I want to express my knowledge about the presence of absence of something. My knowledge is divided into three different cases:

  • I know that the thing doesn't exist.
  • I don't know whether the thing exists.
  • I know that the thing exists.

Sadly, neither of those is the negation of another one. However, I can define four cases, where each case is the negation of another case:

  1. Something is allowed to exist. (allowed)
  2. Something is allowed to be missing. (???)
  3. Something is guaranteed to exist. (guaranteed)
  4. Something is guaranteed to be missing. (prohibited)

I want to describe each of these cases by a single word, which is supposed to clearly distinguish it from the other three cases. As you can see, I already found three of the words. However, in the second case I am unable to find one.

Let me expand on what I mean by the negation. Consider the following table:

| phrase                   | single word | doesn't exist | don't know | exists |
| allowed to exist         |   allowed   |      no       |     yes    |   yes  |
| allowed to be missing    |     ???     |      yes      |     yes    |   no   |
| guaranteed to exist      |  guaranteed |      no       |     no     |   yes  |
| guaranteed to be missing |  prohibited |      yes      |     no     |   no   |

Note, that the first and the fourth case are supposed to be negations of each other, just like the second and the third case. Thus, if I say that something is not allowed to exist (allowed), then it is guaranteed to be missing (prohibited). Also, if I say that something is not allowed to be missing (???), then it is guaranteed to exist (guaranteed).

Thus, my question is: Which single word is able to replace the phrase "allowed to be missing"?

This question can be rephrased to: Which single word is the exact negation of "guaranteed to exist"?

The context is theoretical computer science. Here are two example sentences, which are negations of each other:

  • The connection from x to y is allowed and the connection from y to z is guaranteed.
  • The connection from x to y is prohibited or the connection from y to z is allowed to be missing.

I think the most helpful wording is the one suggested in this answer, using terms from modal logic:

  • The connection from x to y is possible and the connection from y to z is necessary.
  • The connection from x to y is not possible or the connection from y to z is not necessary.

Thanks for all the answers =)

  • Comments are not for extended discussion; this conversation has been moved to chat. – tchrist Jan 19 at 1:00
  • Is there a German word you're looking to replace there? The table itself feels awkward working in an English frame of reference. – Alex H. Jan 20 at 3:13
  • What sort of somethings? An object or a condition? – user2617804 Jan 20 at 11:18
  • VTC: fails to provide a contextual sentence in which the word would be used. As it stands, it's asking for a concept. Try Philosophy.SE – Mazura Jan 20 at 16:13
  • A fill-in-the-blank context can't just be a blank. "allowed to be missing” is just a blank. Where's the rest of the sentence? – Mazura Jan 20 at 16:15

10 Answers 10


The usual mathematical terms for these things (from the study of modal logic) are 'necessary' (for your 'guaranteed') and 'possible' (for your 'allowed'). All you need is negation to get all four possiblities.

  • necessary - it must exist
  • possible - it may exist
  • not necessary - it may not exist
  • not possible - it cannot exist

Depending on your (choice of) logic those two in the middle may be the same.

For a logic of probabilities, where 0 <= p <= 1:

  • necessary: p=1
  • possible: p > 0
  • not necessary: p < 1
  • not possible: p = 0

For example, you can see that 'not possible' is the same is the complement of 'possible'.

This mathematical use of these words follows our informal meaning.

So to your specific questions:

  • Which single word is able to replace the phrase "allowed to be missing?

    With respect to probability, this means that it could be any probability. So any combination that covers all possibilities, 'necessary or not necessary'

  • Which single word is the exact negation of "guaranteed to exist"?

    By negation, there are two possibilities that informal English allows. 1) the set complement, 2) the other point extreme of the spectrum.

    • For the set complement it is 'not necessary'.
    • for the other extreme it is 'not possible'.
  • 9
    It's worth noting that both "not necessary" and "not possible" have single-word equivalents if truly needed ("unnecessary" and "impossible"). – Kamil Drakari Jan 17 at 19:27
  • Thanks for the detailed answer. Even though I don't work with probabilities, the four cases match my four cases (allowed = possible, not necessary = ???, guaranteed = necessary, prohibited = not possible). Here, the not directly clarifies which case is a negation of which other case, so that is nice. By exact negation I meant the set complement, so the case I was looking for is "not necessary". I was hoping to find a single word for each of these cases. However, allowed is already confusing. So maybe I will stick with (not) possible/necessary. They at least seem to be clear. Thanks =) – Stefan Dollase Jan 17 at 20:11
  • single words: unnecessary, impossible (but unnecessary implies something about not being wanted, which the others don't have). – Mitch Jan 17 at 20:35

This is commonly denoted as optional:

available as a choice but not required

(source: Merriam-Webster)

Another example:

The definition of a method, constructor, indexer, or delegate can specify that its parameters are required or that they are optional. Any call must provide arguments for all required parameters, but can omit arguments for optional parameters.

  • 1
    Thanks for your suggestion! However, I think it fails to draw a clear line between the first and the second case. I edited the question to clarify the difference. – Stefan Dollase Jan 17 at 15:27
  • 38
    @StefanDollase That's because there is no difference between the first and second case. If it's "allowed to be there (but doesn't have to be)", that's the same exact thing as "allowed to be missing (but could be there)". Literally the same thing. In English, we call this optional. "Omissable" is likely to confuse some people, whereas "optional" would be immediately understood by everyone. This is by far the best answer, and probably the only answer that fits. – only_pro Jan 17 at 19:20
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    @only_pro This is what I thought, which confused me. "Allowed" and "unnecessary" logically mean the same thing, but pragmatically don't. If a teacher says "you are allowed to bring a calculator to the test", that most likely means something different from "It's unnecessary to bring a calculator to the test." In both cases the calculator is "allowed", but the meaning intended by the teacher is likely difference, ie., maybe something like "It might be a good idea to use a calculator" versus "You really won't need a calculator because it's easy." – Zebrafish Jan 18 at 0:21
  • 1
    @Zebrafish I think what you need to compare is "You are allowed to bring a calculator to the test." versus "You are allowed to not bring a calculator to the test." both mean the same thing logically (second one sounds odd but it's the case of "allowed to be missing") or you can just insert text books and class to the sentence. "You are allowed to use text books in the class." versus "You are allowed to not use text books in the class." they're the same thing. The underlying implication has nothing to do with the logic of the matter. – John Hamilton Jan 18 at 13:42
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    @JohnHamilton Not to mention I don't understand the table. Under "allowed to exist" the "don't know" value is "yes", which is correct, but the value of "exists" is "yes" and the value of "doesn't exist" is "no". If we don't know, how could they have either yes or no as values for "exists" and "doesn't exist"? Haha. – Zebrafish Jan 18 at 14:48

Something that is allowed to be missing is omissible. Wordreference.com defines omissible as:

capable of being or allowed to be omitted


In English grammar the object relative pronoun is omissible:

The book (that) I wanted to buy was sold-out.

  • I think this is a good one. However, I will wait to accept your answer for a few days, hoping to receive several more suggestions. – Stefan Dollase Jan 17 at 17:37
  • 2
    I'd be curious to know if anyone here has ever seen, heard or used the word omissible. I get funny looks when I use promulgate or impracticable, but I can't imagine using omissible. – Flydog57 Jan 18 at 17:48
  • 2
    I have heard "omittable" which is apparently not a word and the actual word is "omissible" (which I have never heard.) I would bet that in 10 years or so, "omittable" will show up in the dictionaries as an alternative to omissible. – Roddy of the Frozen Peas Jan 18 at 20:12
  • @Flydog57 I heard it today in a training session. – Araucaria Jan 19 at 0:26
  • 'Omit' is the normal form of the verb: "I omitted to mention...". It's similar in construction to permit / permissible – DaveMongoose Jan 21 at 10:38

This question is confusing as hell, but I think "unnecessary" or any of its synonyms might fit, depending on what you mean exactly, as I'm confused.

Something is allowed. (allowed to exist)
Something is unnecessary (allowed to be missing)

You also have the condition:

Also, if I say that something is not ??? (allowed to be missing), then it is guaranteed (guaranteed to exist).

If you place "unnecessary" or "unrequired" or "unneeded" where you placed the question marks, does that satisfy your needs? In other words, if something is not "unnecessary", or not "unrequired", or not "unneeded", does it make then make it guaranteed? I don't know.

  • Thanks for your suggestions. These seem to be fitting. However, they all start with "un", so they really are the negation of another word, which generally does not help to understand complex sentences which contain several more logical connectors. What I mean to say is: These are negative terms to express the requested meaning. However, I am looking for positive words. – Stefan Dollase Jan 17 at 17:30
  • "unnecessary" has much more of a connotation of "useless anyway" than "optional" :) – rackandboneman Jan 18 at 20:23
  • @stefan The negation of a word is "not" followed by the word. The word with the opposite meaning is a different word. There are words that start with a negation that can't be removed, e.g. nonsensical. – Matt Samuel Jan 18 at 23:46


"More than you need and therefore not necessary; that can be got rid of."

Cambridge Dictionary


Is your use case Speech? (IE. story telling or writing) Where your intention is to somehow say a character does not know whether a thing exists, but let the reader now it actually does?

The Yes/No nature of the object's existence is hidden by the speaker in most contexts not being able to know the result, so the person speaking the word could never choose the correct one themselves, only an omniscient observer (narrator) could, and it would be in order to signal to the reader the distinction.

In which case you would need to have the narrator remind the person reading of this often enough that they would remember your interpretation of the word.

If so I guess you might use


If this for some list of items, where you DO know whether or not something exists, but you want to denote it's optional, without having a second column.

Or if the user does know the thing exists, and does know whether an optional item exists or not, and wants to denote the item's presence or non-presence and denote the item may or may not exist but it could be created I would use:




Could exist and does right now.





Could exist, but doesn't right now.

As a Note:

These are binary and mutually exclusive states with can exist and can not exist, so only in very specific contexts would this sort of distinction be useful, (as above) but so below we can see why this isn't something that makes a lot of sense to talk about either.


Can Exist: YES / NO

             Does / Does Not (Exist)

So I only came around because I believe you're trying to come up with are ADJECTIVES, but you have the form written as VERBS! That seems to be the heart of the confusion!


I think the word you probably want is absent. If something is absent that means it is not here, and that can either be because it exists elsewhere, or because it does not have the existence necessary to be present anywhere, per the Century Dictionary and Cyclopedia's definitions:

  1. Not in a certain place at a given time; not in consciousness or thought at a certain time; away: opposed to present.
  2. Not existing; wanting; not forming a part or attribute of: as, among them refinement is absent; revenge is entirely absent from his mind.

Although it uses the nominal form of the word rather than the adjectival form, a phrase that very nicely demonstrates this is "The absence of evidence is not evidence of absence", which is a popular phrase in our current century according to Seth Augenstein, in the online Forensic Magazine article When is the Absence of Evidence Evidence of Absence?

The phrase itself is a good example because absence is used both ways. In the first case, we simply do not have the evidence to prove something exists at this point in time, yet that does not necessarily mean it is nowhere to be found as suggested by the second. It is a common rebuttal to the argumentum ad ignorantiam fallacy, particularly where somebody wishes to furnish the missing evidence. Consider this explanation excerpted from chapter 13 of Political Argumentation in the United States: Historical and Contemporary Studies. Selected Essays by David Zarefsky for instance:

Ordinarily the argument from ignorance is regarded as a fallacy in reasoning. It was first given the name argumentum ad ignorantiam by John Locke, and is one of the group of "ad-fallacies" that appeal to irrelevant considerations in order to warrant an inference. The fact that we do not know A to be true is no more reason to conclude that it is false than to regard it as true. The fallacy converts extistential doubt into a conclusive assertion of either truth or falsity.

The only problem with it, in my opinion, is that it is simply an adjective, rather than a past participle (in summary: a verb inflected in past tense form to be used as an adjective) so it would seem out of place among the other examples, but I think an actual adjective should suffice for all practical intents and purposes or at least for the table.


Based on your table, it seems you have 2 booleans, existence and knowledge, giving 4 combinations. You want to name these 4 combinations, but this only makes sense from the perspective of a knowledgable 3rd party.

For example, if I had a ball and a screen and two people sitting on opposite sides of table. Person A either places the ball on the table or not and either places a screen in front of person B or not. From Person A's perspective:

Ball    Screen   Word(s)
YES     NO       Guaranteed / Exposed
NO      NO       Prohibited / Voided
YES     YES      Cloaked / Obscured
NO      YES      Evoked / Hinted

evoke: To cause the manifestation of something (emotion, picture, etc.) in someone's mind or imagination. [ https://en.wiktionary.org/wiki/evoke#English ]


(My initial thought was excused, just from reading the title alone. A person is excused if permitted to be absent.)

However, I think exempt (or exempted) is a better fit. Wiktionary has a nice, concise definition:

Free from a duty or obligation.

In this case, free from the duty or obligation to exist.

This satisfies both conditions set forth by the OP:

  1. Single word able to replace the phrase "allowed to be missing". Although exempt does not connote existence in and of itself, it satisfies the condition equally as well as the single word allowed, which also does not connote existence in and of itself.

  2. Single word, the exact negation of "guaranteed to exist". If something is not allowed to be missing (exempt), then it is guaranteed to exist (guaranteed), just as if something is not allowed to exist (allowed), then it is guaranteed to be missing (prohibited).

It also draws a clear line between the first and second cases. Both allowed and exempt indicate something may be missing, but allowed indicates it would ordinarily be absent if not allowed, and exempt indicates it would ordinarily be present if not exempted.


I would say RELEASED.

According to Macmillan Dictionary https://www.macmillandictionary.com/dictionary/british/release_1?q=Release+#release_1__28:



to allow someone not to have to do something

release someone from something: 

We were released from our classes in order to take part in the celebration.

  • Thanks for the suggestion. However, I don't think it fits my use case. I want to talk about the presence or absence of a thing, but I don't feel like Something is released to exist. is a proper sentence. However, I am not a native English speaker, so please let me know whether this is actually proper English. – Stefan Dollase Jan 17 at 17:44
  • @StefanDollase You're right; "released to exist" doesn't work. One could say "released from the requirement of existing", but this reads as stilted. – Chemomechanics Jan 18 at 3:55

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