The very lack of evidence

The very lack of evidence is thus treated as evidence; the absence of smoke proves that the fire is very carefully hidden. Yes - if it exists at all. But we must first prove its existence. Otherwise we are arguing like a man, who should say "If there were an invisible cat in that chair, the chair would look empty; but the chair does look empty; therefore there is an invisible cat in it."

From 'The Four Loves" -- C S Lewis