Do you concur with the view that natural languages are unlearnable from input alone?

Learning a language is certainly different from learning other arts such as walking,playing,swimming etc.It is different in the sense that humans cannot learn their languages only on the basis of the sensory data or input as is possible in other cases.Then how do humans learn their language?Chomsky ,who represents an anthropological approach to linguistics, put forward an interesting theory called UG by which he shows that human brains are preprogrammed or equipped with some tools called LAD to work collaboratively with the input and  in this way help produce languages.

Language acquisition is undeniably biologically programmed as children all over the world, from varying cultures and linguistic environments produce the same levels of language at the same stages. Chomsky proposes that this is a direct result of Universal Grammar, which is an inherent part of every human mind.
The argument that prevailed before Chomsky is that language is learnable from the input data only.That is to say the children learn their mother tongue by simple imitation, listening to and repeating what adults say .But Chomsky shows that this cannot be supported for a number of reasons.

He mainly bases his theory on the ’poverty of the stimulus’ argument.Chomsky and his followers interpret this poverty of the stimulus from various points of view. The common arguments in support of the poverty of the stimulus are discussed below.

The provery of the stimulus implies that the sensory data available from input are insufficient to enable the child to discover certain rules in the  language it is learning. . Even before the age of 5, children can, without having had any formal instruction, consistently produce and interpret sentences that they have never encountered before. It is this extraordinary ability to use language despite having had only very partial exposure to the allowable syntactic variants that led Chomsky to formulate his “poverty of the stimulus” argument.So, a child cannot learn a language from input only.

To Chomsky the input is deficient or poor in two ways. At first,it is claimed to be degenerate in the sense that it is marred by performance features ,such as false starts ,slips,fragments,and ungrammatically resulting from those and other pressures inherent in real-time oral communication ,and is therefore an inadequate data base for language learning. For Chomsky, acquiring language cannot be reduced to simply developing an inventory of responses to stimuli, because every sentence that anyone produces can be a totally new combination of words. When we speak, we combine a finite number of elements—the words of our language—to create an infinite number of larger structures—sentences.

Secondly and more serious,however,the input is degenerate in the sense that it does not usually contain ’negative evidence’ ,information from which the learner could work out what is not possible in a given language. Speakers proficient in a language know what expressions are acceptable in their language and what expressions are unacceptable. The key puzzle is how speakers should come to know the restrictions of their language, since expressions which violate those restrictions are not present in the input. This absence of negative evidence—that is, absence of evidence that an expression is part of a class of the ungrammatical sentences in one's language proves that a language is not learnable from input only.There are two kinds of negative evidences such as overt and covert.

Overt negative evidence is unavailable to a child because caretakers react to the truth value,not from,of children’s utterances and rarely correct the ungrammatical speech.Covert negative evidence is also unavailable ,since all that learners hear is grammatical utterances. 

In order to understand the negative evidence we can study the following two examples:
For example, in English one cannot relate a question word like 'what' to a predicate within a relative clause (1):
 (1) *What did John meet a man who sold?
Such expressions are not available to the language learners, because they are, by hypothesis, ungrammatical for speakers of the local language. Speakers of the local language do not utter such expressions and note that they are unacceptable to language learners. 

We can also study the following two sentences to learn about the negative evidence.
1,We gave the book to the girl.
2,We explained the answer to the girl.
Apparently these two sentences have the same surface structures,but whereas (1) contains an indirect object and can be rewritten as (3),
3,We gave the girl the book.
(2) contains a prepositional phrase and cannot be rewritten as (4):
4,We explained the girl the answer.

How does the child find out that ’give’ takes an indirect object and ’explain’ a prepositional phrase?How does the child discover  that (4) is ungrammatical?One possible answer is that the adults correct the children ,but the research does show the different thing.It seems logical to assume ,therefore , that there must be some innate principle which prevents the child producing sentences like (4).

Universal grammar offers the solution to the poverty of the stimulus problem by saying that there are certain principles and parameters ,which are inherent in a child.And a child learns his language with the help of these principles.In Chomsky’s view, the reason that children so easily master the complex operations of language is that they have innate knowledge of certain principles that guide them in developing the grammar of their language. In other words, Chomsky’s theory is that language learning is facilitated by a predisposition that our brains have for certain structures of language.

For Chomsky’s theory to hold true, all of the languages in the world must share certain structural properties. And indeed, Chomsky and other generative linguists like him have shown that the 5000 to 6000 languages in the world, despite their very different grammars, do share a set of syntactic rules and principles. These linguists believe that this “universal grammar” is innate and is embedded somewhere in the neuronal circuitry of the human brain. And that would be why children can select, from all the sentences that come to their minds, only those that conform to a “deep structure” encoded in the brain’s circuits.

Universal grammar, then, consists of a set of unconscious constraints that let us decide whether a sentence is correctly formed. This mental grammar is not necessarily the same for all languages. But according to Chomskyian theorists, the process by which, in any given language, certain sentences are perceived as correct while others are not, is universal and independent of meaning.

Thus, we immediately perceive that the sentence “Robert book reads the” is not correct English, even though we have a pretty good idea of what it means. Conversely, we recognize that a sentence such as “Colorless green ideas sleep furiously.” is grammatically correct English, even though it is nonsense.

Similarly, a newborn baby has the potential to speak any of a number of languages, depending on what country it is born in, but it will not just speak them any way it likes: it will adopt certain preferred, innate structures. One way to describe these structures would be that they are not things that babies and children learn, but rather things that happen to them. Just as babies naturally develop arms and not wings while they are still in the womb, once they are born they naturally learn to speak, and not to chirp or neigh.

Subsequent research in the cognitive sciences, which combined the tools of psychology, linguistics, computer science, and philosophy, soon lent further support to the theory of universal grammar. For example, researchers found that babies only a few days old could distinguish the phonemes of any language and seemed to have an innate mechanism for processing the sounds of the human voice.

Thus, from birth, children would appear to have certain linguistic abilities that predispose them not only to acquire a complex language, but even to create one from whole cloth if the situation requires. One example of such a situation dates back to the time of plantations and slavery. On many plantations, the slaves came from many different places and so had different mother tongues. They therefore developed what are known as pidgin languages to communicate with one another. Pidgin languages are not languages in the true sense, because they employ words so chaotically—there is tremendous variation in word order, and very little grammar. But these slaves’ children, though exposed to these pidgins at the age when children normally acquire their first language, were not content to merely imitate them. Instead, the children spontaneously introduced grammatical complexity into their speech, thus in the space of one generation creating new languages, known as creoles. 

Thus,Chomsky set out an innate language schema which provides the basis for the child’s acquisition of a language. The acquisition process takes place despite the limited nature of the primary linguistic data and the degenerate nature of that data. From the way the language-learning proceeds so fast in response to such a relatively slender body of ‘data’ we can hold that the infant must be credited with an innate propensity to follow the grammar of everybody else. To conclude an infant can be treated as a theorist or ‘little linguist’.