Any argument where IF the premises were true, then the conclusion
must be true.
We say an argument is "valid" if
and only if it is a deductive form. The truth (or falsity) of the
premises is irrelevant.
It is possible for an argument to be valid and untrue at the same
time. Let's look at the following argument to illustrate: "If
it is raining, then it is cloudy. It is raining. Therefore, it
is cloudy." Look outside. It may not be raining right now.
But the argument is still a valid deductive argument, because IF
the premises WERE true, then the conclusion would have to be true.
In other words, there is no possible way for the premises to be
true and the conclusion to be false. If it's not raining right
now, pretend. Can you imagine it raining without at least a small
cloud? No, that's not possible. So we know that the argument is
valid.
Invalid arguments are all those which are not valid. That is to
say, all inductive arguments are (deductively) invalid.
