What children experience, what they happen to hear said, what sorts of conversations they get involved in, all vary randomly. And the variation is not just in the forms, but in what defines them as forms. For instance, the CH sound in chew might be resolved as either a T followed by an SH sound or as a complex sound with a complete closure turning into a partial closure. The second analysis happens to work better – more parsimoniously, with less complexity. But the first language learner has to sort this out.

Despite the infinite variation in what children hear, in any given speech and language community, children end up speaking the same language. But how? This mismatch between the variable input and the common output leads to what is known as ‘the logical problem of language acquisition’.

Every language has a way of referring to the various facets of experience, woman, child, mother, water, death, night, silence, known as ‘nouns’. But the way they appear varies widely, mainly in some sort of root form, as in English, or only with sort of additional structure as in Greek and many other languages. Most European languages add an element with a very subtle role defining its relation to the discourse, traditionally known as ‘articles’, but nowadays as ‘determiners’. These additional structures vary greatly from one language to another,

Consider the seemingly simple sentence “He likes sugar”. By what was once known as ‘Do support’ the marking of the present tense is moved to the left in questions and negatives, and realised in a form of the verb do in “Does he like sugar?” or “He doesn’t like sugar”. This way of marking questions and negatives is unusual across languages. It represents the extreme of exceptionality. It is accordingly hard for children to learn.

There are similar problems with the sound structure. All languages allow syllables with one vowel and one consonant like key and toe, where the vowel is the nucleus. Some languages allow no other sorts of syllable. English, as well as allowing words like strength, with three sounds before the vowel or ‘nucleus’ and three after it, also allows L as the nucleus of an unstressed syllabie in words like little. This complexity is plainly hard to learn,

In a more exteme way, at least one language, Tashlhiyt, spoken in North Africa, sometimes called ‘Berber’ to the great dislike of most speakers, allows any phoneme to constitute the nucleus of a syllable. It thus allows sentences consisting entirely of consonants, without a single vowel.

There is thus a scale from the easily learnable to the not-so-easily learnable. These points appear to be distributed somewhat randomly, as though languages are pulled simultaneously in opposite directions, in one direction towards simple and easy-to-learn consistency with simple sound structures,  and in the opposite direction towards precise, well-defined meanings with randomly variable forms and sound structures.

This happens in what is necessarily a ‘learnability space‘.