Absence of proof is not proof of absence.

This incorrect argument is often used, or accepted, because it seems at first sight to be so self-evident that we do not think about its validity. But in safety engineering we have to be concerned with hard physical realities, so we must be able to recognise incorrect statements and false arguments – like this one – even where we want to believe them because they give us a warm and fuzzy feeling that everything is alright, or because they appear to justify cost savings.