Interchangeable But Not Identical - For Now
Imagine you've walked into a restaurant and are at the counter to order. You notice, but think nothing of the fact, that there appears to be two order windows and possibly two separate kitchens. You order a cheeseburger, expecting American cheese and ketchup (among other things), but what you get has mustard and Swiss. When you make a comment about it, you're told that "cheese is cheese" and nothing more is said. Two items-mustard and Swiss-weren't ordered, but one was-a cheeseburger-and so you eat it.