Even a system like Neomycin lacks a lot of knowledge. Why is it true that drug X cures disease Y? In medicine, we know so little that this must just be taken for granted. But in some other domains, we can try reasoning from first principles, deep reasoning. In contrast, Mycin-style reasoning that uses heuristic rules is called shallow reasoning.
For example, a shallow (Mycin-like) system for diagnosing faults in your household wiring might contain rules like ``if the power has gone THEN it's a blown fuse with certainty 80would be equipped with basic electrical physics such as Ohm's Law, plus a wiring diagram of your house, and some knowledge of the strategy to be followed when looking for faults. Obviously, such a system could give a more principled explanation of its advice. The subject is very difficult though; I don't know of any deep reasoning systems that are in commercial use. For an experimental example, see Reasoning from first principles in electronic troubleshooting by Davis. Also in Developments in expert systems.