How to use apt-get to install aptitude…

I would like to have both apt-get and aptitude working properly on my Ubuntu 8.10 install. However, when I run the command apt- get install aptitude I get the following error:

root@lixx-xxx:~# apt-get install aptitude
Reading package lists... Done
Building dependency tree... Done
Package aptitude is not available, but is referred to by another package.
This may mean that the package is missing, has been obsoleted, or
is only available from another source
E: Package aptitude has no installation candidate
root@lixx-xxx:~#

Can someone please provide a working example to help me have both tools installed properly?

Thank you.

-rln