Special data. Lisp programs are encoded as data. Not every data is a program.
The reader does not know anything about Lisp. All the reader knows is data. It does not know about LET, nothing about LAMBDA, has no idea about DEFUN, does not know what an argument list is, it has no idea what a class is, ...
The only thing about the Lisp language the reader actually knows are abbreviations for QUOTE and FUNCTION. Everything else in the reader is about data representation: syntax for lists, numbers, symbols, characters, strings, bitvectors, structures, ... The reader has no parser for functions, definitions, special forms, macros or other Lisp language constructs.
The reader does not know anything about Lisp. All the reader knows is data. It does not know about LET, nothing about LAMBDA, has no idea about DEFUN, does not know what an argument list is, it has no idea what a class is, ...
The only thing about the Lisp language the reader actually knows are abbreviations for QUOTE and FUNCTION. Everything else in the reader is about data representation: syntax for lists, numbers, symbols, characters, strings, bitvectors, structures, ... The reader has no parser for functions, definitions, special forms, macros or other Lisp language constructs.