Retrieve a list of all the internal commands

Question

I learned that there are external commands and internal commands

me@host:~$ time pwd
/home/gaowei

real    0m0.000s
user    0m0.000s
sys     0m0.000s
me@host:~$ /usr/bin/time pwd
/home/gaowei
0.00user 0.00system 0:00.00elapsed 0%CPU (0avgtext+0avgdata 2136maxresident)k
88inputs+0outputs (1major+86minor)pagefaults 0swaps

How could retrieve a list of all the internal commands. Are there distinction between internal commands and builtin commands?

The builtins could be retrieved by

me@host:~$ enable -a | cut -d ' ' -f 2 | nl
     1  .
     2  :
     3  [
     4  alias
     5  bg
     6  bind
     7  break
     8  builtin
     9  caller
    10  cd
    11  command
    12  compgen
    13  complete
    14  compopt
    15  continue
    16  declare
    17  dirs
    18  disown