When we evaluated
fill-column to find its value as a variable,
we did not place parentheses around the word. This is because we did
not intend to use it as a function name.
fill-column were the first or only element of a list, the
Lisp interpreter would attempt to find the function definition
attached to it. But
fill-column has no function definition.
Try evaluating this:
You will create a *Backtrace* buffer that says:
---------- Buffer: *Backtrace* ---------- Debugger entered--Lisp error: (void-function fill-column) (fill-column) eval((fill-column)) eval-last-sexp-1(nil) eval-last-sexp(nil) call-interactively(eval-last-sexp) ---------- Buffer: *Backtrace* ----------
(Remember, to quit the debugger and make the debugger window go away, type q in the *Backtrace* buffer.)