A Priori Proposition - A Posteriori Proposition

• a proposition whose justification does not rely upon experience
• the proposition can be validated by experience, but is not grounded in experience. Therefore, it is logically necessary
• necessary conclusions from first premises

• a proposition whose justification does rely upon experience
• the proposition is validated by, and grounded in, experience. Therefore, it is logically contingent
• conclusions based on sense observation

• “All bachelors are unmarried”
• “7 + 5 = 12”

The justification of these propositions does not depend upon experience: one need not consult experience to determine whether all bachelors are unmarried, nor whether 7 + 5 = 12. (Of course, experience is required to understand the concepts “bachelor”, “unmarried”, “7”, ”+” and so forth. However, the a priori / a posteriori distinction as employed here refers not to the origins of the concepts but to the justification of the propositions. Once we have the concepts, experience is no longer necessary.)

• “All bachelors are unhappy”
• “Tables exist”

Both of these propositions are a posteriori: any justification of them would require one’s experience.