One of the purposes of this book is to explain what makes Lisp different from other languages. On Lisp deals mostly with the kinds of programs you could only write in Lisp. Extensibility, bottom-up programming, interactive development, source code transformation, embedded languages—this is where Lisp shows to advantage.
So when I say that this book is about how to do things that are impossible in other languages, I don’t mean “impossible” in the mathematical sense, but in the sense that matters for programming languages. That is, if you had to write some of the programs in this book in C, you might as well do it by writing a Lisp compiler in C first.