Communication is usually defined as the transmission of information. The term can also refer to the message itself, or the field of inquiry studying these transmissions, also known as communication studies. The precise definition of communication is disputed. Controversial issues are whether unintentional or failed transmissions are included and whether communication does not just transmit meaning but also create it. Models of communication aim to provide a simplified overview of its main components and their interaction. Many models include the idea that a source uses a coding system to express information in the form of a message. The source uses a channel to send the message to a receiver who has to decode it in order to understand its meaning. Channels are usually discussed in terms of the senses used to perceive the message, like hearing, sight, smell, touch, and taste. Communication can be classified based on whether information is exchanged between humans, members of other species, or non-living entities such as computers. For human communication, a central contrast is between verbal and non-verbal communication. Verbal communication involves the exchange of messages in linguistic form. This can happen through natural languages, like English or Japanese, or through artificial languages, like Esperanto. Verbal communication includes spoken and written messages as well as the use of sign language. Non-verbal communication happens without the use of a linguistic system. There are many forms of non-verbal communication, for example, using body language, body position, touch, and intonation. Another distinction is between interpersonal and intrapersonal communication. Interpersonal communication happens between distinct persons, such as greeting someone on the street or making a phone call. Intrapersonal communication, on the other hand, is communication with oneself. This can happen internally, as a form of inner dialog or daydreaming, or externally, for example, when writing down a shopping list or engaging in a monologue.