A posteriori

Knowledge or justification that depends on experience, such as observation, experiment or introspection.

Also: aposteriori, empirical knowledge, a posteriori justification

What it means

A belief is justified a posteriori when its support comes from experience. That grass is green, or that bachelors in one country pay a particular tax rate, can only be known by looking at the world. Most scientific and everyday knowledge is of this kind, and the term is defined by contrast with the a priori. A notable twist came from Saul Kripke and Hilary Putnam, who argued that some necessary truths, such as 'water is H2O', can only be discovered empirically, so being necessary and being knowable without experience come apart. Timothy Williamson has gone further, questioning whether the a priori/a posteriori distinction is as significant as philosophers usually assume.

The questions it belongs to

• 2.2 Sources of KnowledgeWhich ways of finding things out actually work?
• 2.2.1 PerceptionWhen you see a tree, what are you actually seeing?

Thinkers

• Immanuel Kant
• Saul Kripke
• Hilary Putnam
• Timothy Williamson

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

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 Not peer-reviewed.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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