A nonnal adult possesses information stored in memory in the form of statements like 'Socrates taught Plato', 'Frenchmen eat frog legs' ~ 'my brother is taller than lam' and so forth. Some of the statements in our memory are false but most are true. In any case our ability to retrieve information from the stock of statements we believe to be true is useful to us in countless ways. A good memory is a distinct advantage in life. But just as important is our ability to reason with the information we have. We reason by using one or more of the stored statements as premises to derive another statement, a concluSion, which may not previously have been thought of but which may now be added to the store of information in our possession. Logic is the science that studies reasoning. It shows how to reason well and how to distinguish bad reasoning from good reasoning. A unit of reasoning is called an argument or inference. An argument consists of one or more premises together with the conclusion that has been drawn from them. Any argument is either valid or invalid. When an argument is valid, its conclusion is said to follow from or to be entailed by its premises.