• Episode 4: Validity and Soundness

    In their fourth episode, Nick and Dave discuss validity and soundness.


    Dave: You are listening to Critically Minded.
    The podcast for English learners who want to become better critical thinkers. We’re your hosts, Nick . . .
    And Dave. You should be happy to hear that we have completed our introductory discussion of argument analysis. In this episode, we are going to discuss the second category of basic critical thinking skills, argument evaluation.

    Nick: When we evaluate an argument, we judge its strength by looking at several  points. First, whether the premises are true or false. Second, whether the logic is successful in bridging the gap between the premises and the conclusion.
    Today we are going to learn six new words. Really, there are only three new words and their three opposites. And here they are: valid, invalid, sound, unsound, logical, illogical.


    Nick: Let’s look at the word “valid” first. We often hear people say, “He has a valid point.” Or “He has a valid opinion.” It doesn’t really mean much except to say that someone’s opinion seems believable. However, in argument, the word valid means that the premises of an argument lead to a logical conclusion. It doesn’t mean that the premises or the conclusion are true. It only means that the pattern of the argument is follows pure logic.
    Dave: Yes, and that’s an important point. An argument is considered valid when we say to ourselves that IF the premises are true, THEN the conclusion must also be true. For example, listen. You’re leaving for school in the morning and your mother says:

    There is a lot of influenza going around.
    This mask will protect you from it.
    So be sure to wear it on the train.


    Nick: O.K. Now this argument is valid, because, if its premises are true then it must also be true that this mask will protect you from influenza. So again, this argument is valid, but in this case it isn’t sound. Although the argument was valid, it is unsound because the premise, ‘This mask will protect you from influenza’ is not true. Influenza is spread through contact, so washing your hands is far more effective. Note that validity is not based on the truthfulness of an argument, but soundness is.
    Dave: An argument is only SOUND when the reasoning is logical and ALL of the premises are true. If one of the premises, or indeed the conclusion, is NOT true then the argument is unsound.
    Nick: Now here is an example of an argument that is both valid and sound.

    No fish have feathers.
    All sardines are fish.
    Therefore, no sardines have feathers.

    This is sound because it is true in the real world. The premises are true, as is the conclusion.
    Dave: The logic is clear. The premises and conclusion are true, and the conclusion cannot be denied.
    Nick: And if you memorize that example you can use it in the future as a model example. Of course, not every argument is that simple or that clear, but if you hear an argument that you don’t quite understand, break it up into simple parts, and ask yourself, ‘How true is each of the parts?’ ‘Are there any other possibilities?’ and if so, ‘What could they be?’, and ‘How well does everything support the conclusion?’


    Dave: So that first example, with the influenza and the face mask is one way that an argument can go wrong––when, even though the logic is valid, one of the premises is false. But that is not the only way that an argument can be unsound. It may be unsound, when, even though the premises are all true, the conclusion is faulty. How about an argument about musical instruments and families of musical instruments? Listen: All members of the string family of musical instruments have strings. Pianos have strings. Thus, pianos, are members of the string family of musical instruments.
    Nick: Well, that’s not quite right. Although it is true that all members of the string family have strings; and it is also true that pianos have strings, the piano is not a member of the string family. Pianos belong to the percussion family of musical instruments because the strings are hit by little hammers. This argument makes a logical error when it assumes that all things that are similar in one way, like having strings, are similar in every way, like being the member of the same group or family.
    Dave: We could go into more detail about problems in logic here, but it’s a long discussion. It is enough to say that cases of faulty logic are called fallacies. So until next time. You have been listening to–
    Nick: Critically Minded: Critical Thinking for 2nd Language Learners.

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