CRITICAL THINKING – Fundamentals: Deductive Arguments


Hi! I’m Jeff Pynn, and I teach philosophy[br]at Northern Illinois University. In my earlier Introduction to Critical[br]Thinking video, I described the difference between deductive arguments and ampliative[br]arguments. In the next few videos, I’ll talk a bit[br]more about each type of argument. Let’s start with deductive arguments. An argument is a set of statements, called[br]its premises, that are meant to give you a reason to believe some further statement[br]called the argument’s conclusion. In some arguments, the premises are meant[br]to guarantee that the conclusion is true. Arguments like this are called deductive[br]arguments. A good deductive argument can give you a[br]very good reason for believing its conclusion. After all, it guarantees that its [br]conclusion is true. But not all deductive arguments are good,[br]and so there are several things to think about when deciding whether to believe the[br]conclusion of a deductive argument. A good deductive argument really does[br]guarantee its conclusion. Part of what this means is that its[br]impossible for the premises to be true while the conclusion is false. When this is the case, we say that the[br]argument is valid. Now this is a special, technical use of[br]the word “valid.” In ordinary life, we often use this word[br]to mean something like good, cogent, or reasonable. Like if you’re disagreeing with someone[br]about something, and they respond to a claim you make by saying something that[br]seems pretty reasonable to you, you might say, “Well, I guess you have[br]a valid point.” Though that’s what the word often means[br]in ordinary life, it’s not what the word means here. When philosophers say that an argument is[br]valid, they always mean this very specific thing: that if the premises are[br]true, the conclusion must also be true. There are several other Wi-Phi videos that[br]discuss this notion of validity in more detail. To say that an argument is valid is to say[br]something about the relationship between the premises and the conclusion. Namely, that if the premises are true, the[br]conclusion must also be true. But it’s not to say that its premises or[br]conclusion are true. Consider, for example, this argument. Premise 1: Beyonce was born in Paris. Premise 2: Everybody who was born in Paris[br]loves cheese. Conclusion: Therefore, Beyonce loves[br]cheese. Those premises are false. Beyonce was born[br]in Houston, and I’m willing to bet that at least some people born in Paris hate[br]cheese. Still, it’s a valid argument. If the premises were true, then the[br]conclusion would have to be true. But because the premises are false, this[br]argument doesn’t give you a good reason to believe its conclusion, even though it’s[br]valid. Philosophers call a valid argument with[br]true premises “sound.” Like the word “valid,” the word “sound” is[br]term with various meanings in ordinary life, and it can be used to describe some[br]claim as reasonable or compelling. But when philosophers describe an argument[br]as sound, they always mean this very specific thing: that it’s valid, and that[br]its premises are in fact true. Here’s a pretty boring sound argument. Premise 1: Beyonce was born in Houston. Premise 2: Everybody who was born in[br]Houston was born in Texas. Conclusion: Therefore, Beyonce was born in[br]Texas. For more discussion of the concept of a[br]sound argument, see Aaron Ancell’s Wi-Phi video entitled[br]”Soundness.” So, before deciding whether to believe the[br]conclusion of a deductive argument, you need to determine whether the argument[br]is sound. And this, in turn, requires determining[br]whether the argument is valid, and whether its premises are true. Well, how do you tell whether an argument[br]is valid? Sometimes, it’s just obvious. But often,[br]it’s not so obvious. One way to figure out whether an argument[br]is valid is to see if you can think of a[br]counterexample to it. A counterexample is a case, either real or[br]imaginary, where the argument’s premises are true,[br]but the conclusion is false. So, for example, consider this argument. Premise 1: Classical musicians appreciate[br]opera. Premise 2: Beyonce is a pop star, not a[br]classical musician. Conclusion: Therefore, Beyonce doesn’t[br]appreciate opera. Now, suppose that Beyonce’s been listening[br]to opera since she was a little girl, and loves Mozart’s Don Giovanni. Well, then she’d appreciate opera. The conclusion would be false, even though[br]the premises would still be true. It would still be true that classical[br]musicians appreciate opera, and that Beyonce is a pop star, not a[br]classical musician. This counterexample shows that the[br]argument isn’t valid, and so that even if premises are true, the[br]argument doesn’t provide you with a reason to believe its conclusion. There are other, more formal techniques[br]for figuring out whether an argument is valid, which we’ll hopefully be able to[br]discuss in future videos. Now, if you don’t know whether the[br]premises of an argument are true, then even if the argument really is sound,[br]it doesn’t give you a good reason to believe its conclusion. When you know that an argument is valid,[br]but you don’t know whether its premises are true, the argument gives you, at best,[br]a conditional reason to accept its conclusion. If you learn that its premises are true,[br]then you’ll have to accept its conclusion. So, how do you tell whether an argument’s[br]premises are true? Well, this isn’t the kind of thing logic[br]or philosophy can give you much help with. To figure out whether an argument’s[br]premises are true, you need to do some research. This is one reason why being a good[br]critical thinker requires more than just logical ability. It also takes a lot of real world,[br]empirical knowledge. Unless you know enough to know whether an[br]argument’s premises are true, then even if you’re a really brilliant logician and[br]know that the argument is valid, it doesn’t give you reason to believe its[br]conclusion. The more you know, the better you’ll be[br]able to evaluate deductive arguments. Subtitles by the Amara.org community

18 Replies to “CRITICAL THINKING – Fundamentals: Deductive Arguments”

  1. It's such a well-animated video on an important topic that everyone should be familiar with… so why so few views?  🙁

  2. I am studying this for my critical reading class today. It has enough views when it's over 1,000 in my opinion.

  3. Induction is when we get the conclusion which is most probable from the premises. We cannot be certain of this. Deduction is when we are certain from the premises. In the fictitious character Sherlock Holmes, he is actually practicing Induction instead of Deduction. I think Sir Arthur Conan Doyle did this to add to Sherlock's ego.

  4. I have been having a horrible time in my PHI class. I have no clue why I am not picking up on this material. I have a 4.0 for goodness sakes! My professor has been working his bootie off to assist me. I guess I am one of the doomed who is destined to never have any common sense. This video gets an A+ from me, but I do not think I have gotten it any better as of yet.

  5. Schools need to teach this subject starting in grade schools… Students are taught "what" to learn/ think but not "how" to.

  6. If I attack an argument because it commits an informal logical fallacy (begging the question, ad hominem, etc.), am I attacking its logical validity or its soundness?

  7. 4:16 I think this is beautiful because it demonstrates that the rules of logic and deduction were established using induction

  8. The definition of Validity was always an iffy definition. Let there be premises A = True ,B = False ,C=True, Ergo Z, still be valid despite Z has nothing to do with the premises. Just Saying. Soundness had a better definition than Validity. Soundness has to be both syntactically correct and semantically correct.

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