In neuropsychology, linguistics, and the philosophy of language, a \textbf{natural language} or \textbf{ordinary language} is any language that has evolved naturally in humans through use and repetition without conscious planning or premeditation. Natural languages can take different forms, such as speech or signing. They are distinguished from constructed and formal languages such as those used to program computers or to study logic.
\null\hfill -- Wikipedia [\url{https://en.wikipedia.org/wiki/Natural_language}]
\item$65k^3=2.74\times10^{14}$ sequences of 3 words
\item[]
\item[\ra] Second language learner: often struggle to learn more than 3k words after several years
\item[\ra] Native English speakers: 15k to 20k word families (lemmas)
\end{itemize}
%https://www.bbc.co.uk/news/world-44569277
%So does someone who can hold a decent conversation in a second language know 15,000 to 20,000 words? Is this a realistic goal for our listener to aim for? Unlikely.
%Prof Webb found that people who have been studying languages in a traditional setting - say French in Britain or English in Japan - often struggle to learn more than 2,000 to 3,000 words, even after years of study.
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\item[\ra] We can't directly estimate the probability of a sequence by relative frequency!
\item[\ra] Stanley F. Chen and Joshua T. Goodman, \emph{An Empirical Study of Smoothing Techniques for Language Modelling}. Computer, Speech and Language, 13(4), pp. 359-394, 1999.