A Catch-22, coined by Joseph Heller in his novel Catch-22, is a logical paradox arising from a situation in which an individual needs something that can only be acquired by not being in that very situation; therefore, the acquisition of this thing becomes logically impossible. Catch-22s are often spoken with regard to rules, regulations, procedures, or situations in which one has knowledge of being or becoming a victim but has no control over it occurring.
The archetypal Catch-22, as formulated by Heller, involves the case of John Yossarian, a U.S. Army Air Forces bombardier, who wishes to be grounded from combat flight duty. To be grounded, he must be officially evaluated by the squadron's flight surgeon and then found "unfit to fly."
"Unfit" would be any pilot who is actually willing to fly such dangerous missions: as one would have to be mad to want to take on such missions.
But the "problem" is that to be declared "unfit", he must first "ask for evaluation", which is considered as a sufficient proof for being declared "sane". These conditions make being declared "unfit" impossible.
The "Catch 22" is that "anyone who wants to get out of combat duty, isn't really crazy"  Hence, pilots who request a fitness evaluation are sane, and therefore must fly in combat. At the same time, if an evaluation is not requested by the pilot, he will never receive one (i.e. they can never be found "insane"), meaning he must also fly in combat.
Therefore, Catch-22 ensures that no pilot can ever be grounded for being insane - even if he were.
A logical formulation of this situation is:
- 1. (Premise: If a person is excused from flying (E), that must be because he is both insane (I), and requests an evaluation (R));
- 2. (Premise: If a person is insane (I), he should not realize that he is, and would have no reason to request an evaluation)
- 3. (2, Definition of implication: since an insane person would not request an evaluation, it follows that all people must either not be insane, or not request an evaluation)
- 4. (3, De Morgan: since all people must either not be insane, or not request an evaluation, it follows that no person is both insane and requests an evaluation)
- 5. (4, 1, Modus Tollens: since a person may be excused from flying only if he is both insane and requests an evaluation, but no person can be both insane and request an evaluation, it follows that no person can be excused from flying)
 Other uses from the novel
Besides referring to an unsolvable logical dilemma, Catch-22 is invoked to explain or justify the military bureaucracy. For example, in the first chapter it requires Yossarian to sign his name to letters that he censors while he is confined to a hospital bed. One clause mentioned in chapter 10 closes a loophole in promotions, which one private had been exploiting to reattain the attractive rank of Private First Class after any promotion. Through courts-martial for going AWOL, he would be busted in rank back to PFC, but Catch-22 limited the number of times he could do this before being sent to the stockade.
In chapter 6, Yossarian is told that Catch-22 requires him to do anything his commanding officer tells him to do, regardless of whether these orders contradict orders from the officer's superiors. In Chapter 39 an old woman relates that soldiers had claimed that the actual text of Catch-22 did not have to be revealed when carrying out orders related to it, meaning that, "they have a right to do anything we can't stop them from doing". This exchange convinces Yossarian that Catch-22 does not even exist, but because the powers that be claim it does, and the world believes it does, it nevertheless has potent effects. Indeed, because it does not exist there is no way it can be repealed, undone, overthrown, or denounced.
At one point, Captain Black pressures Milo into depriving Major Major of food based on fabricated charges by comparing the situation to Catch-22. He asks, "You're not against Catch-22, are you?" Captain Black wants Major Major punished for not signing a loyalty oath, despite never giving Major the opportunity to sign it.
At another point, after intercourse, Luciana explains to Yossarian that she can't marry him because he is crazy for wanting to marry her, since she is not a virgin.
In chapter 40, Catch-22 forces Colonels Korn and Cathcart to promote Yossarian to Major and ground him rather than simply sending him home. They fear that if they don't, others will refuse to fly, just as Yossarian did.
 Real-life examples of Catch-22
Examples of catch-22 can be found in real life, although none is as hopeless as situations found in the novel. Common examples include the following:
- One cannot get a job in a high-profile occupation without prior experience, but they cannot get experience without getting a job in a high-profile area.
- However, there are many entry-level opportunities available for those fresh out of college. It is merely more difficult to get good jobs without experience.
- One cannot get a loan without established credit, but one cannot establish credit without previously getting a loan.
- There are loans that do not check your credit, prior to lending. Also, those without credit can get the a credit-worthy cosignature.
- Until vendors develop applications for Linux, Linux's market share on the desktop will stagnate. But until the market share of Linux on the desktop rises, no vendor will develop applications for Linux.
- One needs to open a door, but it is locked and the door must be opened to get access to the key, because it was used to lock the door on the other side.
 Significance of the number 22
According to many sources Heller originally wanted to call the phrase, and hence the book, by other numbers, but he and his publishers eventually settled on 22. The number has no particular significance; it was chosen more or less for euphony.
Other sources say that the title was originally Catch-18, but Heller changed it after the popular Mila 18 was published a short time beforehand.
 See also
 False dilemmas and circular logic
Situations which have logical similarities to a Catch-22.
 Non-false dilemma situations
Situations which may be confused with a Catch-22, but have quite different logic or outcomes.
- Chicken or the egg ' a seemingly unbreakable cycle of causation, which has an unknown origin.
- Cornelian dilemma ' a choice between actions which will all have a detrimental effect on the chooser or on someone they care for.
- Deadlock ' in computing, when two processes reach a standstill or impasse, each waiting for the other to finish.
- Double bind ' a forced choice between two logically conflicting demands.
- Hobson's choice ' the choice between taking an option or not taking it.
- Lesser of two evils principle ' a choice between two undesirable outcomes.
- Morton's Fork ' a choice between two equally unpleasant alternatives.
- Paradox ' a statement or group of statements that leads to a contradiction or a situation which defies intuition.
- Game of Chicken - Two participants desire a positive outcome by taking an action, yet if taken by both the result is devastatingly negative.
- Sophie's Choice - a choice between two equally beloved entities, one of which must be destroyed to preserve the existence of the other.