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Recently I saw the movie "Pursuit of Happyness", which is actually quite good, and I noticed the actor (Will Smith) asking the difference between probably and possibly to his son. So I would like to have people's opinion on this question as I am still yet to find an answer myself.

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I would look up a dictionary rather than asking people’s opinions. – Tsuyoshi Ito Dec 2 '10 at 16:01
I would say the difference between probably and possibly is the about the same as the difference between may and might. – Eldroß Dec 2 '10 at 16:07
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@Eldros: I'm not sure that's an accurate comparison, as neither "may" nor "might" indicate any particular probability. See also english.stackexchange.com/questions/7/… and english.stackexchange.com/questions/3785/…. – Steve Melnikoff Dec 2 '10 at 16:57
@JSBangs: good catch, thanks a bunch. – RegDwighт♦ Dec 2 '10 at 21:01
I remember the difference between these two words from the movie "Pursuit of Hapiness", where the son asks his dad about the different between the two words. – Pupil Dec 2 '10 at 22:25

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7 Answers

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Probably

  • chances of happening are high (or at least not minuscule)

Possibly

  • chances of happening are not zero
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"How can this possibly be true?" doesn't exactly reflect a non-zero chance, so no, it's not the full definition of the word. – mojuba Dec 2 '10 at 16:25
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"How can this possible be true?" -> "How can there be a non zero chance of this being true?" I don't see a problem in that. – Let_Me_Be Dec 2 '10 at 16:26
the problem is that "probably" means exactly the same: non-zero chance, and never particularly very high chance. – mojuba Dec 2 '10 at 16:37
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@mojuba Possibly doesn't imply any mathematical probability apart from non-zero. The chance can be high or low, it just can happen (therefore non-zero). On the other hand Probably implies certain not small chance. If you are probably going to win, then the chances of winning are definitely not small. – Let_Me_Be Dec 2 '10 at 16:49
I'd say that if something is "probable", it's implied to be the single most likely outcome. I agree that "possible" isn't necessarily implying any particular level of mathematical probability, although the tone of voice in which someone says "it's possible" can speak volumes about just HOW possible they perceive it to be. – Hellion Dec 2 '10 at 17:36
up vote 6 down vote

These terms actually have fairly precise definitions, although they're sometimes used with a bit of liberty.

Let P = probability of event occurring

Probably => 0.5 < P < 1

Possibly => 0.0 < P << 1

It is evident that in some cases, there is overlap between the two terms.

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conversely, impossible => P = 0 and improbable => P ~ 0 (nearly zero) – bagheera Dec 21 '10 at 17:24
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To say that something is probably the case is to say that that case is probable. To say that something is possibly the case is to say that it is possible. The two are quite distinct.

If something is probable, then it is likely to happen. The chances are not only more than zero; they're better than 50/50 (or at least, the chances are higher that it'll happen than that it won't — since in comparing probabilities, you could have multiple items which have chances better than 50/50 but have one of them which is more probable than the others). Probably indicates that the laws of probability say that it's more likely to happen than not to happen.

On the other hand, if something is possible, then it can happen. Whether it will or not is irrelevant. The chances are better than zero. It doesn't say anything about how probable it is. It could be that the chances are .000000001% or that they're 100%. By saying that something is possible, you're not really saying anything about probability. You're just saying that it could — at least theoretically — happen.

So, if you say possibly, then you're saying that something could be the case, whereas if you're saying probably, you're saying that not only can it be the case but that the chances are higher that it will happen than that it won't.

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I have the same intuition on probably. Note the multiple outcomes case -- if there are 2 red balls, 1 green, 1 blue and 1 pink, you'll probably pick a red ball at random (it's not 50/50, but more likely than the alternatives). – dbkk Dec 2 '10 at 19:09
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This is a bit unclear. Saying that the chances of something are better than 50/50 is exactly the same as saying that it's more likely than not. Relatedly, @dbkk, I disagree with your intuition: in your example, you probably won't pick a red ball at random. – Henry Dec 2 '10 at 20:59
@Henry I see your point. Native speakers can have different intuitions about word usage (or even syntax), especially for the edge cases. – dbkk Dec 3 '10 at 3:57
up vote 1 down vote

Probably implies that the probability of an event is higher than that of the alternatives. In the most common case with 2 alternatives, probably means something is more likely to happen than not. With 5 alternatives, probability of the event is >20%.

Possibly implies a non-zero probability, usually a low one.

Conversational usage may not fit neatly into these definitions, as the intent is often mild exaggeration or irony.

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I guess I said this in response to your comment above, but I don't agree with your definition of probable. Probable means more likely than not; two mutually contradictory possibilites cannot both be probable. With 5 alternatives, no individual outcome is probable. – Henry Dec 2 '10 at 21:10
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Usage of Probably:

Probably, you should be able to do it. = Mostly, you should be able to do it.

 

Usage of Possibly:

Possibly, you should be able to do it. [= If there is a chance (some factor implied), you should be able to do it.]

Possibly, you should be able to do it. [= If it is possible, you should be able to do it.]

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up vote 0 down vote

I think the difference between probably and possibly is somewhat related to may and can. E.g.

  • Probably enough to make you laugh = may or may not be enough, not very sure about it, neither does it matter very much

  • Possibly enough to make you sick = can be enough; warns of the ability (e.g. of a virus) to make you sick, and it's important to know

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As you write it, possibly would have a stronger meaning that propably, but isn't it the other way around? – Eldroß Dec 2 '10 at 16:38
@Eldros: I think in many contexts they both bear the same meaning of some probability in a mathematical sense, but sometimes "possibly" also has some "ability" flavor in it. – mojuba Dec 2 '10 at 16:44
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I disagree with your analysis. For me "probably" is certainly more likely than "possibly", and neither says very much about the importance of the question. – Colin Fine Dec 2 '10 at 17:53
@Colin Fine: I'm not saying which one is more likely to happen, but indication of ability (possibly) and indication of a chance (probably). E.g. just found this phrase on the 'Net: "It's not only possible, it's probable" in a sense that not only someone can do it in principle, but also is likely to do it. – mojuba Dec 2 '10 at 18:12
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This isn't right. "Probable" means the thing is likely to happen. If I say something is "probably enough to make you laugh" it means that I think it's likely, though perhaps not guaranteed. And neither "probable" nor "possible" carries an implication of whether the thing is important or not. – Henry Dec 2 '10 at 18:32
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up vote 0 down vote

Wow... after reading the mumbo jumbo... hahaha I can't remember where I got those words.

Again, spanish helped me.

Probably =

Based on prudent reason - That can be proved - Said when there's good reasons to believe it will happen or will be verifiable.

Possible =

That can be or happen - that can be executed - available media to do something

Possible implies that exist the possibility, let's say 1%.

Probably implies that based on probability the chances to happen are good, let´s say above 50%.

Remember that Probable, probability, are math terms in statistics. Possibility is not used because it works even against all odds. Just having the minimal chance, something is possible. But is probably not going to work, because the chances are too low.

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