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Fallacies: Formal and Informal Fallacies

In this video, Paul describes the distinction between formal and informal fallacies. This distinction is useful for understanding the fallacies in Wi-Phi's Critical Thinking section.

Speaker: Paul Henne, Duke University.
Created by Gaurav Vazirani.

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  • blobby green style avatar for user Tara Zamalynski
    So formal fallacies take correct information that lead to potentially false conclusions because we've made an assumption based on HOW the argument was made?

    And informal fallacies are more about the information provided being wrong somewhere in the argument?

    Not sure I have this one down...
    (5 votes)
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    • leaf green style avatar for user Paul Henne
      Hey, Tara! This distinction can get pretty murky when you first encounter it. Formally fallacies can have true ("correct" as you say) or false premises ("information"). The flaw is with the form of the argument; the truth or falsity of each individual premise has to do with soundness and validity - a topic that we will discuss soon. In order to identify a formal fallacy, you only need to pay attention to the form of the argument.

      An informal fallacy has a defect in the content (the "information"). So, there is an error in the meaning of a premise. In order to identify an informal fallacy, you need to pay attention to the meaning of the content. So, while the form may be valid (that is, there is no formal fallacy), the meaning might elicit a defect in reasoning. Does this help?
      (7 votes)
  • blobby green style avatar for user Cody David Clark
    So, the first example about philosophers was an informal fallacy? Because not all philosophers publish books in philosophy. Publishing books in philosophy is sufficient, but not necessary to be considered a philosopher.
    (5 votes)
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  • leaf blue style avatar for user (●̮̮̃•̃) NADO ☯
    In what way can you use an informal fallacy to your advantage in an argument?
    (4 votes)
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  • duskpin ultimate style avatar for user Bruce William Collins
    Can someone explain what does the 'form' stand for? I understand the concept of the Informal fallacy - there's a problem in the meaning of the used premises. ... . But what does it mean to have a problem in the form of the premises? What is the form of an argument?
    (2 votes)
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    • purple pi teal style avatar for user Maria
      "Form" basically means structure. So if an argument has good form, it has a good structure, or is laid out in a way that makes sense.
      The general form for an argument is:
      Premise 1
      Premise 2
      Here's a sample:
      P1: All rectangles have four sides and four right angles
      P2: All square has four right sides and four right angles
      C: Therefore, all squares are rectangles.
      This is both valid (the structure makes logical sense) and sound (all the premises are true).
      If you have a problem with the form of the premise, the premise might be either false (unsound) or invalid (the premise doesn't make the conclusion is necessarily true).
      Here is an argument with flawed premises:
      P1: All Cats are blue
      P2: Mary has blue hair
      C: Mary is a cat
      This is invalid because having blue hair doesn't guarantee that Mary is a cat. It's also unsound because not all cats are blue. Both of the premises are flawed, so the argument has bad form.
      Let me know if this helps.
      (4 votes)
  • blobby green style avatar for user Jaysen P
    Why is it important to be able to understand and identify fallacies?
    (3 votes)
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  • duskpin ultimate style avatar for user Lee Cowan
    The example at (the bird argument) seems to me to be a formal fallacy, not an informal fallacy:
    Normally, A is B.
    C is an A.
    Therefore, C is B.

    This form is invalid. The premises being true do NOT necessitate the conclusion being true. The first premise makes it possible for A not to be B. However, the conclusion assumes the opposite - and this results in the fallacy.

    Paul considers the word "Normally" as impacting the content of argument but in my opinion it is impacting the form. Which is correct?
    (2 votes)
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  • blobby green style avatar for user DaDumbGuy
    Speaking of both fallacies

    Organics will die. Organics will die.
    John is organic. John will die.
    Therefore, John will die. Therefore, John is organic.

    Does it sound right? Feels like I've done something wrong here.
    (2 votes)
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  • mr pink red style avatar for user madkev.1
    Can I say that all invalid arguments are commiting formal fallacies
    and all valid but unsound arguments are commiting informal fallacies?
    (2 votes)
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  • blobby green style avatar for user canthavemyphonenumber
    To me, it sounds like a formal fallacy means the argument is bad, whereas an informal fallacy means the evidence is bad. Is that roughly correct?
    (1 vote)
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  • male robot donald style avatar for user M Ali
    I'm getting a bit confused, is a valid argument automaticly deductive? And is a invalid argument ampliative (or indeductive)...

    Or are there differences?
    (1 vote)
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    • blobby green style avatar for user lxndr.rvs
      Invalid arguments are not ampliative, they're just bad deductive arguments. I think it comes down to intention of the argument. Is the argument trying to guarantee its conclusion? If yes, it's deductive and you can asses its validity. If no, it's ampliative and validity does not apply. More formally,

      All ampliative arguments do not try to guarantee their conclusion.
      But all invalid arguments do try to guarantee their conclusion.
      Therefore, by definition all invalid arguments are not ampliative.

      The above argument is deductive because I am trying to guarantee that invalid arguments are not ampliative. Now, do you think this argument is valid? Is it sound?
      (1 vote)

Video transcript

hello I'm Paul honey and I'm a philosophy graduate student at Duke University and in this video I'm going to introduce you to formal and informal fallacies first consider the following arguments premise 1 if someone is allergic to peanuts then she doesn't eat peanut butter premise 2 Jane doesn't eat peanut butter conclusion therefore Jane is allergic to peanuts consider this other argument premise 1 a feather is light premise 2 what's light cannot be dark conclusion therefore a feather cannot be dark these arguments probably seemed a bit off to you and you're right each argument contains a fallacy though each has different kind a fallacy as we've seen in other wireless philosophy videos is a defect in reasoning this defect can be unintentional or intentional that is I could simply make a mistake in my reasoning or I could want to trick you into believing something by using deceptive reasoning either way it's probably a good idea that we were able to understand and identify distinct fallacy types so that we can adequately correct any defects in our reasoning but before we talk about fallacy types let's recall what a valid argument is consider this argument premise 1 if someone is a philosopher and she publishes articles in philosophy Tamar Gendler is a philosopher therefore Tamar Gendler publishes articles in velocity this is a valid argument actually although it has no bearing on this arguments validity Jen lawyers published over 20 articles among her other works this form of argument is called modus ponens and it can be represented as the following premise 1 if X then Y premise 2 X inclusion therefore Y and this is a valid argument form the conclusion necessarily follows from the premises remember though this valid form does not mean that the premises are true it would still be a valid argument if Tamar Gendler had decided to become a train conductor for instance instead of a philosopher see the videos on soundness and validity if you have questions about this we can also have an invalid argument form let's return to our first argument this argument as you might have suspected contains a fallacy a formal fallacy and a formal fallacy is exactly what it sounds like a defect in the form of the argument in other words there are certain forms of arguments that are invalid this invalid argument form is a formal fallacy called affirming the consequent this fallacy is apparent if you consider that Jane just might not like peanut butter so simply because Jane isn't like peanut butter she doesn't eat it given just the fact that she doesn't eat it and the conditional on premise 1 we cannot conclude that Jane is allergic to peanut butter this form the fallacy can be represented it in the following way if X then Y Y therefore X and this form a fallacy is one that you don't want to use this is actually one of the fallacies frequently tested on the LSAT so if you are interested in one more practice in identifying this fallacy check out the video on affirming the consequent we will talk more about this and other formal fallacies and upcoming videos but for now we need to see that formal fallacies are fallacies because of the poor form of the argument any argument of these forms will be invalid so anything can be plugged into these bad argument forms and the argument would be about for instance if meow then Splatt Splatt therefore me out is invalid so now that we have a general understanding of formal fallacies what about informal ones let's look at our other example from the beginning of this video notice this fallacy arises out of the content of the argument not out of the form the form may at first glance seem valid that is if we ignored the meaning of the content consider that we can use a seemingly similar argument form to make a valid argument for instance premise 1 rain is wet premise 2 what sweat cannot be dry conclusion therefore rain cannot be dry and here we have a valid argument that seems to have a similar form as a fallacious one and one that lacks defects in its content but the content of our fallacious argument given the two meanings of light light weight and light color yields some problems that is if we understand the meaning of the terms the two meanings like for instance we realize that the argument actually has an invalid argument form initially without considering the meaning of the content it looks like the form of the argument is something like all x ry what is y cannot be z therefore no x RZ but actually given the shifting meaning of light the form is more like all X or Y what is W cannot be Z therefore no x RZ this fallacy is called Co dication and see the video on this if you'd like to know more about it the only way we can understand this defect is by examining the content of the argument if we just looked at the form without understanding the content then we could not detect this fallacy so as we now know informal fallacies occur because of problems with the content of the arguments so an argument might have a seemingly valid form but committin informal fallacy because of a defect in its content hence an argument might have true premises in a seemingly valid form yet also commit a fallacy to reiterate a formal fallacy means that the argument has a defect in its form while an informal fallacy has a defect in the arguments content which might also yield a defect in its form so now that's the difference between formal and informal fallacies you