Absence and evidence

"It depends on Saddam. If he co-operates with the inspectors in allowing
them not just access but telling them what material he has and allowing them
to shut it down and make Iraq safe and free of weapons of mass destruction
then the issue is over, but he is not doing that at the moment."
Tony Blair, 26 January 2003 (Source: the Guardian, 27 January 2003)

The British and American governments have consistently claimed that Iraq has
weapons of mass destruction. But they have not helped their case by rigging
the rules by which their claim is tested. Here’s the problem.

UN weapons inspectors are currently searching Iraq for these weapons. If they
find them, this is obviously evidence that Iraq has them. But if they don’t
find them, then that is only taken as evidence that Iraq is not "telling
them what material he has and allowing them to shut it down". In other
words, if they find weapons that’s proof Iraq has them and they don’t it’s proof
Iraq is hiding them. That means nothing the UN inspectors can do can be accepted
as evidence that Iraq does not have weapons of mass destruction, since only
evidence that it does is counted. Heads I win, tails you lose.

Critics of America and Britain have rightly pointed out that in order to perform
a genuine test of a theory you must permit the possibility of evidence that
would count against it. If you do not, the test cannot be genuine, because a
test that is run with the presumption that nothing could count as a failure
of the test is no real test at all.

The obvious reply to this is that the asymmetry is due to the fact that you
can’t prove a negative. Whilst finding weapons is clear proof that they are
there, not finding them is not proof that they are not. Absence of evidence,
so the saying goes, is not evidence of absence.

In fact this principle isn’t as straightforwardly true as can appear. Often
absence of evidence is evidence of absence. What greater evidence can there
be, for example, of an absence of pizzas in my freezer than the failure to find
evidence of their presence if you have a good look? In law, there is also a
presumption of innocence in the absence of evidence for guilt. It may not be
possible to prove I didn’t kill Colonel Mustard, but absence of evidence that
I did is considered good enough evidence that I didn’t.

The reason for accepting absence of evidence as positive evidence is precisely
because proving a negative is often impossible. So we have to be satisfied with
something less than absolute proof. This something less is absence of evidence
when evidence has been sought where it should be found, if it exists at all.

These difficulties with absence and evidence have led to Britain and the US
getting into a tight spot. The premise of weapons inspections is that absence
of evidence is evidence of absence, just as in a criminal inquiry, absence of
evidence for guilt is taken as evidence for innocence. But Britain and America
don’t actually believe that in this case the absence of evidence is evidence
of absence. And that is because there are just too many ways in which the weapons
being looked for could be concealed.

Ironically, the result is that both sides are claiming the inspections are
a charade. Britain and the US argue that the inspections are a charade because
Iraq is not co-operating and critics of Britain and America say it is a charade
because nothing is allowed to count as evidence of absence.

Both are right. The bad move here is the oscillation between the legal assumption
of innocence that says absence of evidence is evidence of innocence and the
actual facts in this case that suggest it is naïve to suppose absence of
evidence is evidence of innocence. The shifting and confusions between the two
has created a situation in which everyone accuses everyone else of bending the
rules to suit themselves.

That’s not a healthy situation to be in when war is a possibility and lives
are at stake. Extraordinary thought it may seem, intellectual difficulties concerning
the relationship of absence and evidence do seem to be contributing negatively
to the playing out of the current crisis.

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