充足理由律是逻辑学基本原理之一。

在四种基本原理：

• 同一律：事物只能是其本身。
• 排中律：对于任何事物在一定条件下的判断都要有明确的“是”或“非”，不存在中间状态。
• 矛盾律：在同一时刻，某个事物不可能在同一方面既是这样又不是这样。
• 充足理由律：（任何事物都有其存在的充足理由）。人们最容易忽略的就是充足理由律，没有充足理由，整个演绎推理的结论就不牢固。

The principle of sufficient reason states that everything must have a reason or a cause. The modern formulation of the principle is usually attributed to Gottfried Leibniz, although the idea was conceived and utilized in various philosophers who preceded him, including Anaximander, Parmenides, Archimedes, Plato and Aristotle, Cicero, Avicenna, Thomas Aquinas, Anaximander of Miletus, and Spinoza. Some philosophers have associated the principle of sufficient reason with "ex nihilo nihil fit"., Hamilton identified the laws of inference modus ponens with the "law of Sufficient Reason, or of Reason and Consequent" and modus tollens with its contrapositive expression.