What do a priori and a posteriori mean?

Question

What do a priori and a posteriori mean?

Answer

These are both Latin phrases that are used to describe different types of justification for assertions.

The phrase a priori literally translates to ‘from the former’, and is used to indicate statements whose truth is derived from their very definition or logical construction. They must necessarily be true by virtue of the terms that we, as speakers, have collectively agreed upon. A commonly cited example of a proposition with an a priori justification is the following:

• All bachelors are unmarried males.

This statement is necessarily true by virtue of the meaning of the term ‘bachelor’. This justification will also carry over to statements that are derived from it.

In contrast, a posteriori is used to indicate statements which are justified by reference to some other evidence. For example:

• Most bachelors wake up late.

I may be justified in making an assertion like the one above, but this will depend on some fact that is external to the statement itself (whether most bachelors actually do wake up late). In order to demonstrate that I am justified in making this assertion, I may need to gather evidence.

References