It is no exaggeration to regard this as the most fundamental idea in programming: The evaluator, which determines the meaning of expressions in a programming language, is just another program. To appreciate this point is to change our images of ourselves as programmers. We come to see ourselves as designers of languages, rather than only users of languages designed by others.
Abelson and Sussman --
Structure and Interpretation of Computer Programs, Chapter 5, Metalinguistic Abstraction.