选言三段论，也叫做拒取式（modus tollendo ponens，字面意思：通过否定来肯定）是有效的简单的论证形式：

在逻辑运算符记号中：

这里的表示逻辑断言。

粗略的，我们可以说一个或另一个是真；接着我们可以说一个不是真；那幺我们可以推导出另一个必须是真。这种推理叫做"选言三段论"，就是说，首先它是三段论--三个步骤的论证--其次它包含一个析取式，它简单的意味着一个"或"陈述。"要幺P要幺Q"是一个析取式；P和Q叫做这个陈述的离析项（disjunct）。

一个例子：

另一个例子：

In classical logic, disjunctive syllogism (historically known as modus tollendo ponens) is a valid argument form which is a syllogism having a disjunctive statement for one of its premises.

In propositional logic, disjunctive syllogism (also known as disjunction elimination, kneecapper's argument, and or elimination, or abbreviated ∨E), is a valid rule of inference. If we are told that at least one of two statements is true; and also told that it is not the former that is true; we can infer that it has to be the latter that is true. If either P or Q is true and P is false, then Q is true. The reason this is called "disjunctive syllogism" is that, first, it is a syllogism, a three-step argument, and second, it contains a logical disjunction, which simply means an "or" statement. "Either P or Q" is a disjunction; P and Q are called the statement's disjuncts. The rule makes it possible to eliminate a disjunction from a logical proof. It is the rule that: