title: Why can't categories have instance variables?
body: <p>I understand we can use associative references to invoke ivar-like behavior in categories. But what's the specific reason behind not being able to declare new ivars in categories?</p>
  
  <p>Is it because we would invade the private space of the class? Or is there any other reason? If yes, I would appreciate an example that shows the ability to declare ivars in categories breaking whatever it breaks.</p>