Propositional, or symbolic language differs from emotional language in several ways. Instead of communicating feelings, it is the means of transferring ideas from one person to another, and it requires the substitution of a series of sounds or marks for objects, persons, and concepts. This is the essence of language. It is not instinctive but learned and is therefore subject to all the modifying social and cultural influences of the environment. However, the learning process becomes possible only after the nervous system has attained a certain degree of maturation. Mature language function involves the comprehension, formulation, and transmission of ideas and feelings by the use of conventionalized verbal symbols, sounds, and gestures and their sequential ordering according to accepted rules of grammar. Facility in symbolic language, which is acquired over a period of 15 to 20 years, depends on maturation of the nervous system and on education. Many attempts have been made to crystallize the essential difference between human language and that of the higher primates that are able to communicate. Such distinctions, of course, bear on the definitions of language-dependent function, such as thinking, analysis, synthesis, and creativity. Beyond simply the complexity and range of symbolic representation and grammar available to humans in comparison to animals, Chomsky has proposed that the ability to frame recursive ideas (ones that refer to themselves by embedded phrases, such as: “John’s sister’s house”) underlies creativity in human language and an infinite variety of sentences. This has been challenged but is an interesting concept.
Although speech and language are closely interwoven functions, they are not synonymous. Language refers to the production and comprehension of words whereas speech refers to the articulatory and phonetic aspects of verbal expression. A derangement of language function is always a reflection of an abnormality of the brain and, more specifically of the dominant cerebral hemisphere. A disorder of speech may have a similar origin, but not necessarily; it may be a result of abnormalities in different parts of the brain or to extracerebral mechanisms.
The profound importance of language may not be fully appreciated unless one reflects on the proportion of our time devoted to purely verbal pursuits. External speech, or exophasia, by which is meant the expression of thought by spoken or written words and the comprehension of the spoken or written words of others, is an almost continuous activity when human beings gather together. This contrasts with inner speech, or endophasia, that is, the silent processes of thought and the formulation in our minds of unuttered words on which thought depends. The latter is almost incessant during our preoccupations, as we think always with words. Thought and language are thus inseparable. In learning to think, the child talks aloud to himself and only later learns to suppress the vocalization. Even adults may mutter subconsciously when pondering a difficult proposition. As Gardiner has remarked, any abstract thought can be held in mind only by the words or mathematic symbols denoting it. It is virtually impossible to comprehend what is meant by the word religion, for example, without the controlling and limiting consciousness of the word itself. “Words have thus become an integral part of the mechanism of our thinking and remain for ourselves and for others the guardians of our thoughts” (quoted from Brain). This is the reasoning that persuaded Head, Wilson, Goldstein, and others that any comprehensive theory of language must include explanations in terms not only of cerebral anatomy and physiology but also of the psycholinguistic processes that are involved.
Disorders of speech and language may be broadly characterized under four headings; 1. Loss or impairment of the production or comprehension of spoken or written language because of an acquired lesion of the brain. This is the condition called aphasia or dysphasia.