It is not. We do it day in and day out in science and in real life. It is a fallacy and high-school debating line to say what you did. Those of you who make this false argument are in dire need of reading this short article from professor Steven Hales:
https://www.cambridge.org/core/serv...f/thinking-tools-you-can-prove-a-negative.pdf
"But there is one big, fat problem with all this. Among professional logicians, guess how many think that you can't prove « a negative? That's right: zero. Yes, Virginia, you can prove a negative, and it's easy, too."
Remember, in many cases we can't prove a positive either. We simply gather enough evidence to feel good that the positive proof exists. Same is true of negative. There just needs to be enough compelling evidence that a negative proof is made. If I asked you if you exist, you say yes. Maybe you are a robot and don't really exist. But we take it that you do exist. Therefore the proof that you don't exist (a negative) is also made.
You drink bottled water and think it is safe to drink. Yet no one has tested that exact bottle you are drinking. Maybe it is not and you are going to get sick from it. You go about your life not worrying about that. By the same token, when I show you listening tests, explanation of engineering, measurements, etc., the complete position absolutely proves a "negative." That there is no audible effect with this cable.
So don't throw lines at us like this that only gets used to debate with zero value. Save us the nonsense.