理性主義思想家對先驗呢個術語比咗一個合適嘅立足點，例如笛卡爾同埋萊布尼茲，佢哋認為知識可以通過推理獲得，而唔係經驗，數學同埋邏輯真理嘅必然性就係佐證。笛卡爾認為關於自我嘅知識，或者話我思故我在，呢樣就係先驗，因為佢認為一個人無需訴諸過去嘅經驗就可以確認自我嘅存在。萊布尼茨區分咗先驗真理，即理性真理，同埋後驗真理，即係由經驗確立嘅真理。
Rationalist thinkers gave a proper foothold to the term a priori, such as Descartes and Leibniz, who believed that knowledge could be obtained by reasoning, not experience, as evidenced by the inevitability of mathematical and logical truth. Descartes believed that knowledge of the self, or I think therefore I am, is a priori, because he believed that one can affirm the existence of the self without resorting to past experience. Leibniz made a distinction between a priori truth, i.e., rational truth, and posterior truth, i.e., truth established by experience.