To be picky, one also notices the man page says <packages>, which means they are required:
aptitude [<options>...] {build-dep | build-depends | changelog |
download | forbid-version | hold | install | markauto | purge
| reinstall | remove | show | showsrc | source | unhold |
unmarkauto | versions} <packages>...
But for e.g., install, purge, that isn't actually the case.
