What does 'a priori' mean in philosophy?

"A priori" is used in a philosophical sense to describe the prior assumptions upon which an argument or conclusion rests.

For example, suppose someone tells you that all cars are blue. You phone your girlfriend and during the conversation with her she tells you that she bought a new car. You say to her …" so you bought a blue car". You've made the conclusion that her car is blue based on the a priori (prior assumption, premise, or conclusion) that all cars are blue.

When people are making philosophical arguments and someone is questioning the assumptions upon which your arguments or conclusions are based, they will refer to them as a priori assumptions or premises. I would say to you that the conclusion that your girlfriends car is blue is based on the a priori assumption (or premise) that all cars are blue. I would argue that that your conclusion is only valid if the a priori premise (all cars are blue) can be proven, or is in fact a true statement. Or, I might argue that your a priori premise hasn't been proved, or proved to my satisfaction, so I don't accept your conclusion that your girlfriends car is blue.

I could think your philosophic arguments (logic) you have made are rational and correct, but I still may not agree with you because I might disagree with the a priori premises upon which your arguments are based.

A Priori Definition

This Site Might Help You.

RE:

What does 'a priori' mean in philosophy?

It is not a term that is easy to pin down, but here are some thoughts about it:

a priori means that it exists in a given way independent of human perception of it

in short, it means the attribute(s) of a given "thing" BEFORE human perception of the thing. A priori is opposed to a posteriori whereby the first means something like "inherent to" and the second means "percieved in."

These are very much oversimplifications but they should work for most philosophic issues that address the a priori vs. a posteriori distinction (like Kant).

The term "a priori" is usually defined as the reaching of a conclusion through logical deduction from general truths as opposed to reasoning from empirical observation of specific cases, but it has shades of meaning that go beyond this typical definition. One of them is made clearer by constrasting the term "a priori" with "ad hoc." Something that is ad hoc is a thing, rule, law, or argument which applies only to a specific case of something. It is a provisional explanation, action or rule that does not apply in all cases. Something that is a priori is a rule, argument, or regulating set of principles that applies in all cases. In other words, something that is a priori is seen as a general truth that is not conditioned by any one particular experience or instance of something. For example, time and space were once viewed as a priori absolutes in the universe. Every single thing that exists has a place in time as well as in space.

A definition is here: http://www.iep.utm.edu/a/apriori.htm