I would argue that things are "easily" testable when you can control all of the variables. For most of history, we have rarely ever controlled all variables ou

I would argue that things are "easily" testable when you can control all of the variables. For most of history, we have rarely ever controlled all variables outside of the easy cases. Typically, we define away much of the extra stuff. That has not stopped us testing what we can, just be sure to disclose everything else. Obviously, things are more confidently testable if you have controlled all variables, but that would be a crippling condition. Basically, following the link in wikipedia will get you to https://en.wikipedia.org/wiki/Scientific_control https://en.wikipedia.org/wiki/Scientific_control which allows for "A scientific control is an experiment or observation designed to minimize the effects of variables other than the independent variable (i.e. confounding variables)." Which is much less strongly stated than what you have. The idea is still there, of course.