假言三段论又称假言推理。假言推理总是以假言判断为前提来进行推理的。

在逻辑中，假言三段论是服从下列形式的有效的论证:

在逻辑运算符记号中

换句话说，这种论证陈述如果第一个蕴涵第二个，并且第二个蕴涵第三个，则第一个蕴涵第三个。假言三段论的一个例子:

假言三段论有一个好处，它们可以是反事实的(counterfactual): 它们可以是真的，即使前提假设的命题已知是假的。

反事实的前提的可以在有效的假言三段论中使用的例子:

In classical logic, hypothetical syllogism is a valid argument form which is a syllogism having a conditional statement for one or both of its premises.

In propositional logic, hypothetical syllogism is the name of a valid rule of inference (often abbreviated HS and sometimes also called the chain argument, chain rule, or the principle of transitivity of implication). Hypothetical syllogism is one of the rules in classical logic that is not always accepted in certain systems of non-classical logic. The rule may be stated: