What is the basic difference between biases, such as confirmation bias, anchoring bias, etc., and fallacies such as argumentum ad antiquitatem, argumentum ad hominem, argumentum ad ignorantiam, etc.? How exactly do they differ, and how are they related?

  • Biases are what we want to believe. Fallacies are what this desire, often leads us to adopt. Note, biases tend to push us into less obvious fallacies. Ideologies tend to push us toward more overt fallacies.
    – Dcleve
    Commented Aug 5, 2022 at 22:04
  • @Dcleve So biases are the cause of fallacies
    – quanity
    Commented Aug 5, 2022 at 22:06
  • I think so. We want to believe something, then we find a justification that supports our desire, and that is good enough for us in most cases. We don't examine rationales we expect to be true, nearly as closely as we would if the justification were support for something we did NOT want to believe. See Kahneman's Thinking Fast and Slow, which describes the role of System 2's rationality as mostly one of rationalizing what our system 1 concluded unconsciously.
    – Dcleve
    Commented Aug 5, 2022 at 22:30
  • Biases may cause fallacies, but don't necessarily. And fallacies may be caused by biases, but not necessarily.
    – J D
    Commented Aug 6, 2022 at 14:01
  • @JD As for the so-called logical fallacies, most of these are not errors of logic, but the credibility of implicit premises.
    – quanity
    Commented Sep 11, 2022 at 6:53

1 Answer 1


A fallacy is flawed reasoning. For instance, if one concludes that a math proof is correct, not on the merits of the steps of the proof, but because a famous mathematician is providing it, the reasoning is clearly flawed by an appeal to authority. Famous mathematicians can and do make mistakes, and the nature of mathematical proof rests in the consistency of the logic from step to step.

A cognitive bias is systematic error-making in thinking. Let's say you are doing research, but after your PhD advisor looks at your sources, it becomes clear that you are drawing from one set of papers that supports your thesis, while there's an entire set of papers that refutes your thesis. It might be that your choice to do so is consistent with the fact that you routinely avoid any positions critical of your position. In some way, you are confirming your belief through a non-conscious selection of avoiding evidence avoiding disconfirmation.

Both bias and fallacies are types of errors, but a bias is a pattern of making a similar error, and fallacies are errors that apply to a given argument. In other words, fallacies are individual, logical errors in reason, whereas cognitive biases are consistent, psychological errors in thinking.


To address claims in the comments: Biases might cause fallacies, but the important thing about biases is that they are systematic. Everybody makes mistakes errors in reason, but one who does consistently in the same way is biased, and it's important not to conflate the two. Think about lies, and pathological liars. Some people tell lies from time to time, but those who do so with zest, zeal, and compulsion are fundamentally different. And biases aren't what we want to believe, they are what we tend to believe erroneously. I personally have biases, but in a bid to improve my reason very much work hard because I don't want to believe error-riddled reasoning. Many people are comfortable with their biases, but many people work hard to overcome them precisely because they want to be closer to being free of errors.

  • 1
    Perhaps another way to say that is that a bias is a property of the reasoning agent, while a fallacy is a property of a particular argument.
    – Bumble
    Commented Aug 6, 2022 at 14:12
  • @Bumble Absolutely concise and true.
    – J D
    Commented Aug 6, 2022 at 14:12
  • @JD What about my second question . Is abductive reasoning a logical fallacy
    – quanity
    Commented Aug 6, 2022 at 15:39
  • @quanity Yeah, I eliminated that because if you're interested in in-depth answers, you're better off crafting and submitting another question. The short answer is most logicians accept CSPs general argument that we infer explanations about events, but imperfectly. As such, if one claims that one's process of determining an explanation is certain, then it's flawed in a formal, technical sense. Therefore, one way to understand it as an inductive argument about a cause or explanation. Then, strictly speaking, it needn't be certain.
    – J D
    Commented Aug 6, 2022 at 17:16
  • Is it a fallacy? If you claiming that the abduction is deductively certain, yes. Otherwise, it is like a well-reasoned, best guess. Probably right, but possibly wrong. Then, whether or not it is a fallacy depends on the quality of the reasoning in the abduction.
    – J D
    Commented Aug 6, 2022 at 17:18

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