It gets blurry. The bigger question is: can you reason about changes to the system. You need enough overall project and code-local context to do that. Nothing fixes spaghetti code, but tooling can improve you ability to model what is going on. IDEs can give live context inferred via static analysis, or pull together definitions from different files. REPL for introspection can be a big boon. Frameworks forcing conventions help with the principle of least surprise. Good CI/CD practices, create strong feedback loops that improve code quality rather than letting deadlines allow for not fully-understood code to slip through due to aggregate complexity, etc ...