A posteriori knowledge comes from experience. It is knowledge after the fact. You know the sky is blue because you have seen it. You know fire is hot because you have felt it. The knowledge depends on sensory data. Without experience, you would not have it.

The term is Latin for from the latter. It contrasts with a priori knowledge, which is independent of experience. A posteriori knowledge is empirical. It is based on observation, experiment, and evidence. Science is largely a posteriori. It tests hypotheses against the world. The results are known only after the experiment.

Examples of a posteriori knowledge

• The boiling point of water is 100 degrees Celsius at sea level.
• Paris is the capital of France.
• Some swans are black.
• Pain feels unpleasant.
• The law of gravity holds on Earth.

A posteriori knowledge is contingent. It could be otherwise. The boiling point of water could be different on another planet. Paris could have been a different city. Black swans exist, but they might not have. A posteriori truths are discovered, not deduced. They require investigation. They are always open to revision. A new observation can overturn a posteriori knowledge. That is the strength and the weakness of empiricism. It is grounded in reality, but it is never certain. The next experiment might change everything. A posteriori knowledge is the knowledge of the world as it is, not as it must be.