Using the phrase “just in case” as synonymous with “if and only if” is rampant in contemporary philosophy. I remember it first coming up explicitly in one of my logic classes as an undergraduate. The professor said that we should translate the English phrase “just in case” as “if and only if” (or its symbolic equivalent, “<–>”). Since we were supposedly talking about ordinary English, the students got to objecting to the professor’s translation rule. We insisted that in English “just in case” is only used as it is in the phrase “She built a storm shelter just in case a terrible storm came through town.” The professor was dumbfounded.
Well, so was I. Yet for as long as I’ve been studying philosophy, I’ve still been puzzled by it. But I’ve pretty much just put the issue on the backburner. My best guess has been that I’m not crazy, this is a technical turn of phrase used by philosophers (and perhaps other academics), and the professor had just gotten so wrapped up in that usage he lost touch with ordinary English. But I’ve always wondered whether this non-ordinary use is something philosophers really recognize as such. Could it be that young philosophers just assume it’s an ordinary phrase of English that only the highly educated use (and thus they follow suit)? After all, I have never heard it introduced as a special term, despite its ubiquity in philosophy classrooms.
I’ve discussed this issue with other graduate students in philosophy from time to time, but no one seemed to know much about the origin of this odd (I think) use. I got to thinking about this issue again today and did a bit of Google searching to shed light on it. I found this post by a linguist, Geoffrey K. Pullum:
http://itre.cis.upenn.edu/~myl/languagelog/archives/003470.html
He seems as puzzled as I am and draws pretty much the same conclusion that I had: it’s an (American) academic invention. But I wonder what others think, especially those within philosophy. Anyone know anything about this?
Southern California Philosophy 
I think I remember a discussion about ‘just in case’ with philosophers, as well, when I was at the Colorado Summer Seminar in 2007. I don’t know if I’d seen, or noticed, it being used in a special way in philosophy before that. In fact, I think that, in the course of our discussion, many of us took it to mean, not ‘if and only if’, but ‘only if’, at least for a while. So we would have thought “John is a bachelor just in case he is a male” was OK. That just seems very wrong to me now, and I’m having trouble getting back into that frame of mind. I think the idea was that ‘A just in case B’ could be translated as ‘A cannot be true unless B is true’, or ‘A is only true if B is true’, or something like that. And, of course, these are all ways to express ‘A only if B’. Anyway, like I said, I can’t really read it like that now – I’ve been corrupted by philosophy again.
By: Justin on December 8, 2009
at 8:09 am
I think you are right that “just in case”, used as a biconditional, is an academic/technical usage. But consider a slight variation in phrasing:
I will go to the mall just in (the) case (that/where) Susan goes too.
This seems much more natural to read as a biconditional (or at least as giving a necessary condition). I am not sure why, but for me the addition of the definite article makes it distinctively easier to get a “solely” reading of the word “just” (though I suspect the actual change would be with the interpretation of “in case”).
By: Lewis Powell on December 24, 2009
at 6:34 pm
Oh, I think you can also get a conditional reading for some ordinary uses. Fire alarms and elevators make for good examples:
Pull handle in case of emergency.
In case of fire, use stairs.
The “lest” reading is not available in either of these cases. Now suppose we had a set of stairs that were restricted to emergency use. We could imagine a sign on those stairs reading:
Use stairs just [only/solely] in case of fire.
I don’t think ordinary speakers would be confused by such instructions.
By: Lewis Powell on December 24, 2009
at 6:43 pm
There’s not much sense trying to defend the usage, which – whatever its historical origins – is clearly a jargon to try to cut off communication with other intellectuals who might otherwise find it too easy to encroach on the analytic philosopher’s academic turf (every sub-field has them). The philosopher is already hard to understand, so why throw up artificial hurdles except for (conscious or unconscious) protection of one’s sphere of influence, prestige, etc. Clearly everyone outside of analytic philosophy circles (cliques) will take the natural meaning of “just in case” to be “lest,” “as provision against,” etc. It’s funny that the only example that comes close to the “if and only if” reading isn’t “just in case” at all, but “in case” – clearly another idiom. Imagine if warning read “pull handle just in case of emergency”? Who would call that idiomatic? And, incidentally, “in case of fire, use stairs” is not at all a bi-conditional (not if and only if). It certainly doesn’t mean: Take the stairs if and only if there is a fire. (If so, that’s a rule I’ve frequently violated, since I prefer the stairs.) So for that reason as well it doesn’t support the “if and only if” reading of “just in case.”
As to why analytic philosophers mostly use this expression, I’d say it reflects a need to be perceived as insiders and a lack of self-confidence in using the common language against the trend of the group. Jargon dominates every discipline today, and nobody should blame those who practice it. But clearly it is a crowd phenomenon, and a strong character will resist this and communicate to the educated world as much as possible – at any rate, not set up pointless jargon when there are already clear universally understandable expressions in the common language.
By: bill on February 26, 2011
at 8:25 pm
Let me add: I do agree that it is possible to read a bi-conditional in the expression “Use stairs just [only/solely] in case of fire.” But without imagining in advance that the stairs are restricted to emergency use and without the words “[only/solely]” after “just,” I can as easily read it as “lest,” “as a precaution against,” etc. And if we formulate it the way analytic philosophers use it, that is, followed by a proposition, it leans even more in that way: “Use the stairs just in case there is a fire.” Of course, we imagine this sentence spoken or used in discourse, and not pasted on a wall. Here is the context: Looking out from the window on the fifth floor, I see some smoke drifting up from below. I point it out to my office mate. She asks if she should go down to the first floor to get the report. I say: “Okay, but use the stairs just in case there is a fire.”
By: bill on February 27, 2011
at 12:28 am
As a Philosophy undergrad the term is confusing, which is actually what led me to this site from a google search. Lewis’ post sheds some light on the usage of the term, but I agree with the OP that it simply encourages a standard English reading which is confusing for me. Anyway thanks for the post, it may not have answered my question, but at the very least it lets me know I’m not an idiot for asking it in the first place 😉
By: Graham on May 8, 2011
at 1:05 am
I will go to the mall just in (the) case (that/where) Susan goes too.
Nice call Lewis.
The words ‘just’ and ‘only’ often serve the same function in day-to-day English.
A few other people have suggested that ‘just in case’ is comparable to ‘only if’
After adding Lewis’ definite articles, and swapping the word ‘just’ with the word ‘only’, the sentence becomes
“I will go to the mall only in the case that Susan goes too”
Or
“I will go to the mall only in the case that Susan comes with me”
That makes the connection between ‘only if’ and ‘just in case’ much clearer to me.
By: David on November 28, 2012
at 12:35 am
(Disclaimer: I’m not a philosopher but an applied mathematician who’s been using “just in case” for decades.)
I’d be ok with switching to some other phrase such as “exactly when”. However I dislike “if and only if” because it’s wordy, and also because it unnecessarily decomposes the simple concept of logical equivalence as the conjunction of two implications. If p is equivalent to q and q is equivalent to r one can see that p is equivalent to r without any reference to implication.
By: Vaughan Pratt on February 1, 2015
at 6:22 pm
I think the good thing about ‘if and only if’ is it keeps the distinction between necessary and sufficient conditions, even if it is a bit wordy.
The first ‘if’ refers to the the sufficiency of the condition; the ‘and only if’ refers to the necessity.
If we’re doing mathematics and looking for equivalences, then I can totally see why that wouldn’t be necessary.
But if we’re trying to apply logic to get a better understanding of why one thing might be conditional upon another, eg asking whether love might or might not be a necessary or a sufficient condition of happiness, then ‘if and only if’ is a pretty useful tool.
By: charlieduncan on February 2, 2015
at 2:59 pm
Thanks for the perspective from mathematics. Various phrases seem more apt for different contexts. My only beef with “just in case” is that it’s a made up phrase that only a select few in the world know the meaning of. If we want our work (especially in philosophy) to be accessible to the intellectual members of the public, I don’t think we should use made up phrases disguised as real English. So “exactly when,” “if and only if,” they’re all major improvements on “just in case” in my book.
By: Josh May on February 2, 2015
at 3:08 pm
Quine introduced the convention of using ‘just in case’ to mean ‘if and only if’ in his 1940s publications. I can see interpreting ‘in case’ to mean ‘if’ and ‘just’ to mean ‘only,’ so that ‘just in case’ means ‘only if.’ Quine took the different interpretation of ‘just’ as meaning ‘exactly.’ So, he took ‘just in case’ to mean ‘exactly if.’ I’m trying only to explain the origin of this use, not to justify it.
By: praymont on July 26, 2023
at 4:09 am