Use of “Might” and “Might not”

I know "Might" and "Might not" means the lack of certainty, but is there an implied probability in the use of these terms?

In other words, does "I might be coming" imply that "It is extremely likely I will be coming" or is it a case of "It is likely that I will be coming" or moving down even lower "It is feasible that I will be coming" ?

Is the situation identical with the use of "Might not" ? When someone says, "I might not be coming" what approximate probability level are we talking about regarding their "not coming"?

I realize this is a basic question -- sorry about that; Just was not sure about the usage.

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I'd feel a bit diffident about actually editing your question, but people usually say just possible or feasible, rather than slightly possible. Your call. – FumbleFingers May 20 '11 at 0:26
@FumbleFingers Thanks for the correction! Please feel free to edit. – Sai May 20 '11 at 15:21
Talking about 'percentage probabilities' in this context may be a bit futile. We're in an area where speakers frequently intend to be vague or non-commital. – FumbleFingers May 20 '11 at 15:44

The word "might" is used to express possibility in favor of something. Negating it expresses the opposite. Technically, "might" allows for the possibility of "might not" but it doesn't necessarily offer that as a possibility to be considered.

I might come to your party.

This gives the listener some hope that you will attend the party.

I might not come to your party.

This alerts the listener of the sad possibility that you will not attend.

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I think that when I say "I might come to your party," we assume the expected situation, if I do not confirm, is that I am not coming. Think of "I might get home by 7." The part unsaid is that it's more likely I will get home later as usual, and I'm just hoping to get home so early. The opposite is true with "I might not come to your party" or "I might not get home by 7" - we assume that the expected situation is that I'd be coming, or getting home on time, and I'm telling you because this is the exception. – aedia λ May 19 '11 at 21:02
As I've learned, "might" refers to a possible future activity. is there really an expected situation for it? – user8568 May 19 '11 at 21:16
@boob I think there is at usually least an assumption that I wish to contradict with the "might" statement, especially with "might not". Take "It might not rain" - when I say this it implies that I believe you and I understand there is a strong chance of rain. – aedia λ May 19 '11 at 22:42
@aedia: It depends on how it's said. If I say "I might be home by 7:00" I could either be encouraging the listener to hope or, with a certain amount of doubt in the tone of voice, express the opposite. Similar to could: "You could be President!" vs. "You could be President ..." The first is optimistic, the second is doubtful to the point of sarcasm. – Robusto May 20 '11 at 2:13
@matthew-frederick @kit Thanks! I've posted my comments as another answer, including @robusto's note about tone. Feel free to edit... not sure if it got a bit convoluted in the telling. – aedia λ May 23 '11 at 19:06

The probability or likelihood of something happening isn't just with the "might" or "might not" statement, but in the surrounding context. The speaker thinks the listener has a particular assumption and wishes to contradict it.

It might not rain.

When I say this it implies that I think we both understand there is a strong chance of rain. I haven't said very much yet about the new probability of rain. It may be a hope, or I may have new information.

On the other hand, if we'd both been aware of only predictions for good weather, it's hard to think it would make sense for me to say "It might not rain." You'd ask me if I was talking about the weather somewhere else or if I knew something you didn't. But "It might rain" would make sense, even just as a caution to bring an umbrella.

Take another example:

I might be home by 7:00.

If you require an exact answer from me, much as with "I might be coming," you'll have to wait, or insist that I be more specific. I'm not giving you very much information about my certainty with this one statement. But my statement again doesn't occur in a vacuum. My statement implies that the listener and I have an expectation about when I'll arrive home. If I was trying to arrive at 7 or at 8, that can change the meaning from something like "Don't count on me getting there on time" to "I could be early".

As Robusto pointed out (thanks!), the tone of voice can make a difference as well. For example, the listener and I could both have the same understanding - that I am trying to arrive by 7 - but I can say the same phrase and mean different things:

If I say "I might be home by 7:00" I could either be encouraging the listener to hope or, with a certain amount of doubt in the tone of voice, express the opposite.

So when as a speaker, I say

I might be coming

or

I might not be coming

it almost says more about what I think you, the listener, assume, than about what I (speaker) am actually going to do. I'm still unsure about what's going to happen, but I don't want you to get the wrong idea. I'll tell you "No, no, I might be coming" so you don't get the idea I've already decided not to go; I'll tell you "Actually, I might not be coming" so you don't think I've already committed and count on me to bring all the drinks.

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+1 Excellent, this is the way I think of them and is a great way for non-native English speakers to use them. Glad you went with an answer. – Matthew Frederick May 23 '11 at 19:20

I read "I might be coming..." to express the possibility but not necessarily the likelihood of attending, and "I might not be coming..." to express the opposite. If I wanted to express strong possibility, I would say "I will probably [not] be coming...". A lot depends on context; for example, "I might be coming, assuming I grow a second head" is not the same as "I might be coming, unless I get one of my infrequent migraines".

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"Might" and "might not" are vaguer than some other phrases about how likely the event is, and usually mean an intermediate amount of probability. Very soft bounds would be that "I might be coming" is in roughly the 10% to 50% range, though those are very inexact (and of course vary from speaker to speaker and situation to situation). (In particular, "I might be coming" can easily extend a bit above 50%, though as the event becomes more likely, people will start saying things like "I'm probably coming" instead.)

On your list of phrases, "I might be coming" is a bit less likely than "It is likely that I am coming", but more likely than "It is slightly possible that I might be coming".

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"Might be" means the lack of certainty, as you know but about the probability of happening I doubt if there is a exact number, when someone says "I might be coming" means the probability of coming is more than not coming and vice versa. but in both cases the speaker is not sure about what he/she is going to do.