In this article the author argues that LIS has suffered from lack of theoretical foundation. Recently many experts have tried to find a theoretical basis for the subject. Nevertheless they are not satisfied yet and they have not achieved to a consensus. The author argues that it is because they stood on a wrong foot. Content analysis is the main approach to this paper. The author believes that there may be found a powerful theoretical basis for LIS through the retrieval of information and knowledge. The value of this argument is that LIS may obtain a good basis on subject, problems and principles. The author although has had a definition which is newly published (2008) now has changed his mind and proposed a new one. He argues that according to traditional teachings that every scientific discipline must bear a subject, objective and problems, with this new definition all are present and can be well justified.