The sum of the internal angles of a triangle is equal to 180, and it can be concluded that the sum of the two internal angles of a triangle is less than 180. This is called deductive argument, and the conclusion 100% is correct.

For example, birds can fly. This can only be said to be a general situation and does not rule out accidents. This is induction, which classifies the phenomena you observe.

Deductive argument (see deductive reasoning) is two basic methods of argument (deductive argument &; An argument method from general to individual in inductive argument. It deduces conclusions about individual situations from general principles, and the relationship between its premise and conclusions is necessary.

There are many forms of deduction, such as syllogism, hypothetical reasoning and selective reasoning, but the most important one is syllogism. Syllogism consists of three parts: major premise, minor premise and conclusion. Such as the major premise that all metals can conduct electricity, iron is the minor premise of metals, and iron can conduct electricity.

Direct reasoning is the basis of reasoning, and syllogism is the core of reasoning. Syllogism is indirect reasoning that leads to a conclusion from two premises, and its main forms are blunt syllogism, hypothetical syllogism and selective syllogism.

(1) truth syllogism

The blunt syllogism is a kind of reasoning based on two blunt statements with the same item, and then deduces another blunt statement to make a conclusion.

The structure of blunt syllogism is determined by two factors: one is the position of the middle term; The other is the statement form that constitutes the premise and conclusion.

(2) Hypothetical syllogism

Hypothetical syllogism contains hypothetical statements, which is called hypothetical reasoning. Hypothetical syllogism based on sufficient conditional hypothetical statements is the main form of hypothetical reasoning, which is based on other hypothetical statements.

(3) Selective syllogism

Inference that contains alternative statements in the premise is called alternative reasoning. There are two main forms of selective reasoning: one is selective syllogism with selective statement as the major premise and outspoken statement as the minor premise; The other is the dilemma reasoning with incompatible alternative statements as the major premise and sufficient conditional hypothesis statements as the minor premise.

Inductive argument is two basic methods of argument (deductive argument &; An argument method from individual to general in inductive argument. It draws a general conclusion through many individual examples or arguments, and then summarizes their characteristics.

Induction can give examples before drawing a conclusion, or it can put forward a conclusion and prove it with examples. The former is what we usually call induction, and the latter is what we call example. Example method is an argument method to prove the argument with individual and typical concrete examples. Also called factual argument.

Induction and deduction, as two basic forms of argument, the biggest difference lies in the authenticity of the conclusion. As long as the premise is established and the deduction is reasonable, the conclusion is true. Induction is not necessarily, induction cannot guarantee that the premise is true, and the conclusion must be true.