1. Introduction

Natural Phonology was a theory based on the publications of its proponent David Stampe in 1969 and (more explicitly) in 1979. In this view, phonology is based on a set of universal phonological processes which interact with one another; which ones are active and which are suppressed are language-specific. Rather than acting on segments, phonological processes act on distinctive features within prosodic groups. Prosodic groups can be as small as a part of a syllable or as large as an entire utterance. Phonological processes are unordered with respect to each other and apply simultaneously (though the output of one process may be the input to another). The second-most prominent Natural Phonologist is Stampe's wife, Patricia Donegan; there are many Natural Phonologists in Europe, though also a few others in the U.S., such as Geoffrey Pullum. The principles of Natural Phonology were extended to morphology by Wolfgang U. Dressler, who founded Natural Morphology.

2. Autonomy

For the approach favoring direct technological implementations postulated by Gibbon, it seems necessary to assume the autonomy of language because viewing language as a closed, self-contained structure helps in listing and formalizing of the elements involved. Language can, however, be viewed as an autonomous or non- autonomous cognitive faculty. The generativists assume that language is an innate, autonomous faculty, independent of other non-linguistic cognitive abilities. The functionalists are more likely to think that language is a non-autonomous
faculty and they assume that the human ability to use a language is fundamentally not different from other cognitive abilities humans have.
Natural Phonology is a functional theory relating language to other domains of human life. According to the hypothesis that language is not an autonomous cognitive faculty has implications for the representation of linguistic knowledge and processes governing language use. The corollary related to linguistic knowledge is that knowledge of meaning and form on all levels of the language system, i.e. phonology, morphology, syntax and semantics, is conceptual. Although sounds and utterances are physical entities with a formal structure, they must be produced and comprehended. Comprehending and producing speech is possible thanks to cognitive processes, which accept speech sounds as input and produce utterances as output. The corollary related to processes governing language is that construction and communication of meaning by language are governed by the same principles as other cognitive abilities. Speaking and comprehending language involves essentially the same cognitive abilities that humans use in visual perception, reasoning or motor control. The component cognitive skills used for language processing are used for other cognitive tasks, too, and only the configuration of cognitive abilities used for comprehending and producing language is unique. This is not to deny an innate human language faculty. What functionalists deny is an autonomous innate human capacity for language. Functionalists acknowledge an innate component that gives rise to linguistic abilities unique to humans, but the emphasis of functionalist research is on the role of general cognitive abilities in language.
The hypothesis that language is not an autonomous cognitive capacity has an implication for the functionalist approach to language research in the sense of holistic thinking: an adequate model of language has to involve reference to general conceptual structures and cognitive abilities.
Natural Phonology has always emphasized the role of external factors, such as cognitive abilities and physiological predispositions, in phonological processes. It has, however, undergone evolution of its views from postulating complete innateness of natural phonological processes to accepting that processes arise at different stages of development
as a result of self-organization.

3. Explanation in Natural Phonology.

Natural Phonology uses “deductive inferences about grammars based on universal higher-order principles applicable to language as well as to other natural phenomena. Natural
Linguistic preferences are explanatory because they are based on non-linguistic, external evidence.“[W]e can arrive at explanations for the regularities within a certain domain by turning to theories that are not theories for that particular domain (e.g., for grammatical theories, these include: theories of phonetic production, perception, learning, memory, communication, action, semiotic theories, etc.)”
Natural Linguistics proposes a "hierarchic, deductive system withinwhich linguistic preferences occupy a general second rank, below higher principles and above the specific linguistic consequences of preferences"
Higher principles can be cognitive, phonetic, psychological, sociological etc., they are non-linguistic principles, like for example the principle of the least effort. Linguistic preferences include for instance a preference for simple phonotactics, for a CV structure. A linguistic consequence of such a preference is then the absence of clusters in a given language. In a conflict situation between preferences "agents strive towards maximal benefit or expected utility". Solutions for conflicts between preferences are to be found in higher-order universal principles and not in language-internal properties, because "preferences in the use and acquisition of language become frozen in preferences of language structure" . It is also important to note that resolutions of conflicts can be predicted to a certain extent and therefore testable hypotheses can be postulated, but "total predictability is excluded by interlinguistic and intralinguistic language variation" .
According to Natural Phonology, external evidence is vital for explanation. According to its critics, structural explanation would be more useful (e.g. in technological applications), whereas relating preferences to higher-order principles only pushes the problem of explanation back one step. For Natural Phonologists, however, structural description and its formalization are mere descriptions useful before invoking more general, covering laws. These laws will act as facts for a
higher-order areas and they themselves can be explained if they can be subsumed under generalizations which are more comprehensive, i.e. if they can be deduced from some more encompassing laws or principles. What is crucial is that Natural Linguistic preferences in the form of law-like statements make testable predictions.

4. Applications of Natural Phonology.

The importance of considering evidence from other domains of human activity such as cognition, perception, production, memory and learning, as well as providing explanations related to conveying meaning, cognitive and physiological factors is vital in applying Natural Phonology in first and second language acquisition or speech therapy.
Natural Phonology assumes a constructivist conception of acquisition in which the model of self-organizing processes provides a bridge theory for physiology, psychology, neurology and Natural Phonology.

5. Conclusions

It has been argued in this paper that the basis for a discussion of usefulness of any theoretical approach is an agreement on the goals of research. It has been shown that the methods of the study of language and the approach taken, i.e. formalist or functionalist, determine what goals we can aim at. Formalized approaches to language, producing operational computational models are useful for applications in technological domains, as for instance speech recognition or speech synthesis. Functional approaches, however, and thus Natural Phonology, are more focused on language as used by humans. This interest makes them well-suited for applications where the“human factor” with it psychological and physiological embedding is crucial: as in first and second language acquisition or speech therapy. In these domains implicational hierarchies of processes turn out to be successfully applied in teaching children who have difficulties in production in their native language or students striving to make progress in pronunciation in the second language and treating patients with aphasia or dysarthria.

6. References

1. Botha, R. 1981. The Conduct of Linguistic Inquiry. The
Hague: The Mouton Publishers.
2. Croft, W., Cruse, A. 2004. Cognitive Linguistics.
Cambridge: Cambridge University Press.
3. de Boer, B. 2001. The Origins of Vowel Systems. Oxford:
Oxford University Press.
4. Donegan, P. 2004. Review of Bart de Boer: The Origins
of Vowel Systems. Journal of the International Phonetic
Association 34(1), 95-100.
5. Donegan, P., Stampe, D. 1979. The study of Natural
Phonology. In: Dinnsen, D. A. (ed), Approaches to
Phonological Theory. Bloomington: Indiana University
Press.