Where exactly did I say you should understand the code down to how electrons flow through the chip's gates?
I said if you do not understand what you are testing (not the compiler, not the runtime libraries, not the OS, not the instruction decode, not the microcode, not the gates), you can't say you actually tested it. You merely observed what could be a side-effect.
you're drawing an arbitrary line, that was the point.
We specialize and abstract for a reason. No one understands everything about everything surrounded their code, and that includes other software. Don't tell me you've personally been through the code of every single project your projects touch.
A line that encompasses the object you are testing but that you don't fully understand (or that is hidden from you) and nothing more is not arbitrary. The best you can do in these circumstances is say it adheres to a spec for the cases you tested (which could, for all you know, be the only ones that yield correct results).
I said if you do not understand what you are testing (not the compiler, not the runtime libraries, not the OS, not the instruction decode, not the microcode, not the gates), you can't say you actually tested it. You merely observed what could be a side-effect.