'totally novel sentence'

[Esa]

Dear colleagues
        I am about to finish a paper on which I have been working some
time and, just to make sure that I have got everything right, I have to
ask you the following question. In the not-so-distant past it was widely
claimed that speaker-hearers constantly encounter and understand
(utterances of) 'totally novel sentences'. Did anyone of you understand
what was meant by this curious statement? I certainly did not. If I know
the language in question, every sentence that I hear has some obvious
similarities (or analogies) to sentences that I have heard before. I
never hear sentences exemplifying totally novel sentence structures or
containing totally novel grammatical morphemes, and I seldom hear
sentences containing totally novel lexical units. So, to repeat, did
anyone of you ever understand what was meant by this often-repeated slogan?

Esa Itkonen