The subject of investigation in linguistics is human language. Language, in the broadest sense of the word, refers to the totality of those signs of our thoughts and feelings that are accessible to external perception and that we can produce and repeat at will. This definition thus excludes from the concept of language certain aspects of our mental life that are not capable of immediate expression, as well as various reflexes that are independent of our will and are involuntary byproducts of mental states. Since spoken language, that is, language whose expressive signs are words, is of utmost importance for the intellectual development of humanity and simultaneously represents the most common and complete type of language, the term language is generally understood in this sense without further clarification.