If, in a debate, someone categorically claims, "There is no evidence for X", does that not count as an assertion? Can that assertion not be fairly challenged by asking whomever made the claim to support their assertion?
If Joe says "There is no evidence for X", is it my obligation to provide evidence for the converse, or for X, or is it Joe's obligation to support his assertion that there is no evidence for X? Since I haven't claimed in that argument that there is evidence for X, how can it be my obligation to provide evidence for X, when i didn't claim X was true? When all I am doing is asking Joe to support assertions Joe made? Is it my problem that Joe has made an assertion that is probably impossible to support?
Wouldn't demanding me to support the converse be "shifting the burden"? Isn't that in fact a textbook case of shifting the burden?
If Joe is going to use unsupportable rhetoric in an argument, and I can demonstrate it to be unsupportable rhetoric simply by asking Joe to support his assertion, isn't that an entirely ethical challenge to make?