A fallacy is when someone uses wrong or bad reasoning to support an argument. Fallacies can appear in any type of communication between people.

Fallacies are hard to group into categories because there are so many different kinds. They can be divided based on their structure (called formal fallacies) or their content (called informal fallacies). Informal fallacies, which are the larger group, can be further grouped into types such as improper presumption, faulty generalization, errors in linking causes and effects, and issues with relevance, among others.

People often use fallacies when their main goal is to get others to agree with them, even if their reasoning is not strong. When fallacies are used, the reasons given (premises) should be seen as weak, the conclusion should be viewed as not proven (but not necessarily incorrect), and the overall argument should be considered unreliable.

Formal fallacies

A formal fallacy, also called a non sequitur, is a mistake in the structure of an argument.

• Appeal to probability – assuming something is true just because it is likely or possible.
• Argument from fallacy – believing a conclusion is false simply because the argument used to support it is flawed.
• Base rate fallacy – judging the chance of something happening based only on certain conditions, without considering how often it happens in general.
• Conjunction fallacy – thinking that an event that meets several conditions is more likely than one that meets just one.
• Masked-man fallacy – replacing the same words in a true sentence can make it false.

A propositional fallacy is a mistake in arguments that combine different statements. For a combined statement to be true, the truth of its parts must match the rules of logic (such as "and," "or," "not," "only if," "if and only if"). The following fallacies involve rules that do not guarantee true conclusions:

• Affirming a disjunct – assuming one part of an "or" statement is false because the other part is true (e.g., "A or B; A, therefore not B").
• Affirming the consequent – claiming the cause of an event is true because the event happened (e.g., "If A, then B; B, therefore A").
• Denying the antecedent – claiming an event did not happen because its cause did not occur (e.g., "If A, then B; not A, therefore not B").

A quantification fallacy is a mistake in logic where the way premises are worded (e.g., "all" or "some") conflicts with the way the conclusion is worded.

• Existential fallacy – using a general statement to support a specific conclusion.

Logical fallacies in categorical syllogisms (a type of argument with three statements):
• Affirmative conclusion from a negative premise – drawing a positive conclusion from a negative premise.
• Fallacy of exclusive premises – using two negative premises in an argument.
• Fallacy of four terms – using four different ideas in an argument that should only have three.
• Illicit major – using a term in the conclusion that was not properly used in the premise.
• Illicit minor – using a term in the conclusion that was not properly used in the premise.
• Negative conclusion from affirmative premises – drawing a negative conclusion from positive premises.
• Fallacy of the undistributed middle – failing to use a key term correctly in an argument.

Modal fallacy – confusing what is required for something to happen with what is enough to cause it. For example, oxygen is needed for fire, but oxygen alone does not create fire. A condition is necessary if it must be present for something to happen, but it is sufficient if it alone can cause something to happen.

Modal scope fallacy – adding unnecessary claims about necessity in the conclusion.

Informal fallacies

Informal fallacies – arguments that are not logically correct because their reasons are not well-supported.

• Argument from incredulity – believing something is false simply because you cannot imagine it being true, or thinking it must be true because you cannot see how it could be false.
• Argument to moderation – assuming that a compromise between two positions is always correct.
• Continuum fallacy – rejecting a claim because it is not clearly defined or precise.
• Correlative-based fallacies – Suppressed correlative – changing the meaning of a term to eliminate one possible option (e.g., "I am not fat because I am thinner than John").
• Divine fallacy – believing something is true because it seems too amazing to be explained by natural causes.
• Doorman fallacy – judging a job’s value only by its most obvious tasks, not its overall importance.
• Double counting – counting events more than once in probability calculations, leading to incorrect results.
• Ecological fallacy – assuming something about an individual based only on group statistics.
• Equivocation – using a word with different meanings in a statement without clarifying which meaning is intended.
• Etymological fallacy – assuming a word’s original meaning is the same as its current meaning.
• Fallacy of composition – assuming something true about a part of a whole is also true about the whole.
• Fallacy of division – assuming something true about a whole is also true about its parts.
• False attribution – using an unqualified or irrelevant source to support an argument.
• False authority – using an expert with questionable credentials or only one opinion to support an argument.
• False dilemma – presenting only two options when more exist.
• False equivalence – treating two things as equal when they are not.
• Feedback fallacy – believing an evaluation is objective without checking if the source is unbiased.
• Historian’s fallacy – assuming people in the past had the same knowledge as people analyzing their decisions today.
• Historical fallacy – believing a result happened only because of a specific process, even if the process is unrelated.
• Homunculus fallacy – explaining a concept by referring to another concept without explaining its real nature (e.g., saying a "little thinker" inside the brain causes thought).
• Inflation of conflict – claiming no conclusion can be made if experts disagree on an issue.
• If-by-whiskey – using emotionally charged or ambiguous terms to support both sides of an argument.
• Incomplete comparison – not providing enough information to compare options properly.
• Intentionality fallacy – insisting a message’s meaning must match the sender’s original intention.
• Kettle logic – using multiple conflicting arguments to defend a position.
• Ludic fallacy – ignoring unknown random events when calculating probabilities.
• Lump of labour fallacy – believing the amount of work in an economy is fixed.
• McNamara fallacy – relying only on numbers and ignoring quality or subjective information.
• Mind projection fallacy – thinking a statement about an object describes the object itself, not a person’s perception.
• Moralistic fallacy – assuming facts can be determined from moral judgments.
• Moving the goalposts – rejecting evidence and demanding more proof for a claim.
• Nirvana fallacy – rejecting solutions because they are not perfect.
• Package deal – treating unrelated ideas as if they are closely connected.
• Proof by assertion – repeating a claim even when it is contradicted.
• Prosecutor’s fallacy – misunderstanding the probability of a match in evidence.
• Proving too much – drawing an overly broad conclusion from an argument.
• Psychologist’s fallacy – assuming one’s own perspective is objective when analyzing others.