Hume's Fork

Philosophers typically classify all knowledge into two distinct groups, a priori and a posteriori. The Scottish philosopher, David Hume, applied these classifications like two fronts in a war against Christianity. Hume's fork, as it is called, is an aggressive attack on the Christian belief in God. Here is how it works.

A priori knowledge is that which is true by definition. The concept is known without previous experience. Furthermore, a priori knowledge cannot be refuted. It is true always. However, a priori knowledge cannot be experienced in a real and meaningful way. What exists only in the mind as a concept does little to influence the world we live in. Definitions, geometrical concepts, and necessary conditions may reflect how we perceive the universe, but it does not determine it. That is to say that I may experience the color red in a meaningful way, say the sight of blood or a red traffic light, but the fact that red is a color is not something I experience meaningfully. This is also known as deductive reasoning.

Examples of a priori knowledge includes such tautologies as:

Red is a color.

Bachelors are unmarried.

Rectangles have four equilateral sides.

Lions are felines.

A posteriori knowledge is true by experience. An individual must witness the event first hand to determine its validity. As such, a posteriori knowledge can be proved or disproved by experimentation. If I tell you the streets are wet, you can look outside your window and confirm or deny my claim. Furthermore, personal experience is more meaningful than a tautology. Knowing whether or not the streets are wet may affect whether or not you drive, mow the lawn, wear a jacket, and so on. That I say all lions are felines means very little to you. Yet, if I say a lion is behind you, that is a more pressing matter. This is also known as inductive reasoning.

Examples of a posteriori knowledge are:

That shirt is red.

John is a bachelor.

This plate is square.

My cat is a tabby.

The problem Hume had with the Christian belief in God is that they wanted it both ways. Christians want to experience God in a real and meaningful way. However, they also want the presence of God to be irrefutable. Hume's fork claims that knowledge must be one or the other, a priori or a posteriori. God must either exist only in the mind as an irrefutable but ultimately meaningless ideal, or he must exist as something to be experienced in a very real way but also as something that can be proved experimentally not to exist.

Of course to accept either claim undermines faith in God. The Christian must accept God as no more than an abstract concept, or something that can be proven not to exist.

This assumes that knowledge can only exist as one of the two classifications. Immanuel Kant suggested otherwise as did Kierkegaard. There could very well be some form of knowledge that fell in between definition and experience. Also, there is the topic of faith - but that is perhaps a topic for another time.