You're totally correct. I should also consider the fact that you can prove that negative voltages exist. The opportunities for pedantically ignoring the context of the discussion at hand are endless.
You didn't stop at the "discussion at hand". You invoked the general principle of "proving a negative" and Russel's teapot (which is rather naive reasoning in itself).
Well, proving a negative is certainly feasible:
>In some circumstances it can be safely assumed that if a certain event had occurred, evidence of it could be discovered by qualified investigators. In such circumstances it is perfectly reasonable to take the absence of proof of its occurrence as positive proof of its non-occurrence.
