We an have an argument whose premises are true and whose conclusions are true, but which nonetheless is an awful argument:
- All men are mortal
- George Bush is mortal
- Therefore George Bush is a man.
This argument might possible sound good. It's premises are certainly true. It's conclusion is also true. Is it a good argument? Well, no, not really. Here's why:
- All blue bottomed baboons are mortal.
- George Bush is mortal.
- Therefore George Bush is a blue bottomed baboon.
Now this is obviously not true. It shows that even though the premises of the first argument were true and the conclusion was true, the conclusion wasn't true because the premises were true. If it was, then this last argument would also have had a true conclusion.
Because of this problem, we need to know whether a form of argument, or of type of argument, can be good or bad. In other words we need to be able to separate out whether an argument is good from whether it's conclusion and premises are true.
So this means that being true and being a good argument are different things. Consider the following argument.
- Bob's a Blarg or a Varg
- Bob's not a Blarg
- Therefore Bob's a Varg.
Now Blargs and Vargs don't exist, so it's not possible for Bob to be either a Blarg or a Varg. Nonetheless, we can see that this kind of argument, or deduction is a good type of deduction. If there were Blargs or Vargs, and Bob was one and he wasn't a Blarg, then he would indeed be a Varg. So we can see a property of this argument, which is that given true premises its conclusion is true. This doesn't mean that Bob actually is a Varg, however, because Vargs don't exist outside of my imagination. This property of being a good argument is called 'validity'. As we have seen it is not the same as being true.
So now we face a different problem when we see an argument. We may know that if the argument's valid then if the assumptions in the argument are true, the conclusion will be true. However, obviously this means that having a valid argument isn't good enough to guarantee a true conclusion. We need the further property of the assumptions of an argument being true to guarantee a true conclusion. The conjunction of these properties is soundness.
Notice that the premises in argument about the blue-bottomed baboon were true, but the argument wasn't valid. Similarly, the argument about Bob was valid; but it didn't have true premises. In each case we do not have a reliable argument in terms of being able to trust the conclusion because of the argument. For that we need both of the distinct qualities of having true premises and being valid - the quality of being sound!