One thing you probably noticed from the example on the previous page was the fact that the premise might be true (Mary is my wife) and the conclusion may also be true (Mary is the biological mother of my children), yet the inference from premise to conclusion be bad. This is hard for many people to understand, but it is true. You may give a bad reason to believe something even when all your premises are true because your inferential claim is false.
Sometimes a person makes the strongest possible inferential claim. They claim that you ought to infer their conclusion from their premises because the truth of their premises makes it impossible for their conclusion to be false. When the conclusion really does follow from the premises in this way we call the argument valid. When the conclusion does not follow the argument is invalid.It might seem that such an inferential claim could never be right. After all, how could anyone be so sure of anything? Surprisingly, such claims are often correct. if the premise "Mary is my wife" is true, then it is impossible that I am not married. For an argument to be valid, the premises do not have to be true. Nor does the conclusion have to be true.
What has to be true is this: IF the premises are true, then it is impossible for the conclusion to be false.
Arguments with false premises can be valid so long as if the premises were true, the conclusion would also have to be true. Here is an example.
All pigs can fly. Porky is a pig. Porky can fly.
Pigs don't fly, but nevertheless if they did, then we wouldn't be wrong if we inferred that Porky could fly. Of course critical thinkers aren't satisfied with arguments like this. Being valid isn't good enough. We want our arguments to have correct inferential claims and to have true premises as well. Arguments that are valid and have only true premises are called sound arguments.