# Is the slippery slope argument just another reductio ab absurdum? **URL:** \ **Category:** Great Debates\ **Created:** [April 2, 2013, 12:25pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618 "2013-04-02T12:25:45Z")\ **Posts on this page:** 20\ **Page:** 1
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [April 2, 2013, 12:25pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/1 "2013-04-02T12:25:45Z")
Lately, I’ve been exposed to all sorts of argument-ending slippery slope arguments: if you let gays marry, then why not cats and dogs? If you ban private ownership of bazookas, then they’re coming in the black helicopters to seize your handguns, etc. I’ve about had it, notwithstanding the fact that I’ve frequently resorted to its use myself as an all-purpose squelcher of my opponents’ points. Is the slippery slope argument EVER valid? What’s an example of a ssa that works, and one that doesn’t, and the crucial difference between the two? Honestly, I’m just disgusted by the ssa at this point–it’s like an admission, “Yeah, I got nothin’.” ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [April 2, 2013, 12:40pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/2 "2013-04-02T12:40:47Z")
First of all, reductio ad absurdum refers to something different: presume something is true, and then show that it inevitably leads to a logical contradiction. Wiki defines the Slippery Slope logical fallacy as > [@](#): > > Slippery slope (thin edge of the wedge, camel’s nose) – asserting that a relatively small first step inevitably leads to a chain of related events culminating in some significant impact/event that should not happen, thus the first step should not happen. While this fallacy is a popular one, it is, in its essence, an appeal to probability fallacy. (e.g if person x does y then z would (probably) occur, leading to q, leading to w, leading to e.) [67] I don’t think all slippery slope arguments are inherently invalid. The key is “inevitably”. Is it probable or plausible that the slope is indeed slippery? To use the Godwin example, the progressively harsher measures enacted against German Jews as the Nazi regime secured its control is well documented. ---
**Author:** ![Half\_Man\_Half\_Wit](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/half_man_half_wit/32/21766_2.png) [@Half\_Man\_Half\_Wit](https://boards.straightdope.com/u/Half_Man_Half_Wit)\ **Post date:** [April 2, 2013, 12:47pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/3 "2013-04-02T12:47:51Z")
I think there’s nothing _formally_ wrong with slippery-slope type arguments: the form ‘A implies B, B implies C, therefore A implies C’ (and iterations thereof) is certainly valid, so if all the premises are established, it leads to an argument that’s sound. The problem is that the common slippery slopes do not actually substantiate their premises: gay marriage has fuck all to do with marrying cats and dogs. So the problem is not really with the argument in itself, but with the way it’s (mis-)used. Also, I’m not sure I understand what you mean by ‘reductio ad absurdum’. The way I understand that term, it refers to a (valid) argument that establishes the truth of some conclusion by demonstrating the falsity of its negation—i.e. a ‘proof by contradiction’, for example. So I don’t see what it’s got to do with the slippery slope. (Are you just implying that any position established by a slippery slope argument is thereby shown to be wrong? If so, that’d be fallacious: a false argument can nevertheless entail a true conclusion.) EDIT: Or basically what **Lumpy** above said… ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [April 2, 2013, 12:55pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/4 "2013-04-02T12:55:09Z")
First of all, the ssa is a TYPE of reductio ad absurdum argument: it argues that by analogy the thing that is being argued is LIKE something else, that it isn’t really like (it can be argued) but which is sometimes absurd and indefensible. To my mind, defenders of the 2nd amendment are better off conceding that of course the terms are archaic and subject (as is the rest of the Constitution) to modern reinterpretation rather than arguing that it is an absolute that must be interpreted in a particular way and never may a door be opened to any other reading. To argue that something as vague and subjective as “well-regulated” is fixed in meaning forever is a losing argument–I’d rather argue with a reasonable person who defines “well-regulated” differently from me but who grants me that other arguments are possible than a nut who argues he is right, he must be right, and he will not countentance any interpretation that makes possible him being wrong. ---
**Author:** ![TATG](https://avatars.discourse-cdn.com/v4/letter/t/50afbb/32.png) [@TATG](https://boards.straightdope.com/u/TATG)\ **Post date:** [April 2, 2013, 1:01pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/5 "2013-04-02T13:01:23Z")
> [@Half\_Man\_Half\_Wit](#): > > I think there’s nothing _formally_ wrong with slippery-slope type arguments: the form ‘A implies B, B implies C, therefore A implies C’ (and iterations thereof) is certainly valid And then the idea, I take it, is that the argument is A implies C; and C is false, so A must be false. E.g. If cats ought be allowed to marry, then dogs ought be allowed to marry; but dogs ought not be allowed to marry, so cats ought not be allowed to marry. And this seems right; when one uses a slippery slope argument, it is because “your position leads to this consequence _and this consequence is a bad one_ (and so we should reject your position)”. ---
**Author:** ![Dinsdale](https://avatars.discourse-cdn.com/v4/letter/d/97f17d/32.png) [@Dinsdale](https://boards.straightdope.com/u/Dinsdale)\ **Post date:** [April 2, 2013, 1:06pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/6 "2013-04-02T13:06:33Z")
Must a slippery slope lead to the extreme result? Or could it not simply predict additional incremental change. For example, gay marriage may not lead to cats marrying dogs, but could someone opposing all gay rights have contended that recognizing gay rights such as health benefits/civil unions/etc. initiated/continued the slide down the slippery slope to gay marriage? ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [April 2, 2013, 1:25pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/7 "2013-04-02T13:25:25Z")
Yeah, the slippery slope argument is about incremental change that is, if not inevitable, then likely. As any cite will tell you, it’s only a fallacy when no such slope actually exists. But sometimes it does. Imagine if someone had baulked at NYC’s original trans fat ban by claiming we would see the banning of large sized sodas. It would have been poo-hoo’ed away as a slippery slope fallacy. ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [April 2, 2013, 1:29pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/8 "2013-04-02T13:29:06Z")
But the place it “exists” is always in the speaker’s mind. It’s always a projection of some condition that, at the moment, doesn’t actually exist. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [April 2, 2013, 1:39pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/9 "2013-04-02T13:39:41Z")
> [@pseudotriton\_ruber\_ruber](#): > > But the place it “exists” is always in the speaker’s mind. It’s always a projection of some condition that, at the moment, doesn’t actually exist. Sometimes it does. If there is a trend, one can project the trend. My example wasn’t a good one, but now that we have two data points, might we not expect the next thing to be banned would be “x type of unhealthy food”? Plus, the next step along the line needn’t be “absurd”. It doesn’t disprove the premise, but demonstrates that “if a then why can’t we also expect b”? There is a very good reason why SSM won’t lead to “man on dog” marriages, but it might very well lead to legalized polygamy. There is no mutual consent in the former, and so it’s not like SSM in the sense that polygamy can be. ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [April 2, 2013, 1:48pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/10 "2013-04-02T13:48:54Z")
Let’s consider banning slavery. It’s nearly 1850, and I keep hearing that if you free the slaves, the next thing you’ll be seeing is widespread misegenation, and I won’t stand for that. Of course, if blacks marrying whites doesn’t really become a socially acceptable trend until the 21st century, then I’m dead for quite a while, so I shouldn’t get a vote, should I? That point on the slippery slope should be decided by people who are actually living at the time, not their ancestors. I guess that’s reasonable. OK, free the damned slaves. ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [April 2, 2013, 1:55pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/11 "2013-04-02T13:55:36Z")
> [@pseudotriton\_ruber\_ruber](#): > > Let’s consider banning slavery. It’s nearly 1850, and I keep hearing that if you free the slaves, the next thing you’ll be seeing is widespread misegenation, and I won’t stand for that. > > Of course, if blacks marrying whites doesn’t really become a socially acceptable trend until the 21st century, then I’m dead for quite a while, so I shouldn’t get a vote, should I? That point on the slippery slope should be decided by people who are actually living at the time, not their ancestors. I guess that’s reasonable. OK, free the damned slaves. Bad example, since many states made interracial marriage and miscegenation illegal. That sort of puts a damper on things, don’t you think? However, if you really are a white supremacists, then that’s a perfectly valid argument. Freeing the slaves, and doing nothing else, will likely lead to mixed marriages. And when did it become a requirement that the slippery slope had to lead to consequences in that person’s lifetime? Most people are concerned about the type of world they are leaving to their children and grandchildren. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [April 2, 2013, 2:33pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/12 "2013-04-02T14:33:18Z")
> [@pseudotriton\_ruber\_ruber](#): > > First of all, the ssa is a TYPE of reductio ad absurdum argument: it argues that by analogy the thing that is being argued is LIKE something else, that it isn’t really like (it can be argued) but which is sometimes absurd and indefensible. > > To my mind, defenders of the 2nd amendment are better off conceding that of course the terms are archaic and subject (as is the rest of the Constitution) to modern reinterpretation rather than arguing that it is an absolute that must be interpreted in a particular way and never may a door be opened to any other reading. To argue that something as vague and subjective as “well-regulated” is fixed in meaning forever is a losing argument–I’d rather argue with a reasonable person who defines “well-regulated” differently from me but who grants me that other arguments are possible than a nut who argues he is right, he must be right, and he will not countentance any interpretation that makes possible him being wrong. I was going to answer this, but then we’d be on a slippery slope towards turning this into another gun thread. 😃 Slippery slope arguments ask just what is going to bring a progression to a halt once it gets started. I think slippery slope arguments are valid when they raise two issues: the abandonment of a defining principle and an apparent lack of a replacement principle to set a new limit. Gays insist that all gay marriage does is enshrine the permanent committed relationship between two partners who might not necessarily be different genders. But conservatives claim that the founding principle of marriage- the creation of a progenitive unit- has been abandoned and replaced with a definition of “marriage” that effectively means the semi-permanent union of people in a sexual relationship. If that’s the case then it is hard to argue that the new standard wouldn’t allow for polygamy, marriage of teenagers, incestuous marriage, etc. In the case of firearms, if the government has the authority to proscribe guns, then what is there _in principle_ to prevent the government banning them completely- as indeed people have now and in the past advocated that the government do just that? ---
**Author:** ![John\_Mace](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/john_mace/32/185_2.png) [@John\_Mace](https://boards.straightdope.com/u/John_Mace)\ **Post date:** [April 2, 2013, 2:49pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/13 "2013-04-02T14:49:12Z")
> [@Lumpy](#): > > I was going to answer this, but then we’d be on a slippery slope towards turning this into another gun thread. 😃 > > Slippery slope arguments ask just what is going to bring a progression to a halt once it gets started. I think slippery slope arguments are valid when they raise two issues: the abandonment of a defining principle and an apparent lack of a replacement principle to set a new limit. Gays insist that all gay marriage does is enshrine the permanent committed relationship between two partners who might not necessarily be different genders. But conservatives claim that the founding principle of marriage- the creation of a progenitive unit- has been abandoned and replaced with a definition of “marriage” that effectively means the semi-permanent union of people in a sexual relationship. If that’s the case then it is hard to argue that the new standard wouldn’t allow for polygamy, marriage of teenagers, incestuous marriage, etc. In the case of firearms, if the government has the authority to proscribe guns, then what is there _in principle_ to prevent the government banning them completely- as indeed people have now and in the past advocated that the government do just that? I like that. Imagine a slope, where there is a physical barrier. That’s your “defining principle” that prevents us from sliding down that slope. If we remove that barrier, one asks where is the new barrier to stop us? In the case of “man on dog” marriage, that barrier is mutual consent, which the dog is unable to give. ---
**Author:** ![astorian](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@astorian](https://boards.straightdope.com/u/astorian)\ **Post date:** [April 2, 2013, 3:07pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/14 "2013-04-02T15:07:52Z")
Are slippery slope arguments logical? Not always, but they have a tendency to come true. And people are often far too quick to dismiss the actual essence of what slippery slopers of the past were saying. That is, a latter day sophisticate tends to say, “Back in 1956, old prudes said that the world would come to an end if Elvis was allowed to shake his hips on TV. But he did, and the world is still here, so they were obviously fruitcakes.” Thing is, the old prudes who thought that what Elvis was doing would have major consequences were RIGHT! Indeed, the latter day sophisticate KNOWS that, and that’s one of the things he LIKES about Elvis. Elvis WAS changing societal attitudes about sex. The only question is whether or not you LIKE the changes he helped bring about. In 1956, old schoolmarms said “If you allow Elvis to shake about obscenely on TV, things will only get worse. You’ll see MORE provocative dancing on TV, and we’ll hear MORE sexually explicit songs on the radio, and our kids will be encouraged to abandon traditional notions of morality.” Well, is there ANY doubt that, objectively speaking, the old schoolmarms were 100% RIGHT? You’re free to laugh at old-fashioned notions of morality. You’re free to argue that it’s a GOOD thing that sexual morality has changed so completely. But you CAN’T dismiss the slippery slope warnings of the old schoolmarms, because they were right on the money in arguing that Elvis posed a threat to traditional morality. ---
**Author:** ![The\_Second\_Stone](https://avatars.discourse-cdn.com/v4/letter/t/bbe5ce/32.png) [@The\_Second\_Stone](https://boards.straightdope.com/u/The_Second_Stone)\ **Post date:** [April 2, 2013, 3:14pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/15 "2013-04-02T15:14:48Z")
A slippery slope is the initial step leads to further steps. Letting the commies grow in SE Asia will lead to the Phillipines, to Guam, to Hawaii and finally, San Francisco. All those things are presumably plausible all of which are undesireable. A reductio ad absurdum suggests that the the chain of steps with a result that doesn’t really follow. If ketchup is a vegetable, as was attempted for school lunches by the Reagan administration, then so it pepper, mustard and relish, and we can have a balanced meal of just condiments. The result might be technically true, but only in a twisted sense that is not accepted as reality. ---
**Author:** ![astorian](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@astorian](https://boards.straightdope.com/u/astorian)\ **Post date:** [April 2, 2013, 3:16pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/16 "2013-04-02T15:16:42Z")
Are slippery slope arguments logical? Not always, but they have a tendency to come true. And people are often far too quick to dismiss the actual essence of what slippery slopers of the past were saying. That is, a latter day sophisticate tends to say, “Back in 1956, old prudes said that the world would come to an end if Elvis was allowed to shake his hips on TV. But he did, and the world is still here, so they were obviously fruitcakes.” Thing is, the old prudes who thought that what Elvis was doing would have major consequences were RIGHT! Indeed, the latter day sophisticate KNOWS that, and that’s one of the things he LIKES about Elvis. Elvis WAS changing societal attitudes about sex. The only question is whether or not you LIKE the changes he helped bring about. In 1956, old schoolmarms said “If you allow Elvis to shake about obscenely on TV, things will only get worse. You’ll see MORE provocative dancing on TV, and we’ll hear MORE sexually explicit songs on the radio, and our kids will be encouraged to abandon traditional notions of morality.” Well, is there ANY doubt that, objectively speaking, the old schoolmarms were 100% RIGHT? There WAS a very steep, very slippery slope, and Elvis helped push America straight down that slope. You’re free to laugh at old-fashioned notions of morality. You’re free to argue that it’s a GOOD thing that sexual morality has changed so completely. But you CAN’T dismiss the slippery slope warnings of the old schoolmarms, because they were right on the money in arguing that Elvis posed a threat to traditional morality. What about today? A latter day sophisticate is quick to scoff at old-school Christians who argue that legalizing gay marriage will lead to all kinds of other, undesirable changes. The sophisticate will proclaim publicly, “Allowing gays to marry won’t have any effect on heterosexual marriage. Gays jsut want the same kind of monogamy and stability that straights have always had.” That SOUNDS reasonable, but I suspect even the sophisticate doesn’t really believe it. After all, just a week after New York state legalized gay marriage, the New York Times Sunday magazine had a prominent story in which Dan Savage was procaliming that straight married couples need to loosen up on those old-fashioned, archaic ideas of monogamy and fidelity. If old schoolmarms notice that and argue, “Allowing gays to marry WILL change marriage. It WILL lead to the undermining of traditional ideas of monogamy,” it seems pretty evident that they’re right. The only question THEN will be, do you APPROVE of the undermining of traditional notions of monogamy? If, down the road, “Monogamish” straight couples are common, won’t you have to conclude that the slippery slope crowd had a point? ---
**Author:** ![pseudotriton\_ruber\_ruber](https://avatars.discourse-cdn.com/v4/letter/p/a9a28c/32.png) [@pseudotriton\_ruber\_ruber](https://boards.straightdope.com/u/pseudotriton_ruber_ruber)\ **Post date:** [April 2, 2013, 3:18pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/17 "2013-04-02T15:18:16Z")
And therein lies my point: the world will change whether you consent to the changes or not. Slippery slopes arguments are all predicated on change occurring in the future, to which the only possible response is DUH!! Did the Elvis-objectors foresee the Beatles? The assassinations of the 1960s? The Internet? Well, no. They just foresaw the possibility that some bad things might happen as a result of his singing style, so better safe than sorry, let’s oppose all changes to the status quo because of the likelihood of some unforeseen changes coming as a result of this one. ---
**Author:** ![BrainGlutton](https://avatars.discourse-cdn.com/v4/letter/b/82dd89/32.png) [@BrainGlutton](https://boards.straightdope.com/u/BrainGlutton)\ **Post date:** [April 2, 2013, 3:57pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/18 "2013-04-02T15:57:20Z")
RatonalWiki is really into this sort of thing. [Slippery slope:](http://rationalwiki.org/wiki/Slippery_slope) > [@](#): > > The **slippery slope** is a common logical fallacy (and a variant on the argument from adverse consequences) that asks for a prohibition or curtailment on something based on a cascading series of undesired results, e.g. former Senator Rick Santorum’s belief that the US Supreme Court’s decision on Lawrence v. Texas would eventually lead to the legalization of “man-on-dog” sex[1], among other forms of sexual activity generally considered repulsive or obscene[2]. It is also a common argument amongst opponents of drug or alcohol use, based on the idea that simply getting drunk or high once (or even just experiencing a little of the substance in question) inevitably leads to a raging addiction and a failed life. > > While the slippery slope may in fact exist, usually the argument is put forth while ignoring possible mitigating factors (such as informed consent for sex partners in the Santorum example), thus making it generally a somewhat-too-serious form of reductio ad absurdum. > **Parade of the Imaginary Horribles** > > The parade of the imaginary horribles is the list of off-the-wall worst-case scenarios that opponents claim will occur after the legalization or acceptance of a prohibited activity. (I.e., Santorum’s “man-on-dog sex” statement listed above.) These scenarios say more about the psyches of the people who devise them than about the proponents for change. > > This term is accredited to former U.S. Supreme Court Justice William O. Douglas. > **Example of the Slippery Slope Argument** > > Logical fallacies are a dangerous thing! > > First, you start off with something small, like an appeal to authority or “no true Scotsman.” > > Then, its bigger ones, like an argumentum ad hominem, or begging the question. > > That’s not far away from post hoc ergo propter hoc or reductio ad Hitlerum. > > And after that, you’re well on your way to hosting a show on Fox News! > > Avoid logical fallacies at all costs; there’s no telling where they’ll lead! > **Countering the slippery slope argument** > > Frequently, the slippery slope argument has no evidence presented for why one would actually go down the slope, even if it is hypothetically possible. Pointing this out is sufficient to logically counter the argument. However, slippery slope arguments that involve no evidence can usually also slip the other way: if it is argued that gay marriage will lead to man-on dog sex, one could just as well argue that restricting gay marriage will lead to banning marriage altogether. If evidence is presented for why the slip could occur, the argument may be valid, in which case countering it requires refuting the evidence. [_Reductio ad absurdum:_](http://rationalwiki.org/wiki/Reductio_ad_absurdum) > [@](#): > > _ **Reductio ad absurdum** _ is the technique of reducing an argument or hypothesis to absurdity, by pushing the argument’s premises or conclusions to their logical limits and showing how ridiculous the consequences would be, thus disproving or discrediting the argument. > > This has roots in the Socratic method, and has been employed throughout the history of logic, mathematics, philosophy and the philosophy of science. > > As an example, one of the central tenets of homeopathy is the (unfounded) assertion that water retains a “memory” of substances dissolved in it, even when the solution becomes so weak that no trace of the original substance is present. However, if we accept this hypothesis, then any quantity of tap water would have already acquired all the benefical chemicals, and all the harmful ones too. This patently absurd scenario weakens the hypothesis of water memory. > **Validity and fallacious use** > > Reductio ad absurdum is only valid when it builds on assertions which are actually present in the argument it is deconstructing, and not when it misrepresents them as a straw man. For example, any creationist argument that takes the form of “if evolution was real, we’d see fish turning into monkeys and monkeys turning into people all the time” only serves to ridicule itself, since it mischaracterises the theory of evolution to an extreme degree. > > The argument from adverse consequences is a similar but more flawed technique. While reductio ad absurdum rejects an argument on the basis that its logical consequences are so unlikely that the argument cannot possibly be sound, the argument from adverse consequences rejects an argument because its consequences are undesirable, or because accepting it could mean accepting something we would prefer not to acknowledge; in most cases, this is regarded as a logical fallacy. > > Reductio ad absurdum should also not be confused with appeal to ridicule, although both saw extensive use in satire. Appeal to ridicule simply dismisses a position as ridiculous without explaining or arguing why, while reductio ad absurdum actually pursues the logical consequences of an argument. > **In mathematics** > > In mathematics and formal logic, reductio ad absurdum, also known as “proof by contradiction” and “proof by assuming the opposite,” is the establishing of an argument (or theory) by showing that its denial would lead to absurd consequences. > > [If A is not true, then B is true (and therefore C is true’ … etc. ) which is absurd/impossible/not in accord with observation. Therefore A must be true. QED!] > > As this is a logical argument, possibly with many steps between the initial premise and the ultimate conclusion, it is often open to varying interpretations and alternate steps which might not prove the conclusion. In mathematics it has been a key proof since Euclid and is a well accepted method (Often ending with a statement that some property is not equal to itself, thus showing the absurdity). In philosophy and science it is less hard and fast as there is often dispute in the causal relationship (then) between steps of the argument. > > An example found in euclidean geometry. > > Hypothesis: Two straight lines that intersect do so in one and only one point. > > (1) Let “l” and “r” be two straight lines that intersect in two or more points. (We deny the hypothesis.) > > (2) Let “A” and “B” be two of the points where l and r intersect, therefore A and B are both points of l and r. > > (3) (2) is absurd, as it contradicts an axiom (Two distinct points determine one and only one straight line.) therefore (1) is impossible and two straight line can’t intersect in more than one point. ---
**Author:** ![Lumpy](https://sea3.discourse-cdn.com/straightdope/user_avatar/boards.straightdope.com/lumpy/32/446_2.png) [@Lumpy](https://boards.straightdope.com/u/Lumpy)\ **Post date:** [April 2, 2013, 4:04pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/19 "2013-04-02T16:04:38Z")
I thought reductio ad absurdum strictly meant the mathematical sense of establishing a contradiction. Is it accepted to have a broader philosophical meaning (e.g. Socratic method)? ---
**Author:** ![astorian](https://avatars.discourse-cdn.com/v4/letter/a/ad7895/32.png) [@astorian](https://boards.straightdope.com/u/astorian)\ **Post date:** [April 2, 2013, 4:23pm UTC](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618/20 "2013-04-02T16:23:39Z")
> [@pseudotriton\_ruber\_ruber](#): > > And therein lies my point: the world will change whether you consent to the changes or not. Slippery slopes arguments are all predicated on change occurring in the future, to which the only possible response is DUH!! > > Did the Elvis-objectors foresee the Beatles? The assassinations of the 1960s? The Internet? Well, no. They just foresaw the possibility that some bad things might happen as a result of his singing style, so better safe than sorry, let’s oppose all changes to the status quo because of the likelihood of some unforeseen changes coming as a result of this one. Now you’re being obtuse. I have just demonstrated that, in the past, people who predicted that Elvis/rock and roll would surely lead to a breakdown in sexual morality. Did they know the EXACT course those changes would take? Of course not, but they saw Elvis and rock as threats to traditional morality. And they were right. Everything they feared would happen DID happen, and then some. Now, you’re saying, in effect, “So what?” Unacceptable. Let’s get YOUR position(s) straight. 1. Do you believe that marriage is supposed to be permanent? 2. Do you believe that promiscuity and divorce are bad things? 3. Do you believe that the traditional rules of marriage (including the prohibition of divorce and adultery) should apply to gay married couples? If you DO believe in these things, we have a lot to talk about. If you DON’T (and I’m 99% sure you don’t), then we have absolutely nothing to talk about. I really want to know if you’re saying, “Slippery slope arguments are absurd, and all the the bad things people predict will NEVER happen”? Or are you saying, “All those allegedly bad things the slippery slopers are predicting probably WILL happen, but I just want them to shut up anyway, because I think those are actually GOOD things”? [Next page](https://boards.straightdope.com/t/is-the-slippery-slope-argument-just-another-reductio-ab-absurdum/654618.md?page=2)