对立四边形（又译逻辑方阵、四角对当）是来自亚里士多德逻辑或词项逻辑的术语，它明确说明了各种句子类型之间的逻辑关系。

对于主词"S"和谓词"P"，提供了如下规则:

只有前两个规则是亚里士多德在他的著作《解释篇》中陈述的，第三个规则是后人从他的《前分析篇》中补充进来的，第四个规则是后人从前两个规则做出的推论。发人深省的是亚里士多德在《解释篇》中对特称否定命题的陈述为“并非所有的S是P”而不是“有些S不是P”。

In the system of Aristotelian logic, the square of opposition is a diagram representing the different ways in which each of the four propositions of the system is logically related ('opposed') to each of the others. The system is also useful in the analysis of syllogistic logic, serving to identify the allowed logical conversions from one type to another.