INTRO Soundness and validity


okay so soundness and validity. In philosophy what we’re trying to do is give good reasons
for believing stuff so to give somebody a good reason for believing something
you got to give them an argument. Now when I say argument or when
philosophers talk about arguments they don’t have to be things that involve
people being angry or yelling or screaming or anything like that. An
argument has a very special and specific definition. An argument is just a bunch
of sentences. It’s a bunch of sentences where one of those sentences
is the conclusion which is usually marked off by some word like thus,
therefore, hence, in conclusion blah blah blah blah blah. Whereas the other sentences are premises. Now sentences, premises, and
conclusions can be true or false. But arguments can’t be true or false. Truth
and falsity are words we use when we’re talking about premises, not about
arguments. So an argument is a good argument when it forces
you to believe the conclusion –when it’s sound. That just means that you if
you can’t find something wrong with the premises, then you are forced to
believe the conclusion. Now in order for an argument to be sound it actually has
to have two things. First it has to be valid. For an argument to be valid
that just means that if the premises are true the conclusion can’t be false. So
it’s the idea that the argument is like set up so that the premises can force you to believe the conclusion.
If they’re true, then you’ve got to believe the conclusion. But that doesn’t say
anything about whether or not the premises actually are true. So for
example any argument with the format: all x’s are y, a is an X, therefore a is y. Any argument with
that format is a valid argument so the argument all cats are
fish; Hobbes is a cat; therefore Hobbes is a fish is a perfectly valid argument.
But it’s unsound because the first premise ‘all cats are fish’ is false. So
validity has nothing to do with whether the premises are actually true or false. It just is that if the premises are true then
you’re forced to believe the conclusion so therefore soundness is just the
argument being valid and the premises actually being true. So when we talk
about an argument being a good argument we mean the argument being a sound
argument and that’s just to say that the argument is both valid and the premises
are all true. So correspondingly when we try to figure out whether or not
an argument is a good argument we have two questions that we’re going to ask.
First up we’re going to ask whether or not the argument is valid. That means
regardless of whether or not the premises are true or false. They could be
all true but if the argument but if the argument isn’t
valid then we don’t know anything about the conclusion even though the premises
are all true. So first up we have to check whether or not the argument is
valid. Second we then look at each premise and try to decide whether or not
it’s true. If we decide the argument is valid and that all of the premises are
true, then we have to believe the conclusion that means we’ve got
ourselves a good argument

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