I'm interested whether all ravens are black.  Armed with my trusty binoculars, I've traveled to the ends of the earth, viewing every raven I could catch sight of.  Every raven I've seen has been black, and I've seen a lot of ravens.  I infer that all ravens are black.  My inference is a good inference.  The conclusion I reached is one I ought to have reached.  Anyone with the same information ought to reach the same conclusion.

On the table in front of me are a bunch of cards. Some of them show a "4", some a "7", others show "A" and still others show a "D". I believe that each card has either a "4" or a "7" on one side and an "A" or a "D" on the other.  I need to determine whether the following rule is true: "If a card has an "A" on one side, then it has a "4" on the other.  I infer that I should turn over the cards which are showing an "A" and the cards which are showing a "4". My inference is a bad inference.  The conclusion I reached is one I ought not to have reached.  Anyone with the same information ought not reach the same conclusion.

An inference is a relationship between a set of propositions which we call "premises" and another proposition which we call a "conclusion."  The premises purport to provide support for the conclusion.  We've made a good inference when the premises really do support the conclusion.   How do we determine which inferences are good and which aren't?  Good inferences are truth-preserving. If we begin with premises which are true, a good inference will preserve that truth in the conclusion. That is, if our premises are true, our conclusion will also be true.

But how do we know which inferences are truth-preserving?  That's the subject matter of logic.  Logic is the theory of inference, the theory which tells us which inferences are good and which are bad, i.e. which preserve truth and which don't.

A tall order? Well read on! Or do the Wason Selection Task Exercise and learn why the inference in the card example is a bad inference.