Fallacies: Formal and Informal Fallacies

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