One of the common aphorisms we hear in two varieties is, first form:

Absence of evidence is evidence of absence

Second form:

Absence of evidence is not evidence of absence

Either statement seems to make sense on its face; but this is because of a common misunderstanding. In truth, only one form actually works for us within the bounds of reason.

What is evidence? Simple: Evidence is some bit of data or solid knowledge that tends to point towards a specific conclusion.

In the scientific sense, evidence must be consensually experiential and repeatable. If you experience something, but you cannot repeat it and demonstrate it to others, then while the experience can remain meaningful to you, it can't be shared and utilized, and so is not scientific evidence.

In the context of sharing evidence in order to advance everyone's knowledge, your experience then is anecdotal and carries (well, should carry) little or no weight outside of your own mind.

Does evidence only have to point to one conclusion? No. For instance, if I say I have a yellow fruit in my hand and show you just enough of whatever is in my hand so you can verify the color, you can use the evidence I gave you to point towards the idea that I might have a lemon in my hand. But it is also possible I might have a banana in my hand. Or a longan, or a grapefruit. Or a bar of soap.

As is then obvious, valid evidence doesn't have to point to a specific conclusion, or certainty — it serves as an assist to guide us towards right answers. Additional evidence might rule out some choices (the fruit is sweet... not a lemon), and then (never pink inside... not a grapefruit) and so on, until eventually we arrive at the truth of the matter.

Looking back, though, could the color serve as valid evidence towards the final conclusion? Yes.

With regard to the existence of God. Atheists like myself have often been known to say that absence of evidence is evidence of absence; the usual counter is "no it isn't." But let's look at this a little more carefully. Evidence is not proof. Clearly.

The sun rising and setting overhead is evidence for the sun going round us, if you're trying to figure out the answer to that particular question. It's not proof, though. And in fact, the truth is the other way around: the earth goes around the sun. It just looks very similar. The fruit being yellow is evidence that would serve as part of a proof that the fruit was a lemon. Though it wasn't, as we determined later that it was a sweet fruit.

Now keep in mind the significant difference between proof and evidence, and read on: When I say that in the case of God, absence of evidence is evidence of absence, I am not saying it proves there is no God. I am saying that it is evidence for the case, which is something quite different.

I will go further to say that there are a number of issues where this holds true, and they all point in the same direction; and that since we've been looking for such evidence for two millennia, and have come up completely dry in terms of reproducible, consensually experiential proof, that the absence of evidence is profound — it's not just a result of no one looking for it.

When you do look for something, and don't find it, this is not a result that is disqualified as evidence. For instance the question "Is the cup empty of liquid?" can be addressed with evidence by carefully looking in the cup. Nothing found? Then the cup, in fact, is empty of liquid. Century after century of looking for god and gods has likewise turned up nothing. This is, in fact, strong evidence that this particular cup is also empty.

Evidence is not proof. Evidence points the way to some part of the truth, though; and I can easily say that with no evidentiary proof of the Christian god coming to light since the inception of the religion, that one distinctly possible answer is that there is, in fact, no God. Another possibility is that there is, but Christianity significantly assigns characteristics to this God that are invalid. If the latter is the case, then Christianity itself is partially or completely invalid.

Another chunk of evidence is the failure, ever, of humans to identify any supernatural event, anywhere, that is reproducible and consensually experiential — again, absence of evidence is evidence of absence, and since the claims of God are most assuredly supernatural, here's another item that points toward the conclusion that there are no supernatural events or objects, hence the idea of God could be bankrupt from this perspective alone.

So, after all that, here is the correct idea in simple, usable form:

Absence of evidence is evidence of absence, but it is not proof of absence.

But friends, a word to the wise: If you search for evidence where it seems obvious that it should be found, and yet you never find it, you're almost certainly looking for the wrong thing — or you already have your answer.