Does the absence of evidence mean evidence of absence?
Let us consider the problem of the existence of God.
There are basically three possibilities which might be nuanced by probabilities:
- I know God exists (Theism)
- I know God does not exist (Atheism)
- I don’t know if God exists or not (Agnosticism)
For many people today, if we have neither evidence for nor against God’s existence, we should not only reject 1), but also 3) and be atheists.
Quite a few folks would justify that by saying that the absence of evidence is evidence of absence (a principle which will be referred to as PA).
Flying spaghetti monsters and invisible pink unicorns
They often illustrate that by quoting the infamous pink invisible unicorn (which might be lying on the ground besides you!)
Although it is very seldom well articulated, the reasoning seems to look as follows:
- it is certain that the pink invisible unicorn doesn’t exist
- if it is certain, it has to have a justification
- PA is the only possible justification
- therefore PA must be true.
This is the only way I can make sense of the manner Skeptics use such kinds of prowling monsters in public debates.
The first thing which strikes me is that it is completely absurd and hopelessly circular.
We don’t know if PA is true and want to prove it. Now we want to base our proof of PA on our certainty that there is no pink invisible unicorn. But we can only know there is no such beast if PA is true!
But PA faces a far more serious problem: in many situations it leads to quite absurd results…
Let us consider for example that I’ve invented a time-travel machine and fly with it to the ancient Greece.
I meet there an Epicurean philosopher who fervently believes in PA. During the course of our discussion, I explain to him in great details how a kangaroo looks like.
Amused, he glances at me and tells me: “since I have no evidence such a creature exists, I can be almost certain it is not real.“
Would he be justified to held this belief?
Back to the present time: I have no evidence there is a bear-like intelligent being scratching his head at the boundary of the milky way, can I conclude there is no such being?
The absence of evidence is only evidence of absence if one would expect such evidence to be out there.
But once we’ve rejected PA, what are we to do with our best invisible friend and her single pink horn?
The ground for our disbelief shouldn’t be PA, but the self-contradictory nature of the proposition.
I’m completely open to the existence of a pink unicorn somewhere in the multiverse, or of a creature invisible for our eyes, but not of a being having both features.