The best opposite of "mutually exclusive" I can think of is "necessarily accompanying", but it sounds awkward.

Most answers I looked up give words like "concordant" and "accompanying", but these words have more passive definitions that mean things are "compatible", "harmonious" or "in agreement".

I want a word with a harder definition that means things must exist together or not at all.

  • It is not entirely clear what "things" you have in mind. As stated, however, I do not see why two "things" couldn't be mutually exclusive and at the same time (both of them) nonexistent. So, in what sense is the harder definition at the end of your question an opposite to "mutually exclusive"?
    – anemone
    Feb 7, 2015 at 17:54
  • 1
    When two things are mutually exclusive, either one or the other can exist, but not both at the same time. I should have been more clear, and maybe it's not truly an "opposite", but I want to know a way to describe two things that either must both exist together, or not at all. I'm writing an essay for school and want to say that indeterminism and free will necessarily imply each other in that way. Feb 7, 2015 at 18:06
  • When two things are mutually exclusive, it is quite possible that neither exists.
    – anemone
    Feb 7, 2015 at 18:16
  • I've added the philosophical dimension to my answer, but I am concerned that your thesis may be flawed. Indeterminism seems to be a necessary condition of free will, but is free will truly a necessary condition for indeterminism? That question is for another site.
    – ScotM
    Feb 7, 2015 at 22:05
  • Is 'corequisite' a word that's already known? It would have the right meaning, I believe. 'Mutually exclusive' means 'cannot be used together'; I believe 'corequisite' would mean 'both must be used together'. A Google search on 'define:corequisite' says it exists and seems to refer to 'Corequisites are courses that must be taken at the same time.' So, maybe it isn't usually used as an antonym of mutually exclusive, but it is not a wholly unreasonable term to use. Feb 8, 2015 at 2:35

11 Answers 11


In philosophy the expression would be mutually necessary:

Definition: A necessary condition for some state of affairs S is a condition that must be satisfied [in order to obtain] S.

Example of mutually necessary conditions:

Jack and Jill will go up the hill only if they both go up the hill.

  • Jack will not go up the hill without Jill.
  • Jill will not go up the hill without Jack.


  • Jack going up the hill is necessary for Jill to go up the hill.
  • Jill going up the hill is necessary for Jack to go up the hill

Jack going up the hill and Jill going up the hill are mutually necessary.

In a broader context:

Interdependent implies a certain level of mutual necessity:


(Of two or more people or things) dependent on each other:


Codependent relationships exhibit an intensity of interdependence that increases the sense of necessity:

Derivative of codependency:



Excessive emotional or psychological reliance on a partner, typically one with an illness or addiction who requires support:

Symbiotic relationships do not all exibit necessary, but the mutual benefit is a specialized expression of interdependence:

derivative of symbiosis


[MASS NOUN] Biology

1.0 Interaction between two different organisms living in close physical association, typically to the advantage of both.

There is a brand of symbiosis referred to as obligate symbiosis, which does imply the must of the OP:

In animals, a common mutualistic symbiosis occurs between many herbivores and microorganisms of their digestive tracts. Ungulates (hoofed animals) and some other animals eat plant material that is high in cellulose , even though they lack enzymes capable of breaking down cellulose molecules. They obtain energy from cellulose with the help of symbiotic bacteria and protozoa living within their digestive tracts. These microbes produce enzymes called cellulase that break down cellulose into smaller molecules that the host animal can then utilize. Similarly, wood-consuming termites depend upon symbiotic protozoans living within their intestines to digest cellulose. These are obligate symbioses. The termites cannot survive without their intestinal inhabitants, and the microorganisms cannot live without the host. In each of these symbioses, the host animal benefits from the food provided by the microorganism and the microorganism benefits from the suitable environment and nourishment provided by the host.

Read more: http://www.biologyreference.com/Se-T/Symbiosis.html#ixzz3R4tP9zIU

Emphasis mine

Synergetic, a derivative of synergy approaches the meaning you are looking for:



The interaction or cooperation of two or more organizations, substances, or other agents to produce a combined effect greater than the sum of their separate effects:

  • 2
    obligate symbiosis means necessary (or die).
    – Frank
    Feb 7, 2015 at 16:31
  • 2
    +1 for codependent and interdependent. Precisely the words that I was going to suggest. Feb 7, 2015 at 23:26
  • I think that "mutually necessary" does not have exactly the same meaning as "that means things must exist together or not at all.". I understand it as having stronger meaning: that these things are necessary (in mutual way, all at once). However, the "that means things must exist together or not at all" clearly indicate that this is not a must, and these thing may not exit at all - and "mutually necessary" seems to lose that part of the meaning. May 26, 2017 at 19:39
  • For this reason, I like "exclusively mutual" much more: it emphasizes that these things are mutual and that it this "mutuality" is exclusive - meaning that these characteristics are mutual only to themselves and to no more other things. May 26, 2017 at 19:41

It is a twist but there is exclusively mutual.

Urbandictionary has a definition even:

when two events can occur only in conjunction with one another, i.e. neither one can occur without the other


I suggest "mutually implicated" as in the following sentence:

If power and knowledge are mutually implicated, then specific orders of knowledge entail specific kinds of contexts as relevant.

Also, Google shows ~26k results for the expression.


If you're thinking of properties, use 'overlapping' (in that there are some items with both properties.

If you mean that all items must have both properties, then they are, as someone else mentioned, 'necessary and sufficient', or 'equivalent' or 'identical'.

  • Or 'interesecting'.
    – Mitch
    Nov 11, 2016 at 16:49

Based upon the comments provided, I'd use go hand in hand in the verb phrase, rather than look for a modifier.

Indeterminism and free will go hand in hand.


I would say it is "necessarily coexisting"


Consider using the phrase "inherently inclusive".


While codependent is nice, it has a technical meaning in psychology that you don't seem to be wanting (unless you intend to personify Free Will and Indeterminism).

I would use inseparable to mean that one cannot exist apart from the other. One of the dictionary definitions is

seemingly always together

which matches your usage well. Unlike codependent, it works well for any nouns, including objects and ideas.


I think concomitant is a pretty good fit here.

  • 3
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    – choster
    Feb 8, 2015 at 16:50

It is difficult to find an antonym for a construct of multiple ideas, but in this case the question describes the wanted result quite clearly.

Something that has to exist in order for something else to exist is prerequisite.

If A is prerequisite to B and B is prerequisite to A, we can say that the relation, just like in the original, is mutual. So I'd propose mutually prerequisite as an option.


I also looked for this and in the end paraphrased it in the following sentence:

"A and B are neither mutually exclusive, nor does one strictly imply the other."

I preferred this over using non-common expressions, which people may not understand or misinterpret.

  • 1
    But how is that "a word for things that necessarily exist together"? Or are you suggesting "they strictly imply the other"?
    – Joachim
    Nov 23, 2023 at 10:59
  • yes, that is, why I put it in bold
    – Libavius
    Nov 30, 2023 at 9:08

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