In science, as in formal debate, both activities that are grounded in logic and reason, the burden of proof for a theory or thesis rests on the affirmative.
That is, you say it's so, you have to prove that it is.
In science, you try to come up with a replicable experiment, one your peers can do that will produce the same or similar results. If you claim you have achieved fusion in a mayonnaise jar full of seawater, somebody else needs to be able to duplicate that, using your data. If nobody anywhere can, your theory dies the Death of a Thousand Laughs.
If you offer a thesis in a debate, you have to bolster it with evidence that will convince a panel of judges that you more nearly achieved that than not. All the negative really has to do is shake his head and say, "Nope, I don't believe it. Show me."
It's the opposite of the criminal system in this country, where you are presumed innocent until proven guilty. In debate, you are presumed to be full of crap until you demonstrate otherwise.
There are a lot of arguments that debaters use when they don't have the weight of evidence on their side. I won't give you the Latin names, but they involve such things as a) arguing from authority -- Well, my teacher said so, and he is an expert! b) circular reasoning -- You aren't smarter than I am because I am smarter than you! c) Attacks against the person arguing -- Yeah, well, you're an asshole, so we can discount anything you have to say. d) Straw-man, you argue against something that isn't really on the table: I'm against the war in Iraq. Really? Why do you hate our troops? e) rejecting facts as opinion: Well, everybody is entitled to their own opinion! Yeah, but not their own facts. You can believe the sun revolves around the Earth if you want, but no matter how many people believe that, it's still wrong.
I could go on, but if you want to see a list of what's good and not-good in debate, go here. You may find that you are inadvertently doing some of these things when you argue. (And like me, you might find that you do some of them on purpose ...)
Back when I was a private eye, the lawyers had one I liked. If you'd caught hubby sneaking around with a girlfriend and it got to divorce court, the lawyer on the other side, having no real defense, would sometimes ask what we called What-color-were-his-socks? questions. That is, things you didn't know, and couldn't, just to get you saying "I don't know." as many times as they could. The idea was that a judge would hear that and be swayed into thinking you weren't very observant, and maybe you had made a mistake. That's why when you were in the field, you took notes, pictures, and anything else that you could in case you had to defend it in court.
Smart judges would allow two or three of these and then tell the lawyer to move on. Wicked judges would sometimes let it go on for a long time, just because they liked watching a desperate lawyer dance ...
Me, I always like to describe the girlfriend as a "very attractive woman," just to watch her smile if she was sitting in the courtroom. I'm trying to nail her boyfriend for adultery, and she's grinning at my flattery. Look on the boyfriend's face was always interesting when he saw her smile.
Normally, when I proffer a thesis here on my blog, I am assuming the role of affirmative, so I have to offer what I consider evidence. There are all kinds of evidence, and some kinds are better than others. An undoctored real-time video beats a drunk eyewitness most of the time. Something accepted generally as fact -- that Earth/Sun thing and which one revolves around which -- carries more weight than, "Well, that's what my uncle said."
I once got into a wonderful argument with a man over a lunar eclipse. About which way the shadow went across the face of the moon, east to west or west to east. Man said, "Well, that depends on where you are, the angle."
Really? And which planet might you be standing on? Not this one. Not unless the local celestial bodies went into reverse when I wasn't looking.
Opinion based on accepted truth beats hearing it from God on your way home from the Safeway.
Elsewhere on the web, I sometimes take the negative position, and if folks don't come up with something that sways me, I figure they lose the argument. How I get that is this: If both sides of the exchange were printed out and given to a disinterested panel (and "disinterested" is not the same as "uninterested," please note) then I'd win the debate on points.
Had a couple of those lately. Fun stuff when you know you have the winning hand, though sometimes you learn more if you have the loosing side ...