If wizards has some type of magically generated "soul", this would be grounds to treat non-wizards as a soul-less subclass.
Only assuming that this magically generated "soul" has functions or qualities that affect a being's moral worth. It's easy to assume it does because of the usual connotations of "soul", but if this sort of soul (whatever it does) is something that Muggles are able to do without, then we can't jump to that conclusion.
Update: This post has also been superseded - new comments belong in the latest thread.
The second thread has now also exceeded 500 comments, so after 42 chapters of MoR it's time for a new thread.
From the first thread: