Can a lack of evidence be considered proof?

[Asmodee]

It's an interesting question with no single answer.  It depends what you are trying to prove.  Some common sense comes in handy here.

The rain example you gave is one of the "common sense" things.  Lack of rain is proof it's not raining.  It's absolute, definitive proof.

But I have another example where lack of evidence is not proof.  I used to spend time on UFO forums where one believer in alien manufactured crop circles insisted that lack of evidence of human involvement in specific crop circles was proof that aliens did it.  And what if you find proof of human involvement in one?  Well, you throw that one out, of course.  In this case the claimant is selectively choosing from which pools you will look for evidence and attempting to prove that the samples in that pool are "real" because there is no evidence they are not.  This, of course, is ridiculous.  You throw out any data which doesn't fit your premise.  Humans fessed up to that one?  Well, that one is obviously fake, but look at this one.  And by doing that you get to ignore the multitude of examples which show active human involvement in the making of crop circles in general.  You get to ignore the fact that there is a ton of evidence to show that humans do make crop circles and focus just on the lack of proof in your favored example.  You narrow your data set down until it includes only the "undisprovable".  In this case lack of evidence doesn't mean squat.  You've artificially eliminated all the evidence which disagreed with your belief.

When it comes to God things get a little sticky there too.  The short answer is yes, I believe lack of evidence of the existence of any deity is pretty good evidence that no deity exists.  But it's hard to say that confidently because when you think of "God" there's this huge, dark cloud in your head.  You can't see it clearly.  There is so much description of what God is, yet it is still so poorly defined.  While one person may explain away one describing factor in the Bible another may add a describing factor not in the Bible.  So you have to pick out the universal attributes, those which all agree God has.  Some of the biggest are magic and interest in human lives.  The Bible is filled with magic.  God is unchanging.  So it stands to reason that the passage of time would not make God stop doing magic.  A more reasonable explanation is that magic was never real because the fantastical magical stories are made up (duh).  And we see random events every day which some attribute to God's interest in humanity (mostly smiting) and others see the exact same event as showing that there is no God with an interest in humanity.  The major earthquake in Haiti in 2010 is an example of just such an event.

So, short answer, yes, lack of evidence for is evidence against, BUT, that can be manipulated and misconstrued.  You have to be careful that you're not jumping to a conclusion as well.  For instance, lack of human involvement in crop circles means humans didn't do it, it doesn't mean a specific made-up race of aliens did do it.  When you're talking rain, I can walk outside and check your facts.  There's just the one.  But when it comes to more fantastical claim the facts are often obscured.  The "miracle" at Fatima happened a century ago.  I can't go there.  No science teams were there studying it.  There is no data of the event, only claims.  I can go only by what is still around from it.  And therein lies the trap.  Many believe the miracle happened and they believe that they were convinced by "firsthand accounts", but they really weren't.  They were convinced by a small number of people, sometimes only one person, claiming that the firsthand accounts say this or that.  It has been a hundred years.  There are no witnesses alive.  Very few firsthand accounts actually exist.  So we form our opinion essentially on what other people think and how plausible an alternative explanation given is, never mind that there is no more evidence for this alternative explanation than there is for the primary explanation, that it was magic.  But it's more plausible that a few thousand people looking into the sun fucked up their eyes than it is a 2,000 year old dead chick did some magic.

So it's highly situational, sometimes even subjective.  If the claim is simple and easy to check, absolutely lack of evidence for is evidence against.  When they test drinking water for lead they don't use a test which shows how much lead is not there.  They use a test which shows how much lead is there.  Lack of evidence for lead means there is no lead.  But people often want to make it not so simple, not so straight-forward.  They purposely confuse the issue, discard data which doesn't agree with them and use bad arguments to disguise the lack of evidence.  When that happens I would say lack of evidence for, combined with grasping at straws to prove there is no lack of evidence definitively shows not only a bias most severe, but also that the person is desperate to prove something that they, themselves, are having trouble believing.  In this case I think that's actual evidence against.  If you insist you are right, yet your reasons for believing so are so weak that you feel you must manufacture "evidence", you're trying to convince yourself, not me.  If you don't believe your claim, why should I?  This goes for pretty much every "logical proof" ever written.