Perhaps so. Would you care to explain? Your statement here doesn't give me much to go on.

Well I meant it quite literally: if it is not clear which component has the bug, the components are not separate enough. You also say:

> in the real world components are never sufficiently separate

But the separation of components is not an issue of "real world", it is a function of design and implementation. It is absolutely to the developers how independently the components will be implemented; if there is no way to test them in isolation, then they are not really separate components.

Take this website, and your browser, as an example. They are obviously connected, as you're using the latter to access the former, but they are completely independent: you can access other pages with the browser, and you can use other methods (other browsers, or curl, or Postman etc) to access this page. Each can be tested separately, and even when they are used in conjunction they don't directly depend on each other, but rather on a set of standards (HTTP, HTML, CSS etc) understood by both.

Yes, but then you find web pages that attempt to determine which kind of browser requested the page, and changes what is sent back in order to work around the broken-ness of specific browsers. Yes, it's supposed to be a nice clean interface specified by standards. But in the real world, as they say, all abstractions leak. "All" may be an overstatement, but the problem is real. We never separate things cleanly enough.

This statement is true, depending on how you define "enough".

Sure. Look, despite the impression I may have given, I'm not arguing for separating things badly. The cleaner the separation, the better. It really makes a difference.