard to control? And is that actually the reason why you shouldn't press the button?)
Am I on the mark here, or am I just seeing a distinction where no distinction exists?
(Note that I'm only asking about cases where both phrases are legal. I'm not interested in an explanation of cases where one phrase makes sense but the other does not, such as "Are you coming? / Yeah, I guess" and "Are you sure? / No, I'm just guessing".)
Best Answer
The OED does provide a separate definition for I guess; It is definition 6:
The three earliest attested uses are from texts written by John Locke, suggesting that he might have popularized the sense with this kind of meaning. The earliest citation is from Some Thoughts Concerning Education, dated 1692, so it's not an incredibly recent idiomatic turn.
Hence, your intuition was correct when you wrote:
As a final note, I guess isn't limited to this sense of the word implying something close to certainty. It could still be used in a phrase that meant an actual guess, but such a use would be distinguishable through context.
As you can probably imagine, had Jim replied with the present continuous tense of "guess," it would have seemed strange.
This seems to be exactly the distinction you described in the question, so the answer is yes, you're on the mark.