I just read the following quote from Stallman as part of a speech he gave many years ago. He's talking about how it is feasible to implement other programming languages in Lisp, but not feasible to implement Lisp in those other programming languages.

He seems to take for granted that the listeners/readers understand why. But I don't see why. I think the answer will explain something about Lisp to me, and I'd like to understand it.

Can someone explain it?

Here's the quote:

"There's an interesting benefit you can get from using such a powerful language as a version of Lisp as your primary extensibility language. You can implement other languages by translating them into your primary language. If your primary language is TCL, you can't very easily implement Lisp by translating it into TCL. But if your primary language is Lisp, it's not that hard to implement other things by translating them."

The full speech is here: http://www.gnu.org/gnu/rms-lisp.html

Thanks.