Greenspun's tenth rule
This expresses the opinion that the perceived flexibility and extensibility designed into the Lisp programming language includes all functionality that is theoretically necessary to write a complex computer program, and that the core implementations of other programming languages often do not supply critical functionality necessary to develop complex programs.
The rule was written sometime around 1993 by Philip Greenspun. Although it is known as his tenth rule, there are in fact no preceding rules, only the tenth. The reason for this according to Greenspun:
Lisp syntax is barely abstract; Lisp programs are constructed as trees of S-expressions, which are equivalent to the abstract syntax trees that compilers of other languages create internally. As a homoiconic language, Lisp also makes no distinction between code and data; Lisp programs are themselves just Lisp data structures. Lisp programs can thus generate additional Lisp programs themselves via macros. As a result, the programmer can construct complex domain specific languages with relative ease. Lisp advocates such as Paul Graham claim that this allows faster construction of more robust software compared to other languages. Common Lisp, specifically, has quite large standard library with large number of functions with flexible interfaces, that capture many programming paradigms (like
map, etc.). Greenspun's Rule expresses the opinion that these capabilities are necessary for any sufficiently complex software package, and so will be redesigned from scratch in an ad hoc manner in any large project, duplicating effort already exerted in Common Lisp.
Morris's corollary 
…including Common Lisp.
This can be viewed as a commentary on the difficulty of creating an efficient implementation of the large and complex Common Lisp language, or simply a joke about Lisp's eval function. Both Greenspun's rule and Morris's corollary are examples of a characteristic style of hacker humor known as "ha ha only serious".
See also