A priori

Knowledge or justification that does not depend on experience, such as knowing that all bachelors are unmarried.

Also: apriori, a priori knowledge, a priori justification

What it means

A belief is justified a priori when its support does not come from observation or experiment. You can know that all bachelors are unmarried simply by understanding the words, without surveying any bachelors. Immanuel Kant made the idea central to modern philosophy and argued that some a priori truths, such as those of arithmetic, still tell us something new (the 'synthetic a priori'). Philosophers continue to disagree about what grounds such knowledge: some appeal to rational intuition, others to a default entitlement, and critics doubt that intuitions are reliable. Saul Kripke and others have proposed that some contingent truths might be known a priori, loosening the traditional link between the a priori and the necessary.

The questions it belongs to

• 2.2.2 Reason and IntuitionHow, if at all, do you know that a surface cannot be wholly red and wholly green at the same time and in the same respect?
• 2.2 Sources of KnowledgeWhich ways of finding things out actually work?

Thinkers

• Immanuel Kant
• Saul Kripke
• W. V. O. Quine
• Laurence BonJour

Traditions

• Modern European
• Analytic

To read

• An Answer to the Question: What Is Enlightenment? — Immanuel Kant
• Critique of Practical Reason — Immanuel Kant
• Critique of Pure Reason — Immanuel Kant

Sources and further reading

• A Priori Justification and Knowledge (Stanford Encyclopedia of Philosophy)
• The Analytic/Synthetic Distinction (Stanford Encyclopedia of Philosophy)
• A priori knowledge (Britannica)

Written by the Coordinates of Thought editors and checked against the sources above on 29 October 2026. Not peer-reviewed. How we write and check.

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