Argument from adverse consequences
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The argument from consequences or argumentum ad consequentiam if you insist on Latin, is a logical fallacy that the perceived outcomes of a proposition can determine its veracity. An example of arguing from adverse consequences might go like: belief in the theory of evolution leads to eugenics; therefore the theory of evolution is false. Conversely an argument from favourable consequences might go: belief in God leads to an increase in charitable giving; therefore God exists.
The argument takes this form:
- If A is true then it implies, causes, or creates, B.
- B is, either subjectively or objectively, bad, immoral, or undesirable.
- Therefore, A is false.
This is applied fallaciously in arguing whether something is true or not. Just because something is perceived as having adverse consequences if it is true, does not make it suddenly become untrue - such an idea is just a form of wishful thinking. Conversely, when something is perceived as having good consequences if it is true, this perception does not actually make it true. Any argument from consequences is an appeal to emotion.
An example is the following quote from Sun Myung Moon:
“”Some zoologists are saying the monkey is ancestor of the human. Then ask the scholar, "Sir, who is your grandfather? Is he a monkey? Then you are a descendant of a monkey." Will he like that? No way. It will turn him off. Would you like a monkey to be your grandfather?
|— Sun Myung Moon, Way Of Unification (Part 1)|
Of course Moon is grossly (and intentionally) misrepresenting the theory of evolution - none of us humans are a monkey's grandson. But we humans are descended from primates and share most of the monkey's biological ancestry. If, like Moon, some of us find this notion troubling, depressing, or ridiculous, this has no bearing on whether or not it is true. Our conclusions should be based on the scientific method and the evidence available, not choosing whatever conclusion makes us feel better.
Examples 2 & 3
The existence of gravity would make falling from a height onto a hard surface unpleasant, therefore gravity cannot exist.
Reducing greenhouse gas emissions would require policies we don't like, therefore anthropogenic greenhouse gasses don't have any effect on the climate.
 Use in decisions
It is generally not fallacious if used in decision making with something that isn't objectively provable - i.e., is this desirable, not is it true. This is found in political decision making, where the consequences, positive or negative, must be taken into account. There is no "real" political position, such as a tax rate or drug policy.
In these cases, the power of arguing from adverse consequences is directly proportional to the strength of the premise: for example, a proposal to discontinue a particular medicine because there are mild adverse effects (e.g. hair loss) in a small proportion of the cases would generally be considered dubious, while discontinuing the medicine because of serious adverse effects (e.g. early death or deformed babies) would generally be considered reasonable.
Since most acts will have both good and bad consequences, many of them unforeseen and unintended, it requires great care to deploy the argument from adverse consequences properly. Even using adverse consequences in decision making can be fallacious in some cases. The most frequent logical fallacy in this regard is when relatively small adverse effects are used to argue against practices with highly significant positive outcomes, e.g. that racial integration is bad because we no longer maintain separate bathrooms and water fountains which results in job losses for plumbers.
 A similar-looking but legitimate argument
This is of course not to be confused with argumentation by contrapositive, or modus tollens. It is not a matter that the consequence is subjectively/objectively bad/immoral/undesirable if it does not occur in real life. The argument would go like this:
- If P is true then Q would happen/be observable.
is subjectively/objectively bad/immoral/undesirable.does not happen/cannot be plausibly observed
- Therefore, P is false.
An example would be the claims of psychokinesis. Assuming that people can in fact move things with their minds, even in simple acts such as Uri Geller's spoon-bending, there is no way to expect that everyone with such a power would be good or sane. People would be telekinetically snapping each others' necks left and right and we would definitely hear about that sort of thing on the news. Obviously, it can be deduced that, since society isn't a hotbed of psychokinetic violence like in the movies, these powers simply do not exist.
 See also
| Articles about logical fallacies|
|Affirming the consequent||Denying the antecedent|
Argument from adverse consequences
|Fallacious argument styles|