The burden of proof is always on the person asserting something. Shifting the burden of proof, a special case of Argumentum ad Ignorantiam, is the logical fallacy of putting the burden of proof on the person who
denies or questions the assertion. The source of the fallacy is the assumption that something is true unless proven otherwise.

Who says the person asserting something has to prove it?!

ShadowNode (doubtless a member of the B/CC, but that's another matter for another node) makes a claim above:

The burden of proof is always on the person asserting something.
Furthermore, s/he insists that failure to comply is grounds for claiming a logical fallacy!

Do you see a proof of this assertion anywhere? Because I don't see one.

As always, logicians make up "fallacies", but don't bother to prove that they are indeed fallacies.

Food for thought.


Rebuttals

Naturally, the Logician's Guild is busily faking a groundswell of popular support for their views. Doubtless we can expect to find many paralogical people among their supporters. I shall endeavour to empty their hollow claims down below.

Plasma
Plasma's argument is the typical straw man fallacy, which logicians attack elsewhere when it suits their purposes. Specifically, s/he states that some weird "circular argument" would result if 2 people argued, and each shifted the burden of proof.

First, note that Plasma provides no hard, soft or even squishy evidence that such a situation has ever occurred. "Shifting the burden of proof", indeed! However, I am sure that this is perfectly OK to do when arguing on the side of the Logicians.

But even assuming Plasma's implausible and far-fetched scenario, with such a silly argument taking place, note that by e's own "logic" one of the two people arguing is right! So, accepting the ground rules laid by Plasma, we still see that a person who shifts the burden of proof has better than even odds of being right! Just examine Plasma's scenario (exactly 50% chance of being right while shifting the burden of proof) and my scenario (close to 100% chance of being right while shifting the burden of proof). Assign a probability to each, and compute the average.

You get >50%, no matter which probabilities you pick. By shifting the burden of proof, we have a higher chance of being right than by arguing "logically".

Tem42
Tem42 is more insidious. First-off, s/he (rightfully) tries to distance hirself from the Logicians, by claiming to believe hemself a "Philosopher". Immediately after, we are to believe (this time by appeal to authority, i.e. "I'm a philosopher and you're not, so you have to do as I say") that by refusing to prove something we are stating a "belief".

Well, just like the justifications provided by D. Adams for the "42" in Tem's name, Tem42 fails to provide any proof for es assertions! In other words, these so-called "philosophical truths" are (at least in Tem42's method of "logical" reasoning) nothing more than BELIEFS. And we are supposed to believe them by some virtue of their being "self evident". Or maybe because they were uttered by PHILOSOPHERS.

But at least e's not shifting the burden of proof. More like ignoring it.

One point raised by previous noders has been that the assertion "the burden of proof always lies on the person making an assertion" is often assumed, but rarely proved, and as such any person making the assertion is to an extent violating their own principles of argument, by not fulfilling their burden of proof. While to an extent this is a valid concern, it is flawed in that it is possible for one to make a claim or state a fact without evidence. For example, I can simply say "the sky is blue" without any evidence, and it doesn't mean that I am wrong, I'm perfectly right. However, it simply means that I have not proven my case to anyone who has any doubts that the sky is blue.

As for an actual proof of why the burden of proof must lie with the person making an assertion, it is quite easy to show that if the burden of proof were reversed, you could effectively create an infinite loop, whereby one person asserts something with no evidence (and no evidence to prove him wrong either), thereby "winning" the argument. Then, his opponent could simply assert the contrary position, again with no evidence either way, and by our standards have "won" the argument.

For example, if two people are debating a point:
Person 1 asserts with no evidence that position B is correct.
Person 2 denies this, without evidence, and because in this example the burden of proof is on the person denying a claim, he loses the argument.
Thus, the "winning" position must become B.
Person 2 immediately turns around and asserts position A is correct.
Person 1 has no evidence that person 2 is wrong, and so loses, meaning that position A is now the "winning" position.
Then, person 1 turns around and asserts position B and so on.

Thus, if we were to place the burden of proof on a respondent, then unresolvable arugments such as above could occur, and such a thing is clearly unacceptable. Therefore, the burden must fall upon the person making an assertion.

One other criticism which has been raised is that this principle might be contradictory, on the basis that if one person asserts B, and another asserts not(B), then it is no fairer to place the burden of proof on B than on not(B). In addition, it has been argued that to deny a claim simply because there is no proof is Argumentum ad ignorantiam.

However, these criticisms come more from a misunderstanding of the burden of proof than of any shortcomings of this principle. The fact that one fails to fulful the burden in proving B does not mean that not(B) has been proven, it merely means that B has not been proven.

In reality, the effect of the burden of proof is not to prove that an assertion is false, but rather to maintain the status quo against an unproven assertion, without making any claim of if one is necessarily true or false.

For example, consider a situation where there are only two mutually exclusive possible cases, A or B, and there is no initial position, as the given question has never been discussed before. If person 1 asserts position A without evidence, and person 2 denied their claim on the basis they were shifting the burden of proof, then all that person 2 has done is shown that A has not been proven, they have not proven not(A), which would be in this case exactly the same as proving B.

Instead, in cases where there is no initial position, the burden of proof falls on both sides to establish their position, and until either side can do so, the question remains unresolved. The difference between this and Argumentum ad ignorantiam, is that in the latter argues that because A has not been proven, that not(A) must be true, which is indeed a fallacy, but not the same as the principle of the burden of proof.

The problem with shifting the burden of proof is that you're supposed to be doing more than just making a claim. You are supposed to be defending a claim. You can never commit a fallacy by stating a belief; fallacies only occur when you are tying to convince another person that your claim is correct.

If you make a claim, but do not provide any evidence to back it up, then there is no reason for anyone to change their minds. They may happen to agree with you, but you should not expect them to.

If you introduce a viewpoint into a debate and then cannot back it up, this is a bit annoying to the others you are speaking to. If you spend any time insisting that you are right and the others are wrong, you are just being rude*. In philosophy, we call rudeness a fallacy (we have to discourage it somehow).

There are plenty of times when an statement doesn't need to be proven; we might all state our religious beliefs, for example, without having to defend them. But when we start trying to convince others that they should follow our religion, we need to give then some sort of reason (some 'proof'). If you just state loudly that you are right and everyone else is wrong, you will be accused of being illogical, and asked to leave the debate -- and for good reason.

To apply Shifting the Burden of Proof to the religion example:
Person 1: You should believe in my God.
Person 2: Why?
Person 1: Do you have a good reason why you shouldn't believe in my God? (That was shifting the burden of proof).
Person 2: No -- do you have a good reason why I should? (and that was shifting it right back where it belongs).

Note that Person 2 still hasn't stated ier belief; e may or may not believe in Person 1's God. E may also belive in any of a hundred thousand other religions. The burden of proof lies on the one making the claim in this case because there is no other claim.

There may be other debates where there are a set number of possible conclusions; ideally, in these cases each conclusion will be argued for as strongly as possible. In these cases, shifting the burden of proof would prevent one of the options from being explored properly. (Again, blocking someone from looking at your argument and judging it fully is not conductive to a rational understanding of the issue {and it's rude}, and therefore, labeled a fallacy).


* Rude? Yep. You aren't going to trick anyone into thinking that you're right just because you're persistent. You are interrupting what (we hope is) a serious debate. You may not be wrong, but you are rude. And pointless.

Surely, it cannot be the case that both argumentum ad ignorantiam and shifting the b.o.p. are both fallacies! Once again, the Logicians would have us believe we can't have our cake or eat it.

One of argumentum ad ignoramus and shifting the b.o.p. might, hypothetically speaking, be reasonably well-defined types of fallacy. But both? That's absurd!

Let's use Plasma's method to clear up the point. Consider this. Two would-be logicians, Gas and Liquid, are considering a point.

Gas asserts that B is true, without evidence.

Liquid now either goes along with this, or contests the claim. By contesting the claim, he must be asserting that B is false. This is because, as logicians like to point out (as in the principle of the excluded middle, or possibly that of bivalence), B is either true, or false, with no third possibility.

Gas now either "wins the argument", in Plasma's words, claiming no case to answer, or else finds himself on an equal footing with Liquid. One asserts "B is true", and the other asserts "B is false", or to put it more symmetrically, "(not B) is true".

Liquid now claims Gas is shifting the burden of proof, which is not cricket. The crowd (of logicians) goes wild.

Gas immediately cites argumentum ad ignorantiam: Liquid is merely claiming "not B" because (in ShadowNode's own words) "it is argued that something must be false because it hasn't been proved true". The logically-inclined spectators wait with bated breath...

Surely they can't both be right?! Or are we to believe the argument is tied?

Argument over the shifting of the burden of proof often seems to rest upon whether this is a fallacy at all, and not upon what sort and degree of proof ought to actually suffice to engage such a shift. But this is not the only application of this argument. In a civil action in an American law court, for example, the person doing the suing must simply show that it is more likely than not that some claimed wrong was committed against them. There, the inquiry is not whether something is possible or impossible, but of which side's story is the more probable, a burden borne by the person employing the reigns of the court going into the trial by suing at all. But if that person provides enough proof to make their claim the more likely, this, then, shifts the burden of proof, and the other side may still prevail (and, now, may only prevail) by providing even stronger proof negating the side of the one who sued, and instead favoring their own side of the story. But this level of inquiry ought not to be restricted to the sphere of law.

Indeed, most any argument may come to revolve on a back-and-forth proof-shifting display, each side making points and counterpoints designed to make their result seem the more likely, each argument countered by additional proof from the other side designed to shift things back. Now, consider this example, Cynical Cindy and Gullible Gil are watching a horse race. The leading horses are the green-jersey horse and the gold-jersey horse. They are neck and neck in the last lap, when the green-jersey horse suddenly pulls ahead and wins. Now the argument commences, why'd the green-jersey horse win?
Gil asserts: "The green-jersey horse must have had an extra set of legs which gave it that extra speed burst."
Cindy retorts: "What? We didn't see any extra set of legs."
Gil replies: "It was an invisible extra set of legs."
Cindy huffs: "There is no such thing as an invisible extra set of legs."
Gil shrugs: "Prove it."
Now then, knowing nothing more about the world, or about history and biology and physics, we might credit Gil's explanation. We, as well, might shrug and tell Cindy, "Well he's got you there," and chalk it up to a 50-50 possibility, at least, of one or the other being right; or even giving the argument to Gil because something made the green-jersey horse win, and no other explanation has been offered. Here, then, Cindy has two possible routes of contention; one being insistence that the proposition raised by Gil is impossible, the other being demonstration that there exist other alternative and more probable explanations which render Gil's proposal unnecessary and comparatively improbable. Suppose Cindy begins with the first tack:
Cindy: "Every horse historically recorded to have been observed to have been in this or any race has been observed to have, at most, two sets of legs."
Gil: "How about Sleipner?"
Cindy: "Sleep-what?"
Gil: "Sleipner. Odin's horse. It's all in the Eddas, Loki turned into a mare and rutted with Svaðilfari, and the next day Loki gave birth from his womb to Sleipner, an eight-legged horse-- which could also walk on water and on air!! And" (Gil continues, folding his arms triumphantly) "it is, in fact, recorded that Odin sometimes journeyed among mere mortals, disguised as an old man, so it follows, obviously, that if he traveled on Sleipner, those extra sets of legs would have to have been invisible, as well."
Naturally, now, a somewhat agape Cindy realises this argument is threatening to devolve into an unwinnable contest over the historic validity of Norse mythology. She knows that no matter how absurd she finds it, it is impossible to affirmatively and absolutely disprove Gil's inherent claim that Norse mythology is historic fact.

This spiral into absurdity may be avoided by the distillation of what is actually at issue here, which is the assertion of the necessity of additional capacities. Every additional feature or ability can be claimed as a capacity (for even a feature presupposes the ability for the generation and sustenance of that feature). If we claim an additional capacity to be impossible, yes, we do bear the burden of proving that; but if we merely claim it to be unnecessary, unsupported by proofs for which this capacity would offer the only explanation or even the best explanation, then it falls to its proponent to prove the capacity to be necessary, and to prove it as the best explanation.

A claimed capacity may deviate so far from what may be observed that it may lack any foundation in comprehensible reality; it might be internally inconsistent, such could not actually coexist with other asserted capacities; and most damning of all it might offer an incomplete explanation, while a more complete explanation for the proofs offered is one exclusive of the capacity claimed. Denial that a thing exists differs from denial of the necessity that the thing exists, or, even, of the likelihood that the thing exists. One need not prove the impossibility of a thing to prove it so improbable that it ought to be treated as fully accounted for by a more probable explanation, unless evidence is provided disproving the more probable explanation. And, in the same stroke, if more likely explanations may be discounted, then the claimed capacity is only as likely of an explanation as all other equally unlikely explanations.

And so, Cindy's response ought to be a shrug and an alternate positive explanation --
Maybe the green-jersey horse was on steroids; or maybe the gold-jersey jockey was paid to throw the race and slowed down so much that it only looked like the green-jersey horse sped up in comparison; or maybe the green-jersey horse had a better diet, or a better training regimen, or had been holding back up to that point to build excitement. Any one of these is a possible explanation; affirmatively disprove all of those, and all other more mundane possibilities, and your explanation becomes a possibility worth considering -- but even then not a necessity -- because if it is possible that the horse had an invisible extra set of legs, it is as possible that it instead had an invisible set of wings, or an invisible rocket-booster, or an invisible man running behind and pushing.
And that properly shifts the burden of proving probability and necessity to the person claiming an invisible extra set of legs to be the only or best explanation.

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