It's an historically important idea, which Lisp originated. Lisp in Lisp is not just an implementation - it can be the language definition - that was the big

It's an historically important idea, which Lisp originated. Lisp in Lisp is not just an implementation - it can be the language definition - that was the big innovation. McCarthy's original Lisp was defined in Lisp, in academic papers and its manual. And, it didn't take a lot of code -- maybe a page. This shows that the language is very expressive and also compact. The near-contemporary languages FORTRAN and Algol were not defined in themselves. Exhibiting (for example) an implementation of FORTRAN in FORTRAN would not be enlightening - it would be this big complicated thing that would be much harder to understand than Lisp in Lisp.