Saturday, June 6, 2009

Negative Proof

There are two points that are often raised during debates on the existence of the afterlife.

  • Absence of evidence is not evidence of absence.
  • You can't prove a negative.

The first statement, "absence of evidence is not evidence of absence," is true. Lack of evidence of something does't prove it doesn't exist. I have no evidence that you are reading this, but you are.

The second statement is false. It is possible to prove a negative. It can be proved false under the laws of formal logic. If you don't believe this, go to the store and try to pay $5 for $10 worth of goods. When the cashier says you didn't pay enough money, just say "Sorry, you can't prove I didn't pay enough because you can't prove a negative." The cashier will prove the negative (you didn't pay enough money) by counting the money you paid and showing it is less than $10.

But what about this statement:

  • You can't prove spirits and the afterlife don't exist.

If you can prove a negative, how do you do it in this case?

Just because it is theoretically possible to prove a negative doesn't mean it is practical in a real world example. It's easy for the cashier to count the money you paid, but one spirit existing somewhere in the universe would be sufficient to prove spirits existed. How could a skeptic search the whole universe to prove there are none?

One possibility might be for the skeptic to demonstrate the proof of the assertion "some spirits exist" is invalid. He could do this by showing that all the evidence said to prove spirits exist is invalid.

But if absence of evidence is not evidence of absence, then even if a skeptic disputes all the evidence said to show that spirits exist, and demonstrates you can't justify belief in spirits, then he still has not proved spirits do not exist. So why should he go to the trouble of disputing the evidence for spirits? It still won't prove they don't exist.

Does this get the skeptic off the hook? Is he right that he doesn't have to justify his position because it is impossible to prove spirits don't exist? Can he win the debate that easily?

No. In a debate each side must prove their assertions. If a skeptic doesn't believe in spirits, he must explain why. Usually his disbelief is based on rejection of the evidence said to demonstrate the existence of spirits. Therefore in a debate, the skeptic is required to justify his position of disbelief by proving that what is asserted to be evidence for the existence of spirits is invalid. He doesn't have to prove spirits don't exist, he only has to prove there is no justification for belief in spirits.

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