To earn a Ph.D., one must accomplish two things. First, one must master a specific subject completely. Second, one must extend the body of knowledge about that subject.
P. Singer, D. Helic, A. Hotho, and M. Strohmaier. International Conference on World Wide Web, page 1003--1013. Republic and Canton of Geneva, Switzerland, International World Wide Web Conferences Steering Committee, (2015)