Absence of evidence ...
In a valid deductive argument, the conclusions follow necessarily from the premises. This is a proof in the mathematical sense of the word. Provided we know that the premises are true, we can establish with complete certainty that the conclusions are true. For example, identifying a single unicorn would establish without a doubt that unicorns exist. Unfortunately much of the time this type of certainty isn't possible. Consider another example from the realm of mythology: weapons of mass destruction (WMD) in Iraq. Here's what Donald Rumsfeld had to say on the subject in 2002 (the boldface is my addition): There's another way to phrase that and that is that the absence of evidence is not evidence of absence . ... Simply because you do not have evidence that something exists does not mean that you have evidence that it doesn't exist. But surely hunting high and low for WMD month after month and not finding any ( absence of evidence ) supports the inference that there ar...