Thanks. That makes a lot of sense. So while testing a given class, it may have some dependencies, but those may be external resources (a DB, an API, etc), or internal ones. It sounds like the recommendation is only to mock where those external dependencies lie, and leave the internal dependencies. Eventually, as you go down the chain, those internal dependencies will get to external ones (which will likely still need some sort of mock/fake/stub), but you're allowing more of the logic and interaction of the system to be tested, rather than just the logic in the one class that's directly being tested.