There are dozens of different types of fallacies: formal fallacies, informal fallacies, faulty generalizations, red herring fallacies, and so on...

But is there one simple, singular condition that all of these meet that makes them eligible for such a classification?

Such as If x is y, then x is a logical fallacy.

Where x is the proposition and y is the condition that it meets.

  • 2
    Related: Are all paradoxes reducible to one “fundamental” paradox? philosophy.stackexchange.com/questions/28241/…
    – Conifold
    Oct 28, 2015 at 2:27
  • Aren't paradoxes and fallacies different? Paradoxes might be fallacious but not all fallacies are paradoxes?
    – Tony
    Oct 28, 2015 at 3:00
  • 1
    Why do you want such a condition? What benefit does one gain from having a root for the concept of a "fallacy?" Might the condition of "the logic is wrong" qualify as such a root?
    – Cort Ammon
    Oct 28, 2015 at 5:34
  • I raised a similar question about paradox and still find it intriguing. It is hard to separate out the linguistic layers, but I always suspect "self-reference" (liar paradox) or "infinity" (Zeno's race) of lurking in all fundamental paradoxes. To me "fallacy" isn't reducible in the same way. Oct 28, 2015 at 12:28
  • Do you have a list of fallacies you wish to include in the discussion, or are we going to dare to discuss "all fallacies," which opens up so many amusing self referential problems.
    – Cort Ammon
    Oct 28, 2015 at 17:14

3 Answers 3


The fundamental meaning of a fallacy is that it is a defective piece of reasoning that conforms to an identifiable pattern such that all instances of that pattern are defective. If you are asking, is there a single kind of pattern that all fallacies conform to, then no, there are many different types of defective reasoning.

What fallacies do have in common is that they are not cogent, or that the truth or probability of their premises does not provide support for the truth or probability of their conclusion.

It is important to note that the defective nature of fallacies is not the same as invalidity. Being invalid is neither necessary nor sufficient for an argument to be a fallacy.

  • Yes, as phrased the question really just asks for a definition of "fallacy," of which there are innumerable possible instances. But isn't it possible to convert common fallacies into one another, and thereby reduce to something "more fundamental"? I am also wondering if there must be some evolutionary "selection" that suggests common fallacies must retain some rhetorical value, and if that should be part of the definition? Perhaps there is some interesting way to resurrect the question. Oct 29, 2015 at 13:36

Go to the definition. Fallacies are reasons behind invalid steps in a chain of deduction.

If and only if there are deductions dependent upon a rule of deduction that have a model or a possible world where they are false, is the rule a fallacy.

Not that this is particularly useful. But it is what you asked for.


A formal fallacy is an invalid step in a formal argument. However most often we use the term fallacy to mean informal fallacy.

An informal fallacy is any weak/non-cogent move in an informal argument such that it bears enough superficial similarity to a strong/cogent move to be commonly and identifiably mistaken for it.

Note: Informal arguments can neither be valid nor invalid, since those are technical terms defined only in relation to formal arguments. Informal arguments can only be strong (cogent) or weak (non-cogent).

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