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    [Music]
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    Scientists often gather data through
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    observation experiments, archival studies
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    and so on. But they are rarely satisfied
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    with data alone. Scientists want to draw
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    conclusions from those data. They want to
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    use the data to show that certain
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    theories are right and others are wrong.
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    To understand science, then, it will be
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    important to understand when it is
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    legitimate and when it is illegitimate
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    to draw a specific conclusion from what
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    we already know. We need to understand
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    arguments; and that is why, in this
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    the difference between good and bad
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    lecture, we will take a look at logic--the
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    study of argumentation. Let us first
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    introduce some terminology. An argument
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    consists of two parts: the premises and
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    the conclusion. The premises are the
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    things we presuppose and the conclusion
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    is what we conclude from those premises.
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    So let's look at an example:
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    No medieval King had absolute power over
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    his subjects. Louis 7 of France was a
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    medieval King. So Louis 7 of France did
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    not have absolute power over his
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    subjects. Here the first two lines are
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    the premises and a final line introduced
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    by the word "so" is the conclusion. In this
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    argument we assume that medieval kings
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    did not have absolute power and that
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    Louis 7 was a medieval King. And we
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    conclude that he did not have absolute
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    power. As a second piece of terminology
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    we will make a distinction between valid
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    and invalid arguments. A valid argument
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    is an argument in which the conclusion
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    really follows from the premises.
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    Our example about Louis 7 is an example
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    of a valid argument. The conclusion
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    really follows from the premises. It
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    makes sense to draw this conclusion from
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    these premises.
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    As an example of an invalid argument we
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    can take this: No medieval King had
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    absolute power over his subjects. Louis
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    seven of France was a great horseman. So
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    Louis seven of France did not have
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    absolute power over his subjects. We just
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    can't draw that conclusion from those
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    premises. So this argument is not valid.
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    It's invalid. Note that whether an
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    argument is valid or not
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    has nothing to do with whether the
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    premises or the conclusions are true.
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    Perhaps Louis 7 really was a great
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    horseman. Then all the premises and the
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    conclusion of that argument are true and
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    yet the argument is invalid because the
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    conclusion just doesn't follow from the
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    premises. On the other hand it's also
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    possible to have false premises and a
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    valid argument. For instance: No medieval
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    King had absolute power over his
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    subjects. Victor Gijsbers was a
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    medieval king. So Victor Gijsbers did not
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    have absolute power over his subjects.
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    This argument is perfectly valid even
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    though the assumption that I am a
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    medieval King is, as far as I know, false.
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    We can now introduce our final piece of
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    terminology: The distinction between two
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    kinds of arguments. Deductive arguments
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    and inductive arguments. A deductive
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    argument is an argument in which the
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    truth of the premises
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    absolutely guarantee the truth of the
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    conclusion. It's just not possible for
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    the premises to be true and the
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    conclusion to be false.
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    Returning to our original example, we can
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    see that this is a deductive argument. It
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    is true
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    the medieval Kings did not have absolute
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    power; and if it is true that Louis 7 was
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    a medieval King, then it must be true
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    that he did not have absolute power.
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    Or, in other words, if he did have
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    absolute power then one of those two
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    premises must be wrong. I'll come to the
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    definition of inductive arguments in a
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    moment, but first I want to point out two
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    interesting features of deductive
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    arguments: First, if you use deductive
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    arguments you can't make any new
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    mistakes. The only way for the conclusion
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    of a deductive argument to be false is
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    if one of your assumptions is false, so
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    if you already believe something false
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    then your conclusion may end up being
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    false. But if your assumptions are true
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    your conclusions are guaranteed to be
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    true as well.
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    So deductive arguments never introduce
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    falsehoods if they weren't already there.
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    And that makes them very strong and good
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    arguments to use, because they're not
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    very risky. Second, logicians found out
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    already more than 2,000 years ago--and
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    Aristotle played an important role here--
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    that whether a deductive argument is
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    valid or not can be determined just by
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    looking at the form of the argument and
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    ignoring its content. Even if you know
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    nothing about medieval kings and Louis 7
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    you can still see that our example
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    argument is valid. How? Because there's
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    this form: No A is B. C is A. So C is not B.
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    Where A is "medieval King," B is "someone
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    with absolute power," and C is "Louis 7" But
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    we can put anything we like in the place
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    of those letters and the argument will
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    remain valid. For instance, let's choose A
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    "Is a Dutchman" B "is humble" and C "is Victor
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    or Gijsbers" Then we have: No Dutchman
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    is humble. Victor Gijsbers is a
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    Dutchman. So Victor Gijsbers is not
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    humble. Which is another valid argument.
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    Although of course the first premise is
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    false and so is the conclusion. So we can
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    see whether a deductive argument is
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    valid simply by looking at its form
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    without knowing anything about its
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    content. And that is really important
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    because that means that we can see
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    whether something is a good argument
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    without making any prior theoretical
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    assumptions about the content matter. If
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    we believe that scientists first
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    collect data and then come to a
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    conclusion about which theories are
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    right and wrong, this is exactly what we
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    would expect. We only need the data and
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    some valid arguments which can be shown
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    to be valid independent of any theories
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    or ideas, and then we draw our
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    conclusions. It would be great if science
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    worked like that. Unfortunately, and I bet
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    you saw that coming,
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    science doesn't work like that. And it
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    doesn't work like that because the most
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    important arguments in science are not
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    deductive. They are inductive. Remember
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    that a deductive argument is an argument
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    such that the truth of the premises
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    absolutely guarantees the truth of the
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    conclusion. An inductive argument is an
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    argument where the truth of the premises
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    gives good reason to believe the
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    conclusion but does not absolutely
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    guarantee its truth. Again let's look at
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    an example:
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    None of the medieval texts we have
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    studied argues against the existence of
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    God, so no scholar in the Middle Ages
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    argued against the existence of God.
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    That's a valid argument if it's true
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    that none of the texts we have makes
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    this argument, and we have a lot of texts,
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    and it's quite plausible that nobody in
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    that time actually made this argument.
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    But it's indeed only plausible. It could
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    be that the argument was made but
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    somehow it wasn't transmitted to us. So
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    in an inductive argument. The truth of
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    the premises makes the conclusion likely,
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    but it doesn't guarantee it. And that's
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    generally the case in science. We have
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    some limited data. We want to draw a
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    general conclusion from those, and our
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    data makes the conclusion likely but
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    they don't make it certain. So, in science,
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    we are continually making inductive
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    arguments. And, as we will see in the next
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    lecture, induction is a lot more
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    problematic than deduction.
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01:46:39
Claude Almansi edited English subtitles for Sandbox
Claude Almansi edited English subtitles for Sandbox
Claude Almansi edited English subtitles for Sandbox
Claude Almansi edited English subtitles for Sandbox
Claude Almansi edited English subtitles for Sandbox
koma edited English subtitles for Sandbox
koma edited English subtitles for Sandbox
Claude Almansi edited English subtitles for Sandbox
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  • Revision 1 = provided subtitles for Lecture 1.2 of Prof. Scott Plous' Social Psychology course

  • Revision 1 = provided subtitles for Lecture 1.2 of Prof. Scott Plous' Social Psychology course

  • Revision 1 = provided subtitles for Lecture 1.2 of Prof. Scott Plous' Social Psychology course

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