Anyone making an affirmative argument is expected to support that argument with factual evidence, etc. The burden of proof rests with the person bringing an assertion, there is no ‘burden of disproof’ for those who hear it and disagree.
But is it always possible to objectively determine who is making the affirmative argument? I’m pretty sure I’ve seen debates where someone asserts a point; their opponent says ‘prove it!’, and they refuse, restating (or appearing to) the opponent’s objection in a form that itself appears to be an affirmative argument.
So is there always a method by which it can be determined who bears the burden of proof?
Here’s how I understand it, based on having seen discussions like this, but not having actually studied or read up on forensics.
If a statement can be aptly paraphrased in a form “There is at least one X such that Y w.r.t. X,” then the “burden of proof” is on the oe who has made this statement. He must produce the X so described, so to speak.
Not all statements can be so paraphrased. For example, “No unicorns exist” can not be so paraphrased.
Where I am not sure is about universal statements, for example, “Any unicorn would have a horn.” In my opinion, it is the negation of this statement which is paraphraseable in a form like the one I gave, to wit: “There is a unicorn such that it does not have a horn.” I call this latter an affirmative. But I suspect debators would call “any unicorn would have a horn” an affirmative as well.
I am not sure that is the best way to think about it, but it is what I suspect debators would say.
If “All X are Y” is supposed to be an affirmative, I don’t know how it is supposed to be proved.
Anyway, with such statements, if they are affirmatives, then (as I’ve demonstrated) so are their negations, so in such debates, it seems to me, both people should have the burden of proof.
Personal anecdote: A long time ago at a trade union conference I was arguing in favour of a proposition by anticipating arguments that might be raised against it in order to counter them. Another delegate raised a point of order on me, that I was arguing against the motion when I should have been arguing for it. (There was a practice of strictly alternating speakers for and against.) The chair ruled that I was speaking in favour, but the incident does illustrate that sometimes during a debate some people can be uncertain where you are debating one way or the other.
I have my doubts as to whether it is not unlikely that which side is or is not arguing in the affirmative or negative might not in some but never in all circumstances be difficult to resolve without uncertainty.
This is by no means a universal litmus test, but in policy debates - whether the policy be one of state, a trade union or a corporation, etc. - the affirmative is the faction which wishes to enact a change. The negative is the faction which wishes to retain the status quo.
In debates over questions of fact or ethics, etc., I’m not sure the concepts affirmative or negative are particularly useful. Usually we have two contrary positions, as to which one might reasonably say each side has the burden of proof. If neither side carries its burden, the question is simply unresolved.
I’ve generally found that the more affirmative and straightforward the proposition presented, the better. Ambiguity, particularly if avoidable while still getting at the nub of the issue, is your enemy in a good debate.
Rather than debate the proposition, “No unicorns exist,” I would rephrase it to “Unicorns do exist.” Let the proponent marshal his arguments for that!
As many posters have already pointed out (even those posting a single word!), any affirmative proposition can be made a negative and any negative a positive. So the virtually world-wide mythology that “you can’t prove a negative” or that the burden of proof always falls on the affirmative is simply mistaken.
So who has the burden of proof in a general debate? One way to put it (and perhaps not the best) is that the burden of proof falls predominately on the person making the more unorthodox or less well-established claim. This is similar to what PBear42 said, but generalized.
Unfortunately, that doesn’t always help. For example, it’s obvious which is the more orthodox, but which is the more well-established: God or No God? You’d have to have a debate about that before you had the main debate!