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It's been a long and winding road as the
Beatles used to say, but we're finally at
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the last stages of reconstructing
arguments. We've looked at stage one,
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which is close analysis. Stage two which
is get down to basics. Stage three which
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is sharpen edges, stage four is organize
parts, and we are doing stage five which
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is fill in gaps and we'll also get to
stage six which is conclude. Stage five
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really consists of four separate steps.
First we need to assess the argument for
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validity. Then we need to add suppressed
premises. Enough of them to make it valid.
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Then we need to assess those suppressed
premises for truth or falsehood. And then
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we need to qualify the suppressed premises
in order to make them true. The whole goal
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is to make the suppressed premises both
plausible for their truth, and enough to
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make the argument valid. So, these steps
within the stage really do work in tandem
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together to try and make the argument
good. We already learned how to assess
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validity. You simply ask, "Is it possible
for the premises to be true and the
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conclusion false, and if so, the argument
is not valid and if not, the argument is
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valid." And the way you figure out whether
it's possible is you try to tell a story
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or describe a situation and if you can
describe a coherent situation where the
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premises are true and the conclusion is
false, then that show that the argument is
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not really valid. The main topic for today
is, what do you do when you assess the
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argument for validity and you find out
it's not valid?" And the answer is, you
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add suppressed premises, enough of them to
make the argument valid. But that might
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seem like cheating. I mean, you start with
an argument that's no good, it's not
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valid, and then you just throw in some
extra premises in order to make it valid.
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Why is it that's just distorting the
argument and making up something that
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wasn't there? The answer is that it's not
really bad. And if it were bad, we'd all
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be in bad shape because in everyday life,
people always take things for granted.
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They make assumptions. We do it too, and
if we didn't, boy, our arguments would be
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really long and boring. So there's
something to be said in favor of
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suppressing premises, at least, the
obvious ones that people really do take
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for granted. But we can also get tricked.
People can suppress premises that really
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are questionable, and they just don't want
us to see that they're making that
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assumption. So, it's useful to fill out
the argument with suppressed premises to
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make sure it really is valid, because that
brings those assumptions out in the open,
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where we can assess whether or not they're
true or false. Another reason to fill in
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suppressed premises is to understand the
argument better. Because if people
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suppress premises, then they're showing us
some of their footprints along the path.
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But if we really want to know the full
path that their reasoning followed, we've
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got to see every single footprint. So, the
goal of bringing up the suppressed
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premises is to let us trace exactly where
the reasoning is from one step to another.
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So there are two goals: One is to trace
the full path every step and the other
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goal is to see if there are any miss-steps
or they are trying to hid something from
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us by getting rid of one of their
footsteps, so that's the point of bringing
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up suppressed premises. To accomplish
these goals is tricky. You have to find
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suppressed premises that are just strong
enough to make the argument valid but not
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so strong that there gonna be implausible.
Cuz you don't wanna ascribe all kinds of
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suppressed premises to the person that
they didn't really believe, and they
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didn't really need for their argument. So
it's kind of like Goldilocks and the Three
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Bears. You want suppressed premises to be
not too hot and not too cold, but just
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right. Here's an example from a previous
lecture. My wife always gives me either a
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sweater or a board game. This box does not
contain a sweater because it rattles when
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it's shaken so this time she must have
given me a board game. We put this in
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standard form this way. First premise:
This box rattles wh en I shake it and that
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shows you it doesn't contain a sweater.
Third, she always gives me either a
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sweater or a board game. Conclusion, this
time she must have given me a board game.
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Now the first step in this argument is,
this box rattles when I shake it. And the
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conclusion there is it doesn't contain a
sweater. That's the part of the argument
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we want to focus on here and ask whether
that argument is valid. The argument is
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not valid, because it's possible for the
premise to be true and the conclusion
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false. How can that happen? Well, my wife
might be fooling me. She might know that I
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expect either a sweater or a board game,
so she puts a sweater in the box. And then
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she puts little rocks around the outside
so when I shake it, I'll hear something.
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So that's possible, and that shows that
the argument's not valid. Well, how can we
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make the argument valid? The question here
is, can we add a suppressed premise that
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will turn this invalid argument into a
valid argument? Here's one that will do
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the trick. A box that contains a sweater
doesn't rattle when shaken. Now the
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argument looks like this. This box rattles
when I shake it. The box that contains a
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sweater doesn't rattle when shaken, so
this box doesn't contain a sweater. The
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explicit premise is that this box rattles
when I shake it. The suppressed premise is
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that a box that contains a sweater doesn't
rattle when shaken and together they are
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suppose support the conclusion that this
box doesn't contain a sweater, but do they
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really support that conclusion? Is the
argument valid? Well, it's valid only if
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there's no possibility that the premises
are true and the conclusions false.
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Without the suppressed premise we saw this
might be possible, because my wife might
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be fooling me and putting rocks around the
sweater. So let's see if that's going to
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ruin the validity of this argument. No!
Because if the sweater has got rocks
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around it so it makes noise when I shake
it, then the premise that says a box that
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contains a sweater doesn't rattle when
shaken turns out to be fal se.
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So that's not a case where the premises
are true and the conclusions false because
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the premise is false in that case. So, by
adding this premise, we actually succeeded
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in making the argument valid. The problem
of course is that validity is not enough
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for a good argument, as we saw several
lectures ago. You can have an argument
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that's very bad, when the argument is not
sound. What we want really is soundness.
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So, that's why we need the next step,
mainly, check the supressed premises for
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truth. Assess whether they're true or
false. And, if they're not true, then you
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need to qualify them, in order to make
them true. Cuz you don't want to claim
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that the person giving the argument was
assuming this falsehood, when the didn't
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have to. So let's see if there's some way
to qualify this suppressed premise in
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order to make it true. How can we qualify
this premise to make it true? How are we
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going to do that? Let me think. Oh. What
about that little word only. You could add
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that. You could say, a box that contains
only a sweater doesn't rattle when shaken.
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But the word only, what exactly does that
mean? We need to clarify that. What
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exactly is the word only exclude? It
excludes something, that's the function of
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the word only, but what does it exclude?
Well. It probably excludes other things
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that might make the rattling sound like if
my wife put rocks in the box. So we can
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clarify this premise by saying a box that
contains only a sweater and not anything
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else that might make a rattling sound when
shaken, won't rattle when shaken. Well, is
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that premise true? Well, you might quibble
about details but it's close enough for
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now. What we need to do though is to go
back and determine whether, when we put
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that suppressed premise in, the argument's
valid. And the argument now looks like
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this. This box does rattle when shaken,
and a box doesn't rattle when shaken if it
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contains only a sweater and not anything
else that makes a rattling sound. So this
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box doesn't contain a sweater. Is that
valid? Well, no, for the same reason we s
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aw before, because my wife might be a
trickster who puts rocks around my sweater
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in the birthday present box in order to
fool me. Then, the premises can be true,
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and the conclusion false. It's possible
that, the first premise is true. This box
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rattles when I shake it. And the second
premise is true, a box doesn't rattle when
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shaken if it contains only a sweater and
nothing else that makes a rattling sound.
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But it's false that this box doesn't
contain a sweater, cuz it still does
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contain a sweater, and it contains a
sweater in addition to those pesky little
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rocks that make all that rattling noise.
Well, if your argument's not valid, we've
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got to go back to that other step and add
another suppressed premise. Remember how I
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told you how these different steps within
this stage work in tandem and what's
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happening is you've got to check it for
validity, add a suppressed premise,
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recheck for validity, maybe add another
suppressed premise, and that's what we're
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doing now. So what kind of suppressed
premised can we add. Well, we could add my
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wife is not a trickster but basically that
amounts to she wouldn't put rocks in a
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birthday present with a sweater in order
to fool me. So we could make that a little
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more explicit by making the suppressed
premise something like this. If this box
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contains a sweater, then it only contains
a sweater and it doesn't include anything
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else that would make a rattling sound when
shaken. And now we can stick that as an
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extra suppressed premise into the
argument. Now the argument looks like
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this. This box rattles when I shake it. A
box doesn't rattle when shaken, if it
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contains only a sweater and not anything
else that makes a rattling noise when
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shaken. If this box contains a sweater,
then it contains only a sweater and
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doesn't contain anything that rattles when
shaken. So this box does not contain a
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sweater. Now we have an argument that's
valid. And the suppressed premises are
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true, at least given our life's not a
trickster, which she's not, I assure you.
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And it looks like we have a sound rec
onstruction, just what we were looking
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for. Admittedly, this argument is a lot
longer and more convoluted than the
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original, and that shows why people
suppress premises instead of talking the
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way this argument goes. And of course,
many people would be perfectly well
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convinced by the original argument because
they share the assumptions that are in the
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suppressed premises. So why do we go
through all the trouble to go through this
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process and add the suppressed premises?
Remember, the reason is that we want to
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understand the pathway between the
premises and conclusion. We want to
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understand how the reasoning works step by
step by step. And we want to do that
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because sometimes people are going to
include suppressed premises that aren't
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true, and we want to bring them out and
make those assumptions explicit so that we
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can assess them for truth and falsehood.
And when you're talking to somebody you
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trust, you might not have to do that and
it's okay to suppress premises. But when
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you really want to know whether the
argument's any good, that's when you want
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to fill it out with the suppressed
premises. The point of going into detail
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on this example is to illustrate this
stage of reconstruction. You want to
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assess the argument for validity, add
suppressed premises that make it valid.
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Check them for truth. If they're not true,
you qualify them, and then you go back and
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see whether that qualification made the
argument not valid anymore. And you go
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back and forth and back and forth until
you've got a sound reconstruction. The
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same steps are going to apply to all kinds
of suppressed premises. And sure enough,
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there are all kinds of suppressed
premises. So let's go through a few
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examples a lot more quickly in order to
show the variety of suppressed premises
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that are assumed in arguments. Here's one
example. Abraham Lincoln turned 40. On
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February twelfth, 1849. Therefore, Charles
Darwin also turned 40 on February twelfth,
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1849. Now, is that argument valid? No
chance. Of course it's possible for the
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premise to be true and the conclus ion
false. So we have to add a suppressed
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premise. The suppressed premise is, that
Abraham Lincoln and Charles Darwin were
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born on the same day. And they were, it
happened to be February twelfth, 1809. So
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now, we've filled out the argument.
Abraham Lincoln turned 40 on February
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twelfth, 1849. Abraham Lincoln and Charles
Darwin were born on the same day,
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therefore Charles Darwin also turned 40 on
February twelfth, 1849. Now is the
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argument valid? No. It's still not valid.
Cuz Darwin might have died before 1849. So
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we have to add another suppressed premise.
Mainly, that both Abraham Lincoln and
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Charles Darwin lived beyond 40. So now we
have a fuller argument. Abraham Lincoln
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turned 40 on February twelfth, 1849.
Abraham Lincoln and Charles Darwin were
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born on the same day. Both of them lived
beyond the age of 40. Therefore, Charles
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Darwin also turned 40 on February twelve,
1849. Now the argument looks pretty good.
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We had to have two suppressed premises but
we finally have a valid argument. And what
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this shows is that sometimes the
suppressed premises are purely factual
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matters. In this case that they were born
on the same day and that they both lived
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beyond 40. So sometimes we have factual
suppressed premises. Here's another quick
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example. You ought to obey her because
she's your mother. Here, the premise is
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that she's your mother and the conclusion
is that you ought to obey her. Well, is
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that argument valid? No way cause it's
possible that she's your mother but it's
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false that you ought to obey her. When
could that happen? Maybe, she was like
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abusive or stupid or whatever. Then maybe
you ought not to obey her even though she
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is your mother. So we have to add a
premise, namely, you ought to obey your
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mother. Now we can say she's your mother,
you ought to obey your mother therefore
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you ought to obey her, but of course that
a supressed premise you ought to obey your
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mother is questionable because maybe she
was abusive or stupid. So let's add
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another supressed premise that your mother
was not abusive or stupid, of course we a
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lso have to qualify that moral premise
that you ought to obey your mother if
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she's not abusive or stupid. And now the
argument looks like this - she's your
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mother, you ought to obey your mother if
she's not abusive or stupid. Your mother
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was not abusive or stupid. Therefore, you
ought to obey her. An notice that here, we
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added a moral premise about the fact that
you ought to obey your mother under
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certain conditions. Namely, she's not
abusive or stupid. And the second premise
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is she was not abusive or stupid. So, we
have a moral premise and a factual
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premise, both being suppressed in the
argument that you ought to obey her
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because she's your mother. Here's another
. It's the Sabbath, so you ought to go to
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synagogue. Well, that's clearly not valid.
One suppressed premise is that you're
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Jewish. The other suppressed premise is
you haven't been to synagogue already
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today, on this Sabbath. And the third
suppressed premise is a religious norm,
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mainly Jews ought to go to the synagogue
on the Sabbath. And you need that whole
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bunch of suppressed premises in order to
get from the premise, that it's the
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Sabbath, to the conclusion, that you ought
to go to synagogue. And of course all of
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those premises might be questionable. Some
people would question them. Some people
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would deny them, but the point here is to
figure out what's being assumed by someone
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who gave the original argument. And
anybody who says it's the Sabbath, so you
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ought to go to synagogue, seems to be
assuming you're Jewish, you haven't been
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already, and Jews ought to go to the
synagogue on the Sabbath. So what these
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suppressed premises do is they bring out
the assumptions that somebody who gave
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that argument must have had in mind. The
last case is a little bit trickier. It has
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to do with linguistic suppressed premises.
Jen and Bob are first cousins, therefore
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they share a grandparent. Now, in order to
understand that argument, you have to know
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that first cousins always share a
grandparent. That just follows from the
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definition of what a first cousin is. But
i t's not quite so obvious, is that
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biological sisters are female. And so,
there's even more need to bring out that
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linguistics suppressed premise in this
case. But it's still not necessary to make
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the argument valid. It's just not possible
that Janet and Bob are first cousins, and
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they don't share a grandparent. Because
the suppressed premise is purely
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linguistic, so it's necessarily true, so
you can't possibly be first cousins
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without sharing a grandparent. Still, the
point of bringing out linguistic
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suppressed premises is to show every
little step along the way. The argument
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might be valid without those suppressed
linguistic premises, but we won't
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understand why it's valid and why the
reasoning goes through unless we add the
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linguistics suppressed premise. So it's
worth doing that. Shh. Here's a trick.
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Don't tell anybody. Okay? It's just
between me and you. You can always make
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any argument valid just by adding a
suppressed premise that says if the
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premises are true, then the conclusion is
true. But don't tell anybody, because if
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people start doing that, and they start
making the argument valid that way, with
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that suppressed premise, we're never going
to understand the pathway of reasoning. It
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makes the argument valid, but it doesn't
serve the real purpose of adding
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suppressed premises which is to understand
the pathway of reasoning. So you can do
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that. It's a trick. It makes that argument
valid but it doesn't achieve our goal
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because our goal is not just to make the
argument valid, it's to make the argument
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valid so that we can understand the
pathway of reasoning. So it's important to
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know that trick, but don't use it unless
you have to. The examples so far have been
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pretty trivial, I admit it. But the same
points apply in very important context
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such as politic debates, politician can
suppress premises in perfectly legitimate
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ways. They're just trying to save time and
make their arguments more efficient, maybe
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even sometimes clearer because you don't
have to add all those little details. But
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sometimes politicia ns abuse suppressed
premises. They take things for granted
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that they shouldn't be taking for granted.
Here's an example. A politician might
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argue, my opponent is soft on crime
because he's opposed to the death penalty.
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Well, that assumes, as a suppressed
premise, that anyone who's opposed to the
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death penalty must be soft on crime. And
if the politician were to come out and say
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that, it would seem pretty questionable,
and that's probably why he suppresses it.
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And then another politician might say, my
opponent is in favor of the death penalty,
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so he must not have read all the recent
studies that show that the death penalty
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doesn't deter. Well that argument assumes
the suppressed premise that if you've read
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those studies you'ld be convinced by them,
and that the only point of the death
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penalty is deterrence. But the point is
that politicians talking about extremely
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important issues can take things for
granted, that if they were brought into
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the light of day will be questionable, and
that's why they hide them. So when you're
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listening to people give arguments on
important issues in your life, then you
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ought to be looking for these suppressed
premises and asking yourself whether or
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not you really ought to be agreeing with
them about that assumption. Finally, we
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finished reconstruction. Yipee, right? Oh,
no, not quite, because there's one more
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stage, and that stage is drawing a
conclusion. Of course, if we've come up
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with a sound reconstruction, then we know
that the argument is sound, and we know
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that the conclusion is true, because every
sound argument has a true conclusion. But
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if we don't come up with a sound
reconstruction, then what do we say? Well,
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you've got to ask, whose fault is it? It
might be the fault of the argument. Maybe
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we couldn't come up with a sound
reconstruction because there just is no
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sound reconstruction. But maybe we
couldn't come up with a sound
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reconstruction because we just weren't
imaginative enough, or try hard enough.
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Still, if we try really long an hard, and
charitably interpret the a rgument as best
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we can to make it, look as good as we can,
and we still can't make it sound. Then,
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we've at least got reason to believe that
the argument's not sound. Of course, that
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doesn't mean that the conclusion's not
true, because unsound arguments can still
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have true conclusions. But at least we
know that this argument doesn't prove that
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the conclusion is true. And so, this
method of reconstruction can lead us
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either to the belief that the argument is
sound, because we found the sound
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reconstruction, or to the conclusion is
not sound, because we tried long and hard
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to find a sound reconstruction that
didn't, but that's still not going to show
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us that the conclusion of the argument is
false. The point of reconstruction then is
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to reach a conclusion on this issue of is
the argument sound or not. And if we try
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our best and do it as well as we can and
charitably, then we can be justified in
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believing that the argument is sound or
not.