In the police force, two, three, five traitors are detected who are really working for someone else. When we cleanse the police of them, the problem will be simplified a lot. Terrorists will have no one to contact - they will be left without informers.
“There are also two kinds of truths: truth of reasoning and truths of fact. Truths of reasoning are necessary and their opposite is impossible; those of fact are contingent and their opposite is possible.”