When developing applications

[eluser]esra[/eluser]
Any library loaded by codeigniter.php or common.php are CodeIgniter core dependencies. You can browse through those two files to identify those core dependencies. I believe there is a page in the user guide that also identifies those dependencies. All other files could be considered extensions of the core framework. However, I agree with Michael--the framework is very small compared to other frameworks and it makes sense to leave the file set intact, especially if you are using svn for versioning control and want to keep CI updated in a separate branch.

Edit: Also check Base4.php and Base5.php since Loader.php is also a core dependency.