What is now known as classical or traditional logic was first formulated by Aristotle, who developed rules for correct syllogistic reasoning. A syllogism is an argument made up of statements in one of four forms: “All A's are B's” (universal affirmative), “No A's are B's” (universal negative), “Some A's are B's” (particular affirmative), or “Some A's are not B's” (particular negative). The letters stand for common nouns, such as “dog,””four-footed animal,””living thing,” which are called the terms of the syllogism. A well-formed syllogism consists of two premises and a conclusion, each premise having one term in common with the conclusion and one in common with the other premise. In classical logic, rules are formulated by which all well-formed syllogisms are identified as valid or invalid forms of argument.
