a priori

Think of the difference between knowing the two following facts:

1. 2 + 2 = 4

2. The sun is big and yellow.

You know both to be true - they are both knowledge. The difference between them is the diference between a priori and a posteriori knowledge...

2 + 2 = 4 can be said to be true without reference to the outside world. I can know that 2 + 2 = 4 without having to see 2 apples and 2 oranges in my fruit bowl totalling 4 fruit. This is a priori - you know it's true without having to see it in the world - ie, you know it without prior experience.

You only know that the sun is big and yellow because you have seen it (that it's yellow), and that you've been told that it's big. If you had never seen the sun, you'd not know that it was big or yellow. For you to know its size and color, you have to refer to the world. This makes it a posteriori - you only know it based on experience.

Why it this important? Primarily in science: when performing research, clear distinctions need to be made between the two types of knowledge. All a priori knowledge (eg, maths) can be taken for granted, whilst all a posteriori knowledge (eg, observation) must be proved.

You may feel the disinction is simple, but consider medical ethics. Perhaps abortion - some would consider abortion wrong a priori (eg, it is wrong because God says so) , some might consider it acceptable a priori, whilst wrong a posteriori (eg, it is wrong because it is painful to the inoccent foetus). In this case, our belief (that abortion is wrong) is the same, but with the fundamental a priori, a posteriori disinction.

Kant on the distinction between a priori and a posteriori

Kant's clearest formulation of the a priori/a posteriori distinction is found in the following passage:

We shall understand by a priori knowledge, not knowledge independent of this or that experience, but knowledge absolutely independent of all experience. Opposed to it is empirical knowledge, which is knowledge possible only a posteriori, that is, through experience. (Critique of Pure Reason, B2).

This formulation is clear, but still problematic. Kant presents us with a simple either/or. Either a statement can be known absolutely independent of experience or a statement cannot be known absolutely independent of all experience. The apriority of any statement then depends upon whether or not the statement can be known absolutely independent of experience, which is different from whether or not the statement is known absolutely independent of experience. For example, assume, as Kant holds, that "Every alteration must have a cause" is an a priori statement. It is still possible to know this statement a posteriori. For example, perhaps this statement is known by an appeal to authority; many propositions of mathematics are known this way if we accept that a calculator is an authority on mathematics. Or, perhaps causality is known psycho-socially (as Hume might have).

Kant's distinction between a priori knowledge and a posteriori knowledge is clear and straightforward -- a statement P is either n or it is not-n. Difficulties with this distinction arise, however, in the application of these terms to actual statements (as one can see if they ever sit on in an Intro to Philosophy course). It is not always clear if a statement can be known without experience, especially if the statement in question is already known through experience or speculated to be known through custom or some other psycho-social means.

A` pri*o"ri (#). [L. a (ab) + prior former.]

1. Logic

Characterizing that kind of reasoning which deduces consequences from definitions formed, or principles assumed, or which infers effects from causes previously known; deductive or deductively. The reverse of a posteriori.

3. Philos.

Applied to knowledge and conceptions assumed, or presupposed, as prior to experience, in order to make experience rational or possible.

A priori, that is, form these necessities of the mind or forms of thinking, which, though first revealed to us by experience, must yet have preexisted in order to make experience possible. Coleridge.

© Webster 1913.