Discussion Project on A Priori versus A Posteriori

A Priori Knowledge:

• A priori knowledge is knowledge that we can have "prior to experience". We don’t need to observe how the world is to have such knowledge. We can arrive at such knowledge through reason alone (sitting in our armchairs by the fire, as it were, and simply using our powers of reasoning).
• Examples: "All bachelors are unmarried", "All triangles have three sides."

A Posteriori Knowledge:

• A posteriori knowledge is knowledge that we can have only after we have certain experiences. We have to make some observations to gain such knowledge. (We can’t just reflect from the armchair, we have to go out in the world and see how things actually are – or someone else has to do so and tell us about it.)
• Examples: "There is a cup on this table", "Smoking causes cancer."

Try to agree on whether each of the following statements is known to be true (or false) a priori or a posteriori:

1. "Every widower has lost a spouse."
2. "Most widows do not remarry."
3. "The moon is made of green cheese."
4. "7 + 5 = 12."
5. "If anything is colored it is extended."
6. "If anything is colored it is attractive."
7. "In a plane, a circle encloses a larger area than any other figure of equal perimeter."
8. "Every effect must have a cause."
9. "All U.S. citizens are mortal."
10. "Everything that is mortal is subject to death."
11. "You can fool all of the people some of the time, and some of the people all of the time, but you can't fool all of the people all of the time."
12. "If some of the people are fooled all of the time, then at no time are all of the people free of deception."

[This exercise was shamelessly stolen from the website of Ned Beach, U. of Wisconsin, Eau Claire]