hello I'm Paul honey and I'm a
philosophy graduate student at Duke
University and in this video I'm going
to introduce you to formal and informal
fallacies first consider the following
arguments premise 1 if someone is
allergic to peanuts then she doesn't eat
peanut butter premise 2 Jane doesn't eat
peanut butter conclusion therefore Jane
is allergic to peanuts consider this
other argument premise 1 a feather is
light premise 2 what's light cannot be
dark conclusion therefore a feather
cannot be dark these arguments probably
seemed a bit off to you and you're right
each argument contains a fallacy though
each has different kind a fallacy as
we've seen in other wireless philosophy
videos is a defect in reasoning this
defect can be unintentional or
intentional that is I could simply make
a mistake in my reasoning or I could
want to trick you into believing
something by using deceptive reasoning
either way it's probably a good idea
that we were able to understand and
identify distinct fallacy types so that
we can adequately correct any defects in
our reasoning but before we talk about
fallacy types let's recall what a valid
argument is consider this argument
premise 1 if someone is a philosopher
and she publishes articles in philosophy
Tamar Gendler is a philosopher therefore
Tamar Gendler publishes articles in
velocity this is a valid argument
actually although it has no bearing on
this arguments validity Jen lawyers
published over 20 articles among her
other works this form of argument is
called modus ponens and it can be
represented as the following premise 1
if X then Y premise 2 X inclusion
therefore Y and this is a valid argument
form the conclusion necessarily follows
from the premises remember though this
valid form
does not mean that the premises are true
it would still be a valid argument if
Tamar Gendler had decided to become a
train conductor for instance instead of
a philosopher see the videos on
soundness and validity if you have
questions about this we can also have an
invalid argument form let's return to
our first argument this argument as you
might have suspected contains a fallacy
a formal fallacy and a formal fallacy is
exactly what it sounds like a defect in
the form of the argument in other words
there are certain forms of arguments
that are invalid this invalid argument
form is a formal fallacy called
affirming the consequent this fallacy is
apparent if you consider that Jane just
might not like peanut butter so simply
because Jane isn't like peanut butter
she doesn't eat it given just the fact
that she doesn't eat it and the
conditional on premise 1 we cannot
conclude that Jane is allergic to peanut
butter this form the fallacy can be
represented it in the following way if X
then Y Y therefore X and this form a
fallacy is one that you don't want to
use this is actually one of the
fallacies frequently tested on the LSAT
so if you are interested in one more
practice in identifying this fallacy
check out the video on affirming the
consequent we will talk more about this
and other formal fallacies and upcoming
videos but for now we need to see that
formal fallacies are fallacies because
of the poor form of the argument any
argument of these forms will be invalid
so anything can be plugged into these
bad argument forms and the argument
would be about for instance if meow then
Splatt Splatt
therefore me out is invalid so now that
we have a general understanding of
formal fallacies what about informal
ones let's look at our other example
from the beginning of this video notice
this fallacy arises out of the content
of the argument not out of the form the
form may at first glance seem valid that
is if we ignored the meaning of the
content consider that we can use a
seemingly similar argument form to make
a valid argument for instance premise 1
rain is wet premise 2 what sweat cannot
be dry conclusion therefore rain cannot
be dry and here we have a valid argument
that seems to have a similar form as a
fallacious one and one that lacks
defects in its content but the content
of our fallacious argument given the two
meanings of light light weight and light
color yields some problems that is if we
understand the meaning of the terms the
two meanings like for instance we
realize that the argument actually has
an invalid argument form initially
without considering the meaning of the
content it looks like the form of the
argument is something like all x ry what
is y cannot be z therefore no x RZ but
actually given the shifting meaning of
light the form is more like all X or Y
what is W cannot be Z therefore no x RZ
this fallacy is called Co dication and
see the video on this if you'd like to
know more about it the only way we can
understand this defect is by examining
the content of the argument if we just
looked at the form without understanding
the content then we could not detect
this fallacy so as we now know informal
fallacies occur because of problems with
the content of the arguments so an
argument might have a seemingly valid
form but committin informal fallacy
because of a defect in its content hence
an argument might have true premises in
a seemingly valid form yet also commit a
fallacy to reiterate a formal fallacy
means that the
argument has a defect in its form while
an informal fallacy has a defect in the
arguments content which might also yield
a defect in its form so now that's the
difference between formal and informal
fallacies
you