Under normal conditions the act of communication requires the presence of at least two persons: one who sends and one who receives the communication. In order to communicate thoughts and feelings, there must be a conventional( 习惯的 ) system of signs or symbols which mean the same to the sender and the receiver. The means of sending communications are too numerous and varied for systematic classification; therefore, the analysis must begin with the means of receiving communication. Reception of communication is achieved by our sense. Sight, hearing, and touch play the most important roles. Smell and taste play very limited roles, for they cannot receive intellectual expression from fully developed systems of signs and symbols. An example of visual communication is gesture. Although it frequently accompanies speech, there are systems that rely solely on sight, such as those used by deaf and dumb persons. Another means of communicating visually is by signals. Feelings may be simply communicated by touch, such as by handshaking or backslapping, although a highly developed system of hand stroking has enabled blind, deaf, and dumb persons to communicate intelligently. Whistling to someone, applauding ( 鼓掌 ) in a theater, and other forms of communication by sound rely upon the ear as a receiver. The most fully developed form of auditory( 听觉的 )communication is, of course, the spoken language. The means of communication mentioned so far have two features in common: they last only a short time, and the persons involved must be relatively close to each other. Therefore, all are restricted in time and space.