Scientists are sometimes said to assume that something is not the case until there is proof that it is the case  Now suppose the question arises whether a given food additive is safe  At that point, it would be neither known to be safe nor known not to be safe  By the characterization above, scientists would assume the additive not to be safe because it has not been proven safe  But they would also assume it to be safe because it has not been proven otherwise
But no scientist could assume without contradiction that a given substance is both safe and not safe; so this characterization of scientists is clearly wrong
A general statement is argued to be false by showing that it has deliberately been formulated to mislead.