President Donald Trump is threatening to take away the security clearances of a number of former senior intelligence and security officers who have been extremely critical of him. Most Americans were unaware that any ex-officials continued to hold clearances after they retired and the controversy has inevitably raised the question why that should be so. Unfortunately, there is no simple answer.

A security clearance is granted to a person but it is also linked to "need to know" in terms of what kind of information should or could be accessed, which means that when you are no longer working as Director of the Central Intelligence Agency you don't necessarily need to know anything about China's spying on the United States. Or do you? If you transition into a directorship or staff position of a major intelligence or security contractor, which many retirees do, you might need to retain the qualification for your job, which makes the clearance an essential component in the notorious revolving door whereby government officials transit to the private sector and then directly lobby their former colleagues to keep the flow of cash coming.