National defense is considered by many economists as a classic, public good. The primary reason this is such is that first, it is difficult to exclude people from receiving the benefits from national defense, once it has been provided. Secondly, it is not considered a rival, as each member of society is able to "enjoy" the benefits of national defense but not at the expense of others. Therefore, economists state that national defense is therefore not excludable. From a course theoretical perspective, is this correct? If so, why? If not, why not? Are there exceptions to this?