visibility of posts problem…

hi,
I have problems to get my page showing posts for logged in or not logged in users and can’t find a solution:
I want posts either be shown to all people or only to all logged in users. For this, I work with categories. I created a category and set permissions to all user groups. So, when writing an article, I set it either to this category to show it only to logged in users, or I chose another (or no) category to show the post publically.
But this doesn’t work. If I set it to the category for loggedin users, it is not visible to no one, not even the admin…please help!