Debugging, the ART of Detective

I don’t guess, I observe. And once I observe I deduce.

by "Holmes", Elementary, CBS series

The art of debug is also a decent way to observe and deduce. Not a guess.
The architecture of every software is variable to each other, but we can always simplify it as a black box with   input and output. The black box process input and generate output.
Well, actually the message system break modulization attempt,