From POGO:

Undermining the Special Counsel

It is a fundamental tenet of the law, as Edward Coke put it almost 400 years ago, that “no man can be a judge in his own case.” All the more so when the man is president of the United States. And that is why having some independent investigative authority to stand as a bulwark against presidential criminality is important. The special counsel provisions that authorized Robert Mueller’s investigation of Donald Trump are a response to the age-old question: Who will watch the watchmen? How do we make sure that those who are responsible for leading us are not a threat to the citizens of the very union they are bound to protect? In an independent special counsel, our nation hoped it had found the answer.
—Paul Rosenzweig, Senior Fellow, R St. Institute, and former Senior Counsel, Whitewater Investigation