At the basis of human activities there exist concepts， their connections， and their activations， which are best represented and drove by national language expressions. Therefore， the construction of AI systems which involve natural language processing deeply is one of the most important topics in artificial intelligence research. .This paper discusses what kind of knowledge is used in which way and why such usage is necessary in the application systems， such as a dialogue system， a next generation information retrieval system， and a machine translation system， in order to give real satisfaction to all the customers. ..A next generation information retrieval system should provide the customers abstracted or distilled summaries for the clusters obtained from the huge amount of retrieved texts， and also give some additional information， such as how reliable these summaries are， whether there are contradictory information in the retrieved texts， whether the summaries/retrieved texts are true， false or doubtful in reference to the solid/established knowledge， etc..Finally a future direction will be suggested about the next generation AI systems， such that a system should have a self-enhancing capability or self learning ability. For example， natural language systems should have the ability of converting incoming information into knowledge and enhancing knowledge automatically or semi-automatically. ...
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