>> We've looked at a couple of examples of deductive arguments.
We've seen one example of a sound deductive argument.
That proves a conclusion that's interesting, a conclusion that we wouldn't
of heretofore have expected. We've also looked at an example of a
deductive fallacy. Now, I'd like to consider a deductive
argument that's also fallacious, but the fallacy in the deductive argument can be
spotted if we remember our Week five lessons about categorical logic.
The deductive argument I have in mind is, All Babies Are Evil.
By Cameron Smith. Here's how Cameron's argument goes.
Premise one. Humans should not cause discomfort to
other people. Premise two.
Causing discomfort to other people is selfish and cruel.
Premise three. Ignorance is no excuse for being selfish
and cruel. Premise four, humans that are selfish and
cruel are evil. Premise five, all people who live with a
baby are caused to have sleep deprivation by that baby making noise.
Premise six, making noise as other people try to sleep, causes discomfort.
Premise seven. All those who live with a baby complain of
the mess caused by that baby. Premise eight.
Babies never clear up any of the mess that they make.
Premise nine. Humans who do not clear up any of the mess
that they make cause discomfort to other people.
Premise ten. Babies are human.
And so, the conclusion of those ten premises is babies are evil.
Now, at first glance, that argument might appear valid and the ten premises of the
argument might appear true and so, the argument appears to be sound, it appears
to be a valid argument. With entirely true premises.
But I don't think the conclusion is true. So I think either the argument is not
valid, or one or more of the premises is not true.
But which one? Well, I think we can see which one.
If we specify which quantifier is being used in the premises that make a general
statement about humans, are we using the quantifier all or are we using the
quantifier most? If the statements about humans are
understood as statments that use the quantifier all, then we can use Venn
diagrams to show that this argument is actually valid.
This argument is valid if the quantifier that's being used in all the premises
about humans is the quantifier all. But, if the quantifier that's being used
in the premises about humans is the quantifier most, then Venn diagrams don't
demonstrate the validity of this argument. And in fact the argument is not valid.
So whether the argument is valid or not depends on precisely which quantifier is
being used in those premises about humans. Is that the quantifier all or the
quantifier most? If it's the quantifier all, then the
argument is valid. Unfortunately, the premises are not all
true, and they're not all true because it's not entirely true that all humans Who
cause discomfort or pain, are evil. But, if the argument uses the quantifier
most, then while the premises are true, the argument isn't valid.
And Venn diagrams don't help us to show that it's valid.
Because Venn diagrams don't represent the quantifier most, as we saw in Week five.
So either the argument is valid, but not all of its premises are true, or the
argument has entirely true premises, bu it's not valid.
Either way the argument is not sound and so it does not succeed in proving that all
babies are evil. Despite the enormous plausibility of the
argument.