Determining an inference’s or conclusion’s validity or invalidity from two or more statements or premises. Classical or traditional system based on the theory of syllogism. Propose by the Greek philosopher Aristotle (384-322 BC) as a systematic approach in his book ‘Organon’. What is stated, the content, is not the focus of this system. The argument’s structure and form and the inference’s truth based on the premises of the argument are what matter. If the premises are true then the inference, or logical consequence, must also be true. Formal logic has basic principles: (1) Principle of identity: A true premise is true. (2) Principle of excluded middle: a premise is either true or false. (3) Principle of contradiction: no statement is true and false at the same time. Known also as Aristotelian logic. Refer to fuzzy logic and symbolic logic.