Neither Public nor Private
It began with collecting rubbish. Since the 1980s, local communities everywhere have discovered that they could guarantee the service without actually providing it themselves. Private companies removed household rubbish more reliably and efficiently than the public service had done before.
Suddenly there were no delays, no strikes, no bad manners. Since then the principle has been applied to many services: traffic wardens and airport security, then railway lines and flight control systems, hospital buildings and even prisons all came to be run by public-private partnerships, or "PPPs" as they are known in the United Kingdom.
The principle is simple. Government guarantees certain services but private agents provide them. In this way citizens get what they need, but more efficiently and also cheaper. Originally, this was a project of the centre-right; it was part of the privatisation wave which swept the United States and Europe in the Reagan-Thatcher years. Since then, the political left has adopted it and added its own theory.