1. A natural language processing system for processing internal conceptual expressions of a computer not in a natural language so that they are converted to a message in a natural language from an author to a reader in accordance with the relationship between the author and the reader, comprising: input means for inputting information expressed by a concept comprising an internal conceptual expression of a computer not in a natural language that is to be communicated from the author to the reader as a message embodying the concept; a knowledge base having the knowledge of natural languages, general knowledge, and the knowledge of specific domains; selecting means for selecting the expression form in a natural language of a message embodying the concept which is input from said input means by referring to the knowledge base, said selecting means determining the relationship between the author and the reader and determining the expression form of the message to reflect the relationship between the author and the reader by using data in said knowledge base, wherein said selecting means selects the expression form of the message from a plurality of possible expression forms embodying the concept in accordance with the determined relationship between the author and the reader; modulation means for modulating the concept input from said input means on the basis of the expression form selected by said selecting means; and conversion means for converting the concept modulated by said modulation means into the message embodying the concept in the expression form in the natural language selected by said selecting means.