<i>"Greenspun's Tenth Rule of Programming: any sufficiently complicated C or Fortran program contains an ad hoc informally-specified bug-ridden slow implementation of half of Common Lisp." - Philip Greenspun<p>"Including Common Lisp."</i><p>Well, my understanding is that most Common Lisp implementations are actually mostly written in Common Lisp. For that to be possible, you first need to bootstrap the implementation, and to do that you first write an "informally-specified, bug-ridden, slow implementation of half of Common Lisp" in C or something, and when you've done that, you're self-hosting, meaning you can produce newer versions of your implementation that are more formally-specified, less bug-ridden and faster. Eventually you have an accurate and fast implementation of Common Lisp, which is a pretty well-specified language.<p>So there.