The Absence of Evidence

A reader asks:

The absence of evidence is evidence of absence, can this statement be true? If we expect that something will have observable effects, should it exist? And if it doesn't have observable effects, does that mean it probably doesn't exist? The stronger our prior belief that something would, if it existed, have observable effects, the more absence of evidence is evidence of absence. But is it true only where "evidence" for something: a) hasn't been looked for in an effective way and (b) is such that, if it existed, it would not otherwise intrude upon our consciousness?

This statement is generally false. Another way to phrase this principle is: you cannot prove a negative. This was the conceptual problem when the hunt was underway in Iraq for nuclear weapons. Saddam was left with demonstrating non-existence, something he was discliend to due given his war with Iran-- a disinclination that led to the gallows. You can inductively prove only that which does exist, not that which does not exist. We cannot prove, for example, that unicorns do not exist, but in the absence of the evidence of the existence of unicorns, we cannot claim that unicorns do not exist, as they may exist but be undiscovered. Now, in the case where you have a observable domain, as in my computer table, the absence of evidence of my cat on the table is evidence of the absence of my cat on my table.