Interfaith Group

I'd say the problem is it doesn't mention the expectation (or probability) of evidence, given existence (non-absence).

Say the question is whether there is a unicorn in the room with me. Putting aside modern-physics hocus-pocus about parallel universes and 11 (12,13?) dimensions, the expectation of evidence if a unicorn was here is very high (as I can see, hear, and smell, and the room is not very big!). I can say with 100% certainty that is a unicorn where here, I would have evidence. So absence of evidence provides strong evidence of absence.

Now say the question is whether there is a unicorn on earth. Generally, we think that if this were true, we would have evidence. But we are not 100% certain - we can imagine there is a slight chance there is some very remote area where unicorns live, and no-one has seen it or reported it.

When it comes to God and Alien Life, our expectation of evidence might be much lower, using theological or scientific arguments. In this case, the statement "absence of evidence is not evidence of absence" is a valid argument, but might be very weak.

For the mathematically inclined, I think this can be shown using Bayes theorem.