非形式逻辑是对自然语言论证的研究，典型特征是不如形式逻辑善于分析。非形式逻辑的焦点在于分析错误的论证来辨别逻辑谬论，和辨别与分类类似的推理策略等活动。用自然语言分析论证的活动和在形式逻辑系统中表现它们被称为哲学分析，而且有时被当作非形式逻辑的一部分。

报纸的评论文章提供了对非形式逻辑的示例性教科书样的例子(参见 Walton reference)，这些文章通常是简短的并且经常是荒谬的。无论如何，非形式逻辑还用于推理有关人类和社会科学的事件。事实上，多数从已知事实到未知事实的推理使用自然语言，即使组合上数学或统计，可以被当作非形式逻辑的应用，只要它不依赖于额外的经验性论据。

Informal logic, intuitively, refers to the principles of logic and logical thought outside of a formal setting. However, perhaps because of the "informal" in the title, the precise definition of "informal logic" is a matter of some dispute. Ralph H. Johnson and J. Anthony Blair define informal logic as "a branch of logic whose task is to develop non-formal standards, criteria, procedures for the analysis, interpretation, evaluation, criticism and construction of argumentation." This definition reflects what had been implicit in their practice and what others were doing in their informal logic texts.