Common Lisp the Language, 2nd Edition

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# 6.1. Logical Values

The names `nil` and `t` are constants in Common Lisp. Although they are symbols like any other symbols, and appear to be treated as variables when evaluated, it is not permitted to modify their values. See `defconstant`.

[Constant]
`nil`

The value of `nil` is always `nil`. This object represents the logical false value and also the empty list. It can also be written `()`.

[Constant]
`t`

The value of `t` is always `t`.