Consensus has been reached.

I say this on Rubio's authority - the contents of this post are considered to be a part of the definition of this site's scope.

So, I posted this open-ended puzzle a while ago, looking forward to all the fun around it and the upvotes that would pour in.


That question ended up with a score of +1, and 4 downvotes. (now +2, 7 downvotes. Meta effect. Also, I've updated the score multiple times, and I'm tired of it XD. My point doesn't really rely on the current vote count, anyway.)

It was my first and last open-ended puzzle. It also sparked some discussion on TSL, where Deusovi said "I don't think open-ended "puzzles" should be on the site at all, to be completely honest. They're fun games, but IMO they're not puzzles."

To which Rubio replied "I don't mind open-ended puzzles. At least, not if they're any good, and are well constrained," only to say "Ok I think we actually do have a couple [of good open-ended puzzles], but - the bad ones are way more prevalent."

Deusovi mentioned that he didn't have time to bring up the idea of removing open-ended puzzles on Meta, and he was unsure that the proposal would be viewed positively in the first place.

So, armed with my poorly-performing question and Deusovi's agreement, here comes the main point:

Open-ended puzzles are too broad for this site.

Some reasons why:

  1. There is almost never a final answer. Even the tag wiki admits as much! Questions with no final answer are unhealthy for SE's Q&A format. Also, to quote Deusovi again: "puzzles should have only one solution, and anything more than that is either a broken puzzle or not a puzzle at all."

  2. Minimal effort is required on the part of the asker, encouraging lazy questions in general. (Credit to North for this idea)

  3. The community isn't really a huge fan of these puzzles. I'm not the best puzzle-maker, but I'm not too bad, and I thought that my open-ended question was formatted and posed well. It still got 4 downvotes. That's a good indication that the type of puzzle it is isn't very popular.

With that being said, I hereby propose that we lock the "good" open-ended puzzles for their historical significance and close the rest.

Perhaps getting rid of the "open-ended" tag would be in order as well?

  • 3
    $\begingroup$ Or should we just combine open-ended and optimization? $\endgroup$
    – athin
    Commented Apr 20, 2019 at 21:00
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    $\begingroup$ I think the tag should just be reworded instead of gotten rid of as a whole $\endgroup$ Commented Apr 20, 2019 at 21:46
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    $\begingroup$ Related: meta 1, meta 2 $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 8:05
  • $\begingroup$ Huh, the user that gave the only upvote to your question — that was me! Boy, do I feel a little warmhearted, considering that four other users downvoted... but I do believe you are right: open-ended questions are too broad, and questions that are too broad are unfitting, thus so are open-ended questions. Yes, some open-ended questions are popular, but when the third most popular open-ended question is closed, this is saying something. $\endgroup$
    – Mr Pie
    Commented Apr 26, 2019 at 1:24
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    $\begingroup$ @user477343 5 upvotes, 4 downvotes. You were one of 5. (thanks, though!) $\endgroup$
    – Brandon_J
    Commented Apr 26, 2019 at 1:25
  • 2
    $\begingroup$ So now that consensus has been reached, what do we do now? Do we deprecate the open-ended tag? What happens to the questions with that tag? Despite reaching consensus, I don't think we've defined a course of action to take besides that open-ended puzzles should be considered off-topic, and even that doesn't provide any guidelines on what to do. $\endgroup$
    – HTM
    Commented Apr 30, 2019 at 19:30
  • $\begingroup$ That's true. We have defined "off-topic" moving forward, but we haven't said what to do with the old ones. Paging @Rubio ooooh-yi-aoooooh!! (or another mod - I just know that Rubio approved the consensus, so I'm paging him) $\endgroup$
    – Brandon_J
    Commented May 4, 2019 at 15:46

3 Answers 3


(Much of this answer comes from a discussion in chat with Rubio, Brandon_J, and North.)


I see several problems with the "open-ended" format. But first, a definition, to make sure we're all on the same page:

An question is one where multiple answers are expected; answers are ranked by some sort of rule, and the answer with the best score by that ranking is the accepted answer.

(Without the second part of that sentence, it's just a question that is intentionally too broad. These are already disallowed.)

So, what problems do I see? Well:

Dependence on other answers

In questions, an answer can be invalidated by other answers. If Answer A scores 18 points on whatever scale a puzzle used, and then answer B is posted (scoring 19 points), then Answer A is retroactively no longer a correct answer.

Compare this to regular puzzles: if someone posts a strategy for a "prisoners and hats" puzzle, it does not get disproven because someone found a simpler strategy. An answer there can only be invalidated by an inherent problem with that answer, not just "not being as good as this other answer".

Similarly, for another comparison, if someone posts a partial answer (to, say, a crossword), and then someone else posts a full answer, the partial answer does not become wrong. Even if the partial answer was posted after the full answer, the partial answer is still correct.

One goal of Puzzling Stack Exchange is to be an archive of high-quality puzzles that are solvable at any point in time, whether it's immediately after the puzzle is posted or ten years later. This seems to me to be in direct conflict with that: an answer at the start is not necessarily an answer later. Which leads me directly to another reason:

How do you know the best answer is the best?

With these questions, you can almost never know when an answer is correct. It's always possible someone can come along a year later and one-up an already-posted solution. So these questions are implicitly of the form "What's the best way to do X... that the PSE community has thought of so far?". That's why the ranking system exists: it allows you to pick out of the answers of the PSE community, not

So the top answer is in the same position as all of the other answers were: it's always waiting to be outdone. questions don't have a clear correct answer, and having a clear correct answer is a quality necessary to be a puzzle. And speaking of which...

These aren't puzzles. They're games.

questions may be fun things to think about. But they're not puzzles. As Rubio put it:

The danger - and I think we're correct to avoid it, and it's something Deus alluded to in an earlier comment - is that instead of a puzzle what we really have is a game: find the best thing you can come up with. (I've even seen some that are: best solution in days is the winner)

There is no "solution" to these questions, and that's by design. They're meant to be fun things to think about, but they're fundamentally not a thing that can be solved. Unless you have a proof of optimality (which the question is generally not designed for), all you can do is incrementally improve on previous answers and hope that nobody will improve on yours.

We don't treat analogous situations the same way.

I've already pointed out that we don't allow other questions without a single best answer. (Note that the ranking system doesn't satisfy this: it allows you to find a single best answer out of the ones posted by PSE members, but not a best answer in general.) We don't allow other questions whose answers change based on the answers that PSE members have posted -- that wouldn't be a self-contained puzzle. It's why was deprecated.

Consider a question that starts like this:

Here are 20 lines taken from various riddles. What object fits the most of these lines?

Assume, for the sake of argument, that whether something fits a line is generally not up for debate: they're fairly clear qualities like "this object is smaller than a breadbox" or "this object is green".

This question should probably be closed. Yet this seems completely analogous to, or even better than, questions as we have them. Any problems this question has apply equally to questions, yet we allow the latter.

The inevitable problem of HNQ

questions attract many answers. This is one of the criteria used to pick questions for the Hot Network Questions list, which is the first impression many Stack Exchange users get of Puzzling. We're putting our worst foot forward here, and showing questions that are (hopefully) not representative of the rest of the site's content.

There's unfortunately no way to show that this is driving people away without something to compare to. But I know that if my first impressions of the site were questions, I probably wouldn't have stayed.

So why do we make this exception? Why do we allow the tag to absolve askers of all responsibility for making sure their questions have solutions, and let them break some of the core principles of PSE (solvability at any point in time, a single best answer, self-contained puzzles)? I don't believe that there's any benefit to keeping these.

A proposal

In the aforementioned discussion, we eventually came to a conclusion that would draw a fairly clear line, while still allowing well-designed optimization questions. There are three things that might be called "open-ended" questions, that we'd still like to allow:

  • Optimization questions

These should have a provably best answer (ideally, not one that needs heavy programming or mathematics to prove, in the same way that we currently don't want puzzles that require heavy programming or mathematics to solve). Answers should come with justification of why they are optimal; an answer without this is not a full answer, but more of a comment.

  • Questions about whether something is provably optimizable

These are allowed, but answers are of the form "yes" or "no", with justification. Again, "you can get this much but I'm not sure if you can do better" is a comment, not an answer.

  • Questions about puzzle design that might have many possible answers

Here, we can follow the same rules as other sites on the SE network. These questions should not be asking for solutions to puzzles, but for the best way to accomplish some sort of design for a puzzle.

  • 1
    $\begingroup$ Soooo.....when do we reach consensus? $\endgroup$
    – Brandon_J
    Commented Apr 23, 2019 at 12:26
  • 3
    $\begingroup$ "...then Answer A is retroactively no longer a correct answer." How so? It's not wrong unless it doesn't solve the problem posed in the question; it's just no longer the optimal one. Someone can still solve the puzzle later and try to optimize their answer; they just might not come up with the most optimized answer. I don't see a non-optimal answer as any different than a partially solved crossword. The answerer got partway there, just not all the way. (I don't know what's best for this site, but that point just seems wrong to me.) $\endgroup$
    – jpmc26
    Commented Apr 23, 2019 at 20:57
  • $\begingroup$ @jpmc26 The partial crossword is part of the completed crossword, but the non-optimal answer to the open-ended problem might have nothing to do with the optimal answer. $\endgroup$
    – noedne
    Commented Apr 24, 2019 at 1:50
  • $\begingroup$ @noedne Are you sure? Less optimal answers are often important steps in reaching the optimal one. $\endgroup$
    – jpmc26
    Commented Apr 24, 2019 at 4:59
  • $\begingroup$ @jpmc26 not all open-ended questions are optimization problems. Optimization problems have a provably optimal solution, while most open-ended questions have an arbitrary set of criteria to determine the “best” solution. Optimization is OK; open-ended is not. $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 6:29
  • $\begingroup$ @PiIsNot3 The definition states that, "answers are ranked by some sort of rule." This rule is the "function" to be optimized, even if it's not presented as a formal mathematical function. By definition, it would seem that all open ended questions must be optimization problems of some kind, though they may not be provably optimized. Regardless, my point is that answers are still not invalidated by a new, higher ranked (according to the rule) answer being given. They're only invalid if they fail to meet the other constraints. $\endgroup$
    – jpmc26
    Commented Apr 24, 2019 at 6:46
  • 1
    $\begingroup$ @jpmc26 The big issue with that is no single rule or even set of rules is enough to guarantee there be a single correct answer. For “longest word/sentence” type problems, the answer changes depending on what dictionaries you use and which words you allow. For “formation of numbers” problems, you have to define which operations to allow, what constructs are valid/invalid, etc. etc. Most questions tagged open-ended don’t have that level of rigor and instead devolve into a game of “what loopholes can I exploit that give a technically correct answer.” $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 6:59
  • $\begingroup$ In contrast, optimization problems are often grounded in math and logic, two subjects that have very clear-cut rules that don’t allow many loopholes. So that’s the difference between optimization and open-ended (or at least, what my understanding of the difference between the two is). $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 7:01
  • $\begingroup$ @PiIsNot3 The core of your point seems to be: "Most questions tagged open-ended don’t have that level of rigor and instead devolve into a game of 'what loopholes can I exploit that give a technically correct answer.'" That's a fine objection to the tag, but it has nothing to do with making one answer invalid by posting another one. As I said, I don't have any answers for whether the tag is bad or not; I just found the first argument to be very flawed, which hurts the persuasiveness of this answer. $\endgroup$
    – jpmc26
    Commented Apr 24, 2019 at 7:01
  • $\begingroup$ @jpmc26 It’s an unwritten rule on PSE that all puzzles must have a justifiably best solution. Allowing accepted answers to be beaten by “better” ones goes against this rule, as Deusovi alludes to. Perhaps that section of the post should be moved to the top to make the point clearer. $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 7:05
  • 1
    $\begingroup$ Let us continue this discussion in chat. $\endgroup$
    – HTM
    Commented Apr 24, 2019 at 7:10
  • 3
    $\begingroup$ @Deusovi I'd like to ask you to read over our chat conversation. I think you may find our discussion helpful in evaluating your answer. These two messages are perhaps of the utmost concern: 1, 2 (though the context around them is also important, of course). $\endgroup$
    – jpmc26
    Commented Apr 24, 2019 at 7:51
  • 3
    $\begingroup$ @Barker Yes, I agree that it is a direction the site could go. I never claimed we couldn't do it due to Stack Exchange standards, just that it would be bad for the site and go against some of the goals we have. $\endgroup$
    – Deusovi Mod
    Commented Apr 26, 2019 at 22:56
  • 4
    $\begingroup$ @Barker: codegolf.stackexchange.com has actually been strongly deprecating all the puzzles (i.e. one intended solution with no open-endedness); the "programming puzzles" in its name turn out to be better handled at Puzzling, because we're so set up for handling open-ended posts. I can't see any reason why having a clear contrast between the two sites would be a bad thing in this regard, because things like the site rules on meta need to be quite different in the two situations. (A simple example: on codegolf accepting answers is rare and looked down on, here it's almost mandatory.) $\endgroup$
    – ais523
    Commented Apr 29, 2019 at 7:00
  • 1
    $\begingroup$ @Shinjo We've tried mandating a sandbox for often-poorly-made puzzles before. It did not go well. $\endgroup$
    – Deusovi Mod
    Commented Apr 29, 2019 at 9:28

What is an open-ended puzzle?

If we're going to declare these off-topic, then let's have a clear definition so that the policy doesn't get misapplied. Deusovi has attempted to do this in his answer above, but I want to make it even clearer. (The reason for this answer is that I've already seen discussion over how/where it should apply, and I want to avoid a worst-case scenario where we ban all problems or "what's the most possible X" puzzles.

An question is one where there is no absolute "best possible" answer, so that any answer could conceivably be outdone in the future - the green checkmark will never be guaranteed to stay permanently in one place.


  • Growing anagrams - What is the longest possible list forming a sentence? is . The currently accepted answer has 16 words, but there's no way to "prove" that a given answer is optimal. Any accepted answer could conceivably be beaten by one with even more words.

  • Numbers on a Blackboard is NOT . The accepted answer proves that a particular number is optimal, but even if that answer hadn't been posted, the question still wouldn't be open-ended. It's clear that there will be an optimal number, and it just requires some mathematical ingenuity to prove it. Even if there had been several answers finding successively lower bounds, those would count as partial answers rather than open-ended attempts at the solution.

In an open-ended puzzle, any answer posted could be the best one, but even the accepted answer could be sub-optimal. If an absolute best answer exists and seems likely to be provable, then it's not open-ended: even if some answers are posted with successively better but non-optimal solutions, those are partial answers with increasing progress towards an upper/lower bound, not just iterations in an open-ended answer stream.

An open-ended puzzle could conceivably have arbitrarily many answers. That's literally what "open-ended" means: there is no upper bound on the potential for more answers, each one better than the last. (Here "arbitrarily many" might be literally "infinitely many" or it might be just "too many to reasonably fit on one Stack Exchange question". We don't want questions with ten billion answers posted either.) But if there's a question seeking an optimal number, and it's logically proven to be e.g. between 152 and 208, that's not open-ended either.

At the end of the day, this means we're likely to discriminate a little by tag. A "find the most X" puzzle is much more likely to have an absolute, attainable, provable, optimal solution than a one. That's just the nature of the beast. But hopefully the above discussion will help prevent the policy from being applied too zealously.

  • $\begingroup$ I agree with your ideas, but I most certainly disagree on GrowingAngrams being without an upper limit. As there is a bound on the length of English words, this will be the absolute upper bound of growing anagrams as well, wouldn't you agree? (And if there is a the argument of 'new' words being possibly invented, then one could easily restrict it by pointing to one of the "english-dictionary-lists" people have used for the answers anyway.) So, good arguments, maybe not the most convincing examples. $\endgroup$
    – BmyGuest
    Commented May 13, 2019 at 8:02
  • $\begingroup$ @BmyGuest No offence to your question (which I upvoted years ago), but I'm not sure if that "bound on the length of English words" falls into the category of "too large a bound to reasonably fit in a SE post". I'm happy to edit my answer and replace that example by another one if you can find something more clearly open-ended. $\endgroup$ Commented May 13, 2019 at 8:06
  • $\begingroup$ No offence taken. (And I think thanks to your post, the questions has recently seen a bit of traffic ;c) ) But your answer made me wonder a bit, and I'm actually becoming less and less convinced of the opinion. I'm thinking of coming up with a counter-arguement in this discussion, but am unfortunately running low on time right now. $\endgroup$
    – BmyGuest
    Commented May 13, 2019 at 8:09
  • $\begingroup$ Or maybe not. Considering that "consensus" has been reached. (Even if I might not consent ;c) ). $\endgroup$
    – BmyGuest
    Commented May 13, 2019 at 8:10
  • $\begingroup$ @BmyGuest Personally I'm not sure if I like this consensus either, but I came here too late to turn around the decision, so it seems this is our new policy. I'm just here to make sure it's not applied over-zealously ;-) $\endgroup$ Commented May 13, 2019 at 8:26
  • $\begingroup$ @Randal'Thor looking at your example "Growing anagrams" which is still an open questions, (and regarding my current closed question), I thought the example was a type of open-ended question that was NOT off-topic. $\endgroup$ Commented Mar 16, 2020 at 23:36
  • 1
    $\begingroup$ @ChristopherTheodore The thing is, we didn't go around closing all the old open-ended questions (although maybe we should). We've just been applying this policy proactively, not retroactively. Which of course is confusing to newcomers who see old well-received not-closed open-ended questions. Sorry about that :-/ $\endgroup$ Commented Mar 16, 2020 at 23:44
  • $\begingroup$ @Randal'Thor no problem. I think the policy is flawed (not directing that at anyone with malice). This very topic uses a corrupted definition of the word puzzle, as a main element for the basis for the "consensus", of what puzzles are. This is my reason for stating it's a flawed policy. I happen to like games that test ingenuity. The just aren't riddles, or logic problems, etc.. I do get there are some issues with open-ended puzzles.. anyways. $\endgroup$ Commented Mar 16, 2020 at 23:53

Some misgivings

Friends, puzzlers, PSE denizens, lend me your ears!

Despite the heading above, I am not trying to overturn the consensus that open-ended puzzles are a bad idea. But I'm not sure we have total clarity on exactly which puzzles are bad and and why, and in particular one thing that earlier discussion might make one think is the key feature seems to me not to be much of a reason to reject a puzzle.

(I shall be disagreeing with Deusovi quite a bit in what follows. That should be taken primarily as an indication of how clearly and unambiguously Deusovi has laid out the case.)

Specifically, consider the following remark of Deusovi's, quoted by Brandon in this question: "puzzles should have only one solution, and anything more than that is either a broken puzzle or not a puzzle at all". And the following, near the start of Deusovi's answer here: he gives a definition beginning "An open-ended question is one where multiple answers are expected [...]" and says "Without the second part of that sentence, it's just a question that is intentionally too broad. These are already disallowed."

I don't think that the mere fact of having multiple acceptable answers makes something not a puzzle, or even not a good puzzle.

The OED's definition of "puzzle" says things like "A puzzling or perplexing question; a difficult problem" and "Something devised or made for the purpose of testing one's ingenuity, knowledge, patience, etc." I think these get to the heart of what a puzzle is, and having a unique answer is not essential to it.

Consider, for instance, some of SlowMagic's recent word-transformation puzzles. You have (for instance) two words, and you want to turn them into two other words, by means of some repertoire of operations by which you can move letters between the two. For instance, this one and this one, both currently on a well-deserved +10. The solutions aren't (so far as I know) unique, but finding a solution at all is "a puzzling or perplexing question", "a difficult problem", and "something devised for the purpose of testing one's ingenuity".

For another example, consider puzzle 74 from H E Dudeney's "Canterbury Puzzles", called "The Broken Chessboard". It presents a set of 13 pieces and invites you to put them together to make an 8x8 chessboard. Dudeney apparently thought his solution was unique, but later Sam Loyd found another and Laurie Brokenshire a third; but finding any solution is enough of a challenge that it's a perfectly reasonable puzzle even though it has (it turns out) exactly those three solutions.

I think any definition of "puzzle" that excludes these things is a bad definition; and any criterion of acceptability-on-PSE that excludes them is a bad criterion.

Similarly, any definition of "solution" that would make a valid sequence of transformations accomplishing one of SlowMagic's goals, or a valid way to assemble a chessboard from Dudeney's pieces, unacceptable unless accompanied by a proof that one couldn't do it better, is a bad definition.

(Not because there's anything wrong with proofs. I'm a mathematician. I like proofs. I like questions that demand proofs. But it's OK not to prove everything.)

The foregoing does not constitute a rejection of the consensus here, because "open-ended puzzles" as considered here are not simply those that have multiple solutions. The definition near the start of Deusovi's answer begins as quoted above, but goes on: "answers are ranked by some sort of rule, and the answer with the best score by that ranking is the accepted answer". And Deusovi's proposal, which seemed to attract some measure of consensus, was that such puzzles should be forbidden here except for three special classes; I would class the first two together under the heading "finding optimal solutions with proof", and the third class is of questions about puzzle design that are not themselves puzzles.

It seems to me that if a puzzle would be acceptable here although possibly solvable in multiple ways, then adding a further criterion by which to choose among those possibly-multiple solutions shouldn't make it unacceptable.

(By this point, yeah, I suppose I have arrived at a definite disagreement with that consensus. You should have expected that ever since I started with "Friends, puzzlers, PSE denizens ...". But when I say that Deusovi is an honourable man, I really do actually mean it.)

And yet ... Clearly something is wrong with questions like Brandon_J's that started all this, which just invites "solvers" to construct "as many pairs of alliterative ailments as possible". (I would apologize to Brandon for saying this, but he's already said as much himself.) As Deusovi says, it's more game than puzzle. But, as you can see above, I don't quite agree with the diagnosis of how it's wrong. What, then, is the actual disease?

One feature that SlowMagic's and Dudeney's puzzles have (and would continue to have if we added some sort of ranking to the solutions), and Brandon's lacks, is that finding any solution at all is a challenge. That element of challenge is, for me, the most important thing that distinguishes puzzles from things that are not puzzles. Finding phrases like "fever and flu" or "cough and cold" is not a challenge, and that's why Brandon's puzzle is actually not a puzzle but a game.

Finding more and more and more pairs is a challenge, though. Does "find lots of pairs", or "find a hundred pairs", or "find all possible pairs", constitute a decent puzzle? Well, "find lots of pairs" lacks another quality that for me is near the heart of puzzle-ness (though not quite so central as the element of challenge): definiteness. Not in the sense of having a unique answer, which as I've said above doesn't bother me so much, but in the sense of being able to tell unambiguously whether something is an answer or not. I think "find a hundred pairs" is a puzzle, but it's an incredibly boring one. And "find all pairs" is indefinite in two slightly different ways: first, "ailment" is a category with fuzzy edges; second, there's no good way of checking whether an alleged accounting of all alliterative ailments actually achieves absolutely all.

So the position I'm coming to is that mere open-endedness as such is no obstacle to being a puzzle, or even a good puzzle, but that a puzzle of this kind is only any good if it would still feel like a puzzle without that criterion for ranking solutions, and the ranking criterion should be seen as secondary, as a way of choosing between multiple acceptable answers if they should show up.

I'd like to look at some of the other objections raised to open-ended puzzles. Deusovi lists some interesting ones.

"An answer can be invalidated by other answers." (Because if the goal is to find not just any X but the best X, then something can cease to be the best merely because someone else did better.) I am not convinced that this is very bad. I have fairly often seen the following on Stack Exchange sites (including PSE, though I think it happens more often on others): someone asks a question; someone gives a good (and, literally, acceptable) answer; someone else comes along and gives an even better one -- one with clearer explanations, deeper insights, helpful illustrations. Even questions that don't explicitly ask for "the best X" can have better and worse answers, and sometimes an answer is good enough to get accepted but turns out not to be the best, and another is accepted instead. There's nothing wrong with that.

"With these questions, you can almost never know when an answer is correct." (Because if the goal is to find the best X, then the only way to know whether it's been achieved is to have a proof, and proof is hard.) Again, that's true, but if we take these questions to be saying "find an X; best one gets the green checkmark" then this isn't really a problem any more than the fact that any answer to any question might turn out to be outshone by some brilliant later answer. Deusovi says, further, that "having a clear correct answer is a quality necessary to be a puzzle"; if that's taken to mean a unique clear correct answer then, as I've already said, I can't agree.

"We don't allow other questions without a single best answer." But we do! This one asks for a Life configuration achieving a certain goal, which can be achieved in multiple ways, and the poster even says "The puzzle has been solved, but please don't be discouraged from posting alternative solutions. They are still very valuable, as they seem hard to come up with." This question, by the way, is on a mere +177 points.

"We don't allow other questions whose answers change based on the answers that PSE members have posted -- that wouldn't be a self-contained puzzle." True enough, but again if we think of these questions as "Find an X; best one gets the green checkmark" then the answers aren't changing based on other answers posted; all that changes is whether an answer is worth posting, and there's precedent for that: we reject answers (to any question) that merely repeat what's already in other already-posted answers.

"Open-ended questions attract many answers, which makes them likely to land in HNQ, which gives a bad first impression of PSE." Bad questions with many answers are bad, for sure, but the solution to that is to downvote them into oblivion. I do agree that open-ended puzzles are likely to exacerbate the problems PSE has with the HNQ algorithm, but the real problem here is that the HNQ algorithm doesn't understand PSE and picks unrepresentative questions, and I don't think we should let that drive our definition of what is and isn't an acceptable puzzle here.

So, after all that -- and I do apologize for its length -- where have I landed? I think most open-ended questions are bad, I think the consensus we've arrived at is better than just allowing them all, but I can't agree with the reasons given, and I worry that those reasons will lead to poor decisions about other matters -- such as what to do about answers to optimization-type questions, a meta question about which was what prompted me to write all this. And I think the current consensus, while better than "anything goes", doesn't exclude quite the right things.

If it were just up to me, I would say:

  • Open-endedness as such is no crime.
  • To be a puzzle, something needs to be challenging and it needs to be clear what constitutes a correct answer to it.
    • Most bad open-ended puzzles fail on one or both of those criteria, and that's what makes them bad.
  • There are two quite different sorts of "find the best X" puzzle, and we should try to be clear about which sort any given puzzle is.
    • Sometimes it means "find any X; best one gets the green checkmark".
      • These are only any good if "find any X" is a puzzle -- if it's challenging and it's clear what is and what isn't an X.
      • For these, something that's demonstrably an X is a good answer, and worthy of acceptance (though a better answer might come along and dethrone it). It's not necessary to prove that no other answer could do better (though such a proof might be interesting, and make the answer a better answer as well as ensuring prompt acceptance).
      • For these, the ranking criterion is just an alternative to saying "first correct answer gets accepted" or "whichever answer I happen to like best gets accepted" or whatever.
    • Sometimes it really means "find the best X", and the point of the question is to show that your X is best.
      • These are only any good if figuring out what X is best is a puzzle (challenging and clear).
      • For these, a good answer needs to be demonstrably best, not just demonstrably X.
  • All else being equal, having a unique answer is a very good feature for a puzzle to have. For many classes of puzzles (e.g., sudoku, chess puzzles) there is a firmly established convention that every puzzle's solution should be unique. But it isn't a thing that distinguishes puzzles from non-puzzles, as (I think) challenge and clarity do.
  • I am entirely in favour of vigorously closing (or, as an inferior but acceptable alternative, downvoting into oblivion) puzzles that are not challenging or not clearly defined or both.
  • 2
    $\begingroup$ To be absolutely clear: the "friends, Romans, countrymen" framing is just for fun, and I do not in the least bit intend any deeper analogy; e.g., I am not suggesting that those who disagree with me are dishonest, or that they are in any way like assassins, or that they deserve to be torn apart by an angry mob. $\endgroup$
    – Gareth McCaughan Mod
    Commented Sep 19, 2019 at 0:51
  • 1
    $\begingroup$ No offense remotely taken :) Now I gotta take some time to read this very thorough-looking answer. And also look up what the heck a denizen is - I feel like it gives me a strong advantage the next time I play hangman. $\endgroup$
    – Brandon_J
    Commented Sep 19, 2019 at 0:54
  • $\begingroup$ IIRC, we came to the conclusion we did in the chat reluctantly, knowing that we would have to close a few good puzzles for the benefit of clearing the far larger number of lousy "puzzles" like mine. We couldn't really find a better distinction. As far as "challenging" goes - now that's a hard metric to pin down, IMO. Also, if I read correctly, a computer can show which answer in the Life puzzle is the fastest. $\endgroup$
    – Brandon_J
    Commented Sep 19, 2019 at 1:13
  • 1
    $\begingroup$ To summarize my abrupt summary - we came to the conclusion for pragmatic purposes, rather than purist purposes. $\endgroup$
    – Brandon_J
    Commented Sep 19, 2019 at 1:14
  • $\begingroup$ That's reasonable. For reasons I don't now remember, I didn't see much of the chat discussion, so it's entirely possible that my answer here is missing some relevant things... $\endgroup$
    – Gareth McCaughan Mod
    Commented Sep 19, 2019 at 1:41
  • 1
    $\begingroup$ Currently writing up a response, but as a preliminary comment - how do you propose to distinguish what is "challenging"? Should we add a "too easy" close reason? That seems problematic and pretty subjective. $\endgroup$
    – Deusovi Mod
    Commented Sep 19, 2019 at 6:30
  • $\begingroup$ Answer invalidation: I've responded to this many times already. Someone coming along long after an answer is accepted and posting a better answer in a 'normal' question doesn't make the previous one invalid, just "not as good". But in a "what's the optimal way to do this" question, a better answer does invalidate a worse one. $\endgroup$
    – Deusovi Mod
    Commented Sep 19, 2019 at 6:46
  • $\begingroup$ (To not clutter the comments, the rest of my response is in this chatroom.) $\endgroup$
    – Deusovi Mod
    Commented Sep 19, 2019 at 6:47
  • 1
    $\begingroup$ I need to read this a few more times to respond fully, but one issue is that "challenging" is very subjective. What's trivial to you and me might be very challenging to someone with, for example, less mathematical background. We don't want - and I realise this is not what you're suggesting - to gatekeep out all puzzles which aren't so hard as to provide a challenge to all of our best puzzle solvers. (Edit: I hadn't refreshed the page and now see that Deus made the same point half an hour ago.) $\endgroup$ Commented Sep 19, 2019 at 7:01

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