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    "For example, the string terminator in C is the null pointer.", no, the string terminator in a C string is the character NUL, it just happens to share a numeric value with (most) NULL pointers (the integral value 0). Commented Apr 16, 2012 at 15:29
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    The null pointer in C doesn't have a numeric value. Casting a null pointer to an integer type yields some unspecified value (not necessarily 0), and a literal 0 is a null pointer constant, but pointers are not numbers. Commented Apr 16, 2012 at 19:33
  • As some answers note, Lisp is dynamically typed, and allows you to stick anything you want in the cdr cell -- but the resulting object is then known as an "improper list". This is arguably a philosophical distinction, but it has practical consequences, since an extra value must be handled differently from regular list elements. Commented Apr 16, 2012 at 22:36
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    Since nobody answered it and since I’m too lazy, I suspect the real reason is that it makes sense, mathematically: Peano’s axioms. Commented Apr 17, 2012 at 1:00
  • Nobody answered what? I mentioned the mathematical slant in my response 3 hours earlier. Every list has an empty list as its suffix, similarly to how every set has the empty set as subset, or every sequence has the empty sequence as a subsequence. And since Lisp lists (the data structure) are suffix-based, ... Commented Apr 17, 2012 at 21:31