二元对立是在批判理论上，一对相反的理论，它们通常会以以阶级形式出现。在结构主义理论中，二元对立论，是解释人类基层思想，文化与语言的一种相当有力的工具。相反，后结构主义者认为二元对立并不是人类思想最基本的组织，只是西方思想的加工品。

二元对立最经典的例子，是理性（rational）与感性（emotional）的二分，而在西方哲学中，理性一向比感性获得更高的评价。另一个例子，是存在（presence）与缺少（absence）的二分，同样地，前者在西方哲学中的地位远高于后者。这些「高等理念」的相似性，例如前述的理性、存在，和很多其他的如男性（与女性对立）、说话能力（speech）（与写作能力（writing）对立），被视为一种称为逻辑中心主义的西方哲学思想的影响。

A binary opposition (also binary system) is a pair of related terms or concepts that are opposite in meaning. Binary opposition is the system by which, in language and thought, two theoretical opposites are strictly defined and set off against one another. It is the contrast between two mutually exclusive terms, such as on and off, up and down, left and right. Binary opposition is an important concept of structuralism, which sees such distinctions as fundamental to all language and thought. In structuralism, a binary opposition is seen as a fundamental organizer of human philosophy, culture, and language.