We cannot but admit that language is a cognitive system. That is, language exists only insofar as people who use a particular language have a set of grammatical rules for it in their heads.
However, it isn’t enough to say merely that we have grammatical rules in our heads. In order for language to be a system of communication—a system that allows us to share our thoughts with others—we have to be able to use it to transmit messages. We must be able to use those grammatical rules to produce something in the world: something that others are able to perceive and interpret. Therefore, every language must have a modality or a mode of communication.
A language’s modality tells us two things: how it is produced, and how it is perceived.

It is likely that most of the languages with which you are familiar are auditory- vocal (sometimes also called aural-oral), which means that they are perceived via hearing and produced via speech. Auditory-vocal languages include English, Russian, Portuguese, Navajo, Korean, and Swahili, among many others. Auditory- vocal languages may also be referred to as spoken languages. Throughout history, there has been a commonly held—though entirely incorrect—view that language is one and the same as speech. This misconception is often spread when the terms speech and language are used interchangeably. From this confusion, people may conclude that only spoken languages may properly be described as being languages.