H.P. Grice, an American language philosopher, in 1967 who proposed the cooperative principle of speech communication at Harvard University, which is a major breakthrough in the field of pragmatics. But sometimes people deliberately against to the principle of cooperation in the process of language communication. In other words, people often do not use direct way to express words in verbal communication, always use the indirect way of diversion, only to let the other party equivocate, not to talk to each other frankly. To explain the linguistic phenomenon, the later scholars, British linguist Geoffrey Leech etc proposed politeness principle, which is tonic with Grice's cooperative principle, to supple and perfect the co-operation principle. Geoffrey Leech thinks that the speaker and the obedient person should try to say politely, and minimize impolite expression in all verbal communications. 

1983, in a book which called "pragmatic principles", Leech (1983) systematically put forward the "Politeness Principle" theory. According to Leech's design, "Politeness Principle" should be under the framework of 6 principles, tact, generosity, appreciation, modesty, agreement and sympathy. The so-called "tact" refers to the speaker's way of way to achieve his goal in verbal communication. Leech thinks that this rule is first important, and it is a common rule to guide people to succeed. The concrete expression of "tact" is to reduce cost to other or to give more benefits to other. "Generous" which means to be generous to others, the specific approach is: the speaker should abide by the "own little fight for interests" and "give yourself more trouble" sub standard. Leech thinks this is the "Politeness Principle" of the second criteria of performance. When others represent or stand for something or a point of view, you do not say anything against each other. This is the appreciation, but also people strive to abide by a principle in the conversation. Leech lists the basic performance of this rule, which is, "said the praise to others", put another way, "said the praise to others as much as possibly." You should keep the modesty when you talk with other people, which is a successful dialogue guideline. The approach is to minimize advantages of self instead of maximizing disadvantages of self. The meaning of agreement principle is to speak less disagreement between self and other, or to maximize agreement between self and other, and this session will be able to continue. In the end, the successful communication should follow a principle of sympathy. The hearer tries to reduce the indifference of dialogue, or should try to express the sympathy for what the speaker says, making the dialogue between the two sides reach a tacit understanding. Researching carefully the relationship between these 6 criteria, you will find that they have some overlap with each other. These six principles belong to the principles of pragmatics, but they also have a strong guiding role in the writing of letters in foreign trade practice. Polite business English letters attach importance to each other's point of view, respect for each other's rights, aspirations and needs, meet the other's psychological, and can produce a pleasant feeling, thus contribute to and expand foreign trade. Therefore, we should pay attention to the adhibition of politeness principle in Business English letter writing.