A July 2022 renovation of the tag and others has changed the context of this question. It is now obsolete. Please see the following question and the link under "Conclusion." If we can't burn the "reality check" tag, can we rename it?

We currently have 83 questions tagged both reality-check and science-based, and another one question tagged both reality-check and hard-science.

The tag wikis excerpts for the tags say, respectively:

reality-check: For questions asking whether or not a particular concept is realistic in a given context.

science-based: For questions that require answers based in hard science, not magic or pseudo-science.

hard-science: All answers to this question should be backed up by equations, empirical evidence, scientific papers, other citations, etc.

There seems to me to be a progression here, with being the most relaxed but still requiring some degree of maintenance of suspension of disbelief (you can plausibly have a question involving dragons and unicorns tagged reality-check), and on the other end of the scale requiring any claim in an answer to be backed up by citations (which means the unicorns are probably out the barn door).

Thus, any "higher requirement" tag would seem to imply the lower requirements as well. You could hardly have answers to a question that wouldn't pass the requirements of , for example, but answers to a reality-check tag don't have to meet the standards of a hard-science question.

Since these tags don't really serve to categorize the question, and each question is limited to a maximum of five tags, these "meta tags" should be used sparingly, and there has been plenty of discussion right here on our meta if we should have them at all, with vocal proponents of both sides. Using more than one of the above tags on a single question however doesn't seem to me to make sense in any situation.


Said alternatively, does it ever make sense to use more than one of these three tags on a single question?

The aim of this question is not to decide the fate of our current 84 questions. If we decide that the tags do imply one another, then we can later decide what to do with the questions we currently have that are in that case redundantly tagged.


5 Answers 5


I partially disagree and partially agree that they imply each other, fortunately it's easy to demonstrate in this case.

A asks "is this possible/feasible/plausible".

A asks "answer this using hard science".

Now if we ask a question:

I have a rocket weighing 10 tonnes, with 5 tonnes of that being propellent and efficiency similar to our current best performing rockets. What is the maximum acceleration and top speed of that rocket?

That question is and/or , entirely answerable, but not .

On the other hand if I asked:

I have a rocket weighing 10 tonnes, with 5 tonnes of that being propellent and efficiency similar to our current best performing rockets. Would that rocket be able to get into an orbit around mars?

You could usefully use both tags here. This question is both "is this possible/does this make sense" and "answers should be based in science

Where I do agree with you though is that implies . Those two are much more closely related.

  • $\begingroup$ In which situation does tagging the same question both reality-check and science-based or hard-science add anything over just using science-based or hard-science? After all, that situation is what prompted my question. $\endgroup$
    – user
    Commented Apr 14, 2015 at 9:23
  • $\begingroup$ I edited the question to clarify that part as well. Sorry for partially invalidating your answer. $\endgroup$
    – user
    Commented Apr 14, 2015 at 9:30
  • 1
    $\begingroup$ @MichaelKjörling Updated the answer as one of my examples already covered this. hard-science implies science-based, but reality-check is a different axis really. $\endgroup$
    – Tim B
    Commented Apr 14, 2015 at 10:09
  • $\begingroup$ It's worth nothing that your first example is not really a question for WB, and more for Physics Exchange. $\endgroup$
    – Mermaker
    Commented Feb 5, 2016 at 10:24
  • $\begingroup$ @ThalesPereira True enough. The intent was to make a simple but understandable example. I'm open to an improved suggestion :) $\endgroup$
    – Tim B
    Commented Feb 5, 2016 at 10:28
  • $\begingroup$ It appears to me that most cases of Hard-Science that doesn't include a reality check would be better for Physics.SE or similar. Those questions would not be asking about building a world, just to solve some math. If you need to build a world and fix the math for it, then reality-check kinda becomes implicit - if the math for what you want fails, the whole proposal sinks. I think that people who use Hard Science should be prepared to get "this won't work" as an answer instead of trying to force the laws of reality to bend to their storytelling. $\endgroup$
    – Mermaker
    Commented Feb 5, 2016 at 10:41


I understand where Michael K. is coming from when he understands these as hierarchical or a progression of relaxed to ever tightening requirements. I see these tags as three separate domains of inquiry, rather than a hierarchical structure of inquiry, and that any one or more than one can be applied to a query.

  • A reality-check asks simply "does this sound reasonable"; "does this make sense". No particular framework of reality is suggested or assumed.
  • A science based question asks for an answer with scientific foundation.
  • A hard science question asks that answers contain appropriate scientific data a/o citations to back it up.

All three of these tags describe the nature of the response: what it should look like, how much research should go into it, how detailed and technical the answer has to be. What none of these tags seem to do is describe the nature of the query or better the nature of the object of the query.

I do not see these as being hierarchical.

  • Hard science does not assume science based. The domain of this kind of answer is providing examples of data, equations, journal article citations, etc. that either support or deny the object of the query.
  • Science based does not assume reality check. The domain of this kind of answer is providing general scientific principles and how they might apply to the object of the query.
  • Reality check is not the bottom rung of the ladder. The domain of this kind of answer is that of common sense, ordinary logic, pattern matching, error teasing.

To use the unicorn as an example, in combination with the nature of the tags themselves, I'll show how I think these tags can handle any kind of question. First, I'll assume that we agree that dragons (e.g. Smaug) and unicorns (fairy stories) do not exist on Earth as depicted.

So if someone asks the question "I think narwhals are cool. Would it make sense for a horse-like animal to grow out a long tooth?, which later humans will confuse for a horn (< Lat. cornu, horn) --- thus, a unicorn!" I think any of the three tags could apply:

  • The reality check tag would seek an answer like "Yeah, seems plausible given that the world you're describing has similar beasts derived from mythological sources. You already mentioned narwhals, so why not horses too?"
  • The science based tag would seek an answer like "The narwhal tusk is an elongation of the left canine tooth that protrudes through the animal's lip. Horses do have canine teeth. Because the evolution of the tusk is reasonably well understood (descent from a toothed artiodactyl, placement & growth of the tusk within the skull allow for positive identification), we can at least posit a potential scion of an early equid developing similarly."
  • The hard science tag would seek an answer like (and I'm just making this up, because I'm not a paleobiologist) "I read in PaleoBiology This Week about a now extinct form of water deer (Hydropotes antiquus) the right canine of some specimens having been found to be considerably enlarged and twisting with a slight helical twist and that the left canine is diminished in size. There is evidence, according to Dr. Li Hsien, that these early cervids used their elongated tusks for dominance display as evidenced by bony scar tissue around the muzzle area of male H. antiquus."


I disagree with M.K. as regards Using more than one of the above tags on a single question however doesn't seem to me to make sense in any situation.

I don't see any issue with using one or more together, because 1) they're not asking for the same thing; and 2) I don't view them as a hierarchy! (I would agree: if they were hierarchical in nature, then Reality Check would be the basic form; Science Based would subsume a positive reality check; Hard Science would subsume the assumption that the answer must be based in known science.) If the querent had placed all three tags, then the respondent would simply be required to provide, basically, all three types of answers!

  • As to the Reality Check, ABC; as to the Science Based, PRS; as the Hard Science, XYZ.

In contrast, had the query been about dragons (as seen in the Hobbit), then we can still answer the question given these tags, though likely with different results. The reality check would most likely be positive, as dragons only have to make sense within the setting or world in which they have been posited.

A Science Based query about dragons can certainly be asked and answered. In the history of Earth, there have been enough large beasts to demonstrate that size is not at issue. Some animals have scales & feathers, so those features of dragons should pose no difficulty. No known animal breathes fire, but since we're not required to provide scientific data, and whether or not magic & pseudoscience work in that world or setting, we can at least offer sound scientific speculation as to potential mechanisms for fiery breath. A bold respondent may even posit an in-world scientific answer (that accounts for magic as a natural force, if enough data is known).

A Hard Science query about dragons will, of course, run into a wall. Big beasts, sure! We know about dinosaurs. Scaly or feathery beasts? Sure! We know about dinosaurs. Fire breathing beasts with four feet, two wings, tough scaly armour and perhaps decorative feathers? Nope. No such thing known; no way for it to be able to fly. The Hard Science answer will be along the lines of "What-if speculation aside, there is no way for known to biology or physics or chemistry to allow for the particular combination of characteristics you want in your dragon; so the answer to your question is NO."


  1. Should hard-science always imply science-based, without that having to be made explicit? --- NO. I think it's clear there is no implication.
  2. Should science-based always imply reality-check, without that having to be made explicit? --- NO. Same: no implication.
  3. Does it ever make sense to use more than one of these three tags on a single question? --- YES. The three tags can validly be used in combination. Should they be so used is a different matter!
  • 1
    $\begingroup$ i disagree with your interpretation of hard-science. In my opinion hard-science is never reasonable without the implication of science-based. And yes, you can have hard-science about dragons. If you were to give a detailed enough outline of what they are gonna look like or what they are made of or how their fire-spouting is supposed to work, somebody could give you a detailed analysis of certain aspects of that. Like structural integrity, biochemistry forming the flames, etc. To me (and most others) hard-science is science-based with a much higher scientific standard. $\endgroup$ Commented Sep 13, 2018 at 9:45
  • $\begingroup$ Fair enough (as to how I view the situation)! As to hard science and dragons, there I would disagree: the hard science tag explicitly requires answers to be backed up with "equations, empirical evidence, scientific papers, other citations". In other words, you'd have to have some kind of fire-breathing animal sometime in Earth's history to support an answer regarding fire breathing dragons. At least in my opinion, a journal article about combustibles produced in organic life forms and another article about fire [cont.] $\endgroup$
    – elemtilas
    Commented Sep 13, 2018 at 22:12
  • $\begingroup$ doesn't cut the cheese. That's what I mean by "different domains". R.C.'s domain is basic sensibility; S.B.'s domain is scientific plausibility; H.S.'s domain is scientific certitude, backed by hard evidence. $\endgroup$
    – elemtilas
    Commented Sep 13, 2018 at 22:13
  • $\begingroup$ I agree with reality-check and the science-tags being different domains, just not with hard-science and science-based. I think they are just qualifiers for a certain scientific standard. $\endgroup$ Commented Sep 13, 2018 at 22:15
  • $\begingroup$ @ArtificialSoul --- Fair enough! I'll happily accept partial agreement! Though I'd wonder what distinguishes those "certain scientific standards"? And why bother with two tags? $\endgroup$
    – elemtilas
    Commented Sep 14, 2018 at 1:45
  • $\begingroup$ Science-based is the more simple version, were everything has to be scientifically reasoned, but no elaborate data, calculations or papers have to be presented. Scientific reasoning suffices. For hard-science you would need to be more precise. Let's take the example of a dragon. A hard-science question might want to delve into structural integrity of the bones of a creature. Answerers would be required to argue based on math and/or papers with detailed comparisons to real creatures. A science-based version might just point out how other things have solved similar issues without references. $\endgroup$ Commented Sep 14, 2018 at 6:17
  • $\begingroup$ In my opinion you should compare average hard-science answers with average science-based answers. Because as far as i have seen hard-science is usually significantly more detailed and bring more references. It is basically a descriptor tag of how detailed OP wants to know and how accurate it has to be. $\endgroup$ Commented Sep 14, 2018 at 6:19

It's long after the fact, but based on a comment to my recent question, I thought I'd chip in.

L.Dutch pointed out that my use of both the and tags shouldn't have occurred. I removed the tag to comply, but after reading the descriptions and the question/answers here...

I simply don't agree

The tag imposes a specific restriction on an answer. The answer cannot contain magic, or fantasy elements, or even modified physics if permitted within the OP's world. It mandates that the science known on Earth, the here-and-now, is required for the answer, but not that it must be proven to an absolute degree. This permits "bending the rules" a bit, or the use of science to create a plausible if not perfectly rationalized explanation.

The tag imposes the very same restriction on an answer: no magic, no fantasy, not allowed to use the OP's modified physics... but in this case, the science must be proven. No opinions. No bending of the rules. Either science permits the answer or it does not.

In contrast, has a similar restriction, but it is not identical. The answer must be given within the framework of the OP's world. Not our own. Not anyone else's. Just the OP's. It requires the presentation of a scenario that can be judged within the context of the OP's world. Without that scenario, the question is incomplete and must be closed as "unclear."

Thus, in reference to my recent question, I had both the [tag: reality-check] and the tags. My question was being asked within the context or current or emerging technologies that can and must be known and the answer absolutely had to reflect the abilities of technology.

Now, having said all that, it could legitimately be said that my question failed the mandate in that a specific scenario (a solution for killing the planet's biosphere) had not been presented, but was instead asking for solutions. I humbly admit that, in that regard, I failed to use the tag correctly.

However, I disagree that is mutually exclusive to either the or tags. It's identifying a specific type of question that demands a specific type of analysis to properly answer the question. And in that context, it can and could be combined with either of the other two.

  • $\begingroup$ I support HS+RC - here is why - answer not necessarily positive, it may be refuting OP's situation, same as SB+RC. Namely, the answer can be - it is no possible and here is why, and that why how it presented is a matter of HS or SB tags. RC - it can be yes or no answer - "is it possible?" yes it is, or, no it isn't. And therre should be a reasoning why, which can be tuned by HS and SB $\endgroup$
    – MolbOrg
    Commented Jun 6, 2019 at 1:35

I agree with TimB's answer, and just to add a few points:

  • , as the wiki says, needs to be backed up.
  • does not need to be backed up but should be free of flights of fancy.
  • is, as you say, the most relaxed, and answers only have to be logical, not necessarily scientific. Flights of fancy are fine.

We should not have to specify more than two of these on any one question, and in most cases one should be sufficient. The most important part here is that does not fit with , and the two together on one question may be grounds for immediate tag editing (unless it's unclear which is meant, in which case the author should be consulted).

In most cases, one of these tags should be sufficient: if it's really necessary, more details can be specified in the question.

  • 1
    $\begingroup$ Actually I disagree with hard science and reality check being incompatible - see the example in my answer of a question that could be both. However hard-science does make science-based redundant. $\endgroup$
    – Tim B
    Commented Apr 14, 2015 at 11:35
  • $\begingroup$ @TimB your example is for science-based and reality-check, instead of hard-science $\endgroup$
    – ArtOfCode
    Commented Apr 14, 2015 at 11:37

My reading puts Science-Based and Hard-Science as the ends of a scale of proof requirement while Reality-Check simply asks for internal logic of a given scenario without asking a burden of proof for real world applications.


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