Where to put classes of business logic?

[eluser]meglio[/eluser]
[quote author="xwero" date="1222350510"]
If you don't want to change your class you can include it as you would in plain php and instantiate it yourself.[/quote]

I understand that this is possible, but I do not know where can I include class? That is what I'm asking

[quote author="xwero" date="1222350510"]
Another solution is to bend the rules an put the class in a helper this way you have the advantage it doesn't gets loaded more than once and you instantiate it yourself.[/quote]

So I can declare classes in *_helper files? Hm... that is where I was confused. In the CI documentation I just found that helpers are simply colletions of functions and not of classes. So returning back to my reasons, that is why I'm asking this question here.