Appeal to Nature

Natural Law Fallacy

This fallacy involves assuming something is good or correct on the basis that it happens in nature, is bad because it does not, or that something is good because it "comes naturally" in some way. This is fallacious because it turns "natural" into an ideal state without any meaningful reason, effectively using it as a synonym for "desirable" or "normal."

Bob: "My father is terribly ill at the moment, but the doctors say this new treatment will save his life."
Alice: "That treatment is unnatural. You need to accept that it's your father's time rather than trying to fight it."

Obviously, Bob's father is unlikely to consider himself better off dead than alive. This fallacy is sometimes combined with Retrospective Determinism, arguing that a given event was "just the way things are" and hence should not be regarded as negative. "It's nature's way." See below for this variant.

This is an expression of the is/ought dichotomy which separates what is from what ought (should be) and had generally defied philosophers to use one to prove the other.

It can also can arise from a fallacy of ambiguity since the words "normal" and "natural" have two meanings: "what is", and "what should be".

In politico-religious discussion, the phrase: "homosexuality is (not) normal/natural" epitomizes this semantic and logical problem.

This is related to Science Is Bad. Frequently results in All-Natural Snake Oil.

Examples of Appeal to Nature include:

Film

In Troll 2, an evil witch is able to convince someone to drink a steaming green broth that has just turned someone else into green goo because "it is made from vegetable extracts".

Literature

Here's an example from The Bible commonly used to demonstrate why one should always interpret its passages carefully: Proverbs 17:8 tells us that bribery works. Whoa! The Bible is telling us to bribe people? Um, no, pay attention. What Proverbs 17:8 tells us is that bribery works. It doesn't say that therefore we should bribe people to be successful. Yes, this passage might be a rather cynical observation, but that's all it is: an observation.
Discworld:
- In Carpe Jugulum, King Verence is talked into drinking brose after being told "It's got herbs in", on the assumption it must be healthy. He spends most of the remainder of the book foaming at the mouth and randomly attacking inanimate objects. This, however, turns out to be useful. It should be noted that brose is what the Nac mac Feegle, six-inch pictsies who can drink their weight in lamp oil with no ill effects, drink to get their spirits up before marching into battle.
- Similarly, the popular drinks Scumble (made of "mostly apples") and Splot containing such vaguely defined ingredients as "tree bark" and "naturally occurring mineral salts".
- Pratchett has a lot of fun with this trope; both Verence and his wife Magrat fall prey to it on a regular basis, usually for the worse (in Witches Abroad, teetotaler and lightweight Magrat drinks a third of a bottle of absinthe because she vaguely recognizes it as involving wormwood, after which point she, Granny Weatherwax, and Nanny Ogg start calling it "herbal wine"). In another book, Ankh-Morpork's notorious CMOT Dibbler is making himself a killing off a particularly desperate dandruff sufferer selling herbal shampoo "now with more herbs!" One character notes, "throw a bunch of weeds in the pot and you've got herbs."
- In The Fifth Elephant, when Colon says he's opposed to "unnatural things" like Sonky's contraceptives, the Patrician replies "You mean you eat your meat raw and sleep up a tree?"

Live-Action TV

The Sarah Jane Adventures, where aliens convince millions of people to drink a new energy soda that contains alien parasites called "Bane" simply by claiming that Bane is "organic" (and by extension "healthy").
Eureka had an episode where everyone was becoming dumber, and the supposedly-a-genius farmer didn't think the additives she were using were bad, since they were "organic"...In a town of super-geniuses, granted lacking in common sense sometimes, this seemed rather glaring in its stupidity.
Parodied in a Fry and Laurie sketch where a doctor is offering his patient cigarettes as a cure. "They're herbal are they?" asks the patient. "Yes, a naturally-occurring herb called tobacco, I believe."
- Another Fry and Laurie sketch had a bedtime drink containing "nature's own barbiturates and heroin".

Tropes

See All-Natural Snake Oil for a lot of examples of this.
This is the underlying logic of The Social Darwinist and often the Evilutionary Biologist; there are various versions, typically some variant of:
- The strong deserve to rule over and / or destroy the weak, because it's nature's way.
- Mankind has perverted the course of nature, so society needs to be destroyed / someone needs to genetically engineer a killer something to prey on man / whatever.

Real Life

This is often used with regard to social issues; for example, the more extreme opponents of feminism argue that the natural order is for males to be dominant, so women should not be allowed the same rights as men. (This argument is itself unsound, as many species, especially bugs, will attest to.)
- Not just bugs. In quite a lot of species the lowest-ranking females is well above the most powerful males. Some species, such as the whiptail lizard, have even done away with males all together and created a Real Life One-Gender Race.
The idea of the superior "Noble Savage" has popped up repeatedly for centuries. The superiority of the primitive person or beasts over civilized man has been a repeated trope. Of course the fact that 25% of men died from war, 20-50% of children never left childhood, and that polygamy was invented and heavy repression of women was common primarily to repopulate after those last two points is conveniently ignored.
A famous example from mathematics is Giovanni Saccheri's attempt to prove the parallel postulate. In his book, Euclid Freed of Every Flaw, Saccheri assumed the postulate was false and tried to derive a contradiction. Instead, he derived results that got stranger and stranger (but remained logically consistent), finally concluding that they were "repugnant to the nature of straight lines". Saccheri didn't know it, but he was developing what we now call hyperbolic geometry—a fruitful field of study that just doesn't work the same way Euclidean geometry does.
Eric Schlosser mentions this in Fast Food Nation: sometimes artificial things are better for you than natural ones. The example he uses is almond flavoring; extracted naturally, it contains trace amounts of cyanide.
- That's the nature of "natural" and "artificial" ingredients, at least as defined under United States law. Often, the active chemical is identical, the difference being that the "artificial" ingredient is synthesized directly from its components as a pure substance while the "natural" ingredient is extracted from some naturally occurring source but usually includes contaminants that aren't removed in the extraction process.
Lots of "herbal" supplements. The idea being that because they are "herbal" they can't be harmful. Belladonna (also called deadly nightshade), which is a poison, is an herb (in small amounts, it can be used as a soporific, but still). This also ignores the fact that anyone can be allergic to a plant that is not usually poisonous, making it harmful to him/her.
- In an extension of this, multiple supplements are now claiming (word for word) "It's all natural, so there are no side effects." Depending on the product, this is either a case of misleading truth (it's natural AND there are no side effects), a case of Blatant Lies (it's natural, and there are some side effects, but that's not because it's natural) or a case of selective omission (it's all natural, and there are no side effects... There are no PRIMARY effects either, this is basically a placebo to help you psychologically while you do the rest of the stuff we tell you, THAT'S what makes you healthy.)
In German, the word "Chemie" (literally "chemistry", but in this case a more accurate translation would be "chemicals") is often used to refer to certain food additives and basically any other substance that the speaker considers to be "unnatural". The fallacy is that, technically, water is a chemical too, and so is everything else. So if you're condemning the use of "chemicals", you are basically against every substance known to man, the healthy ones as well as the unhealthy ones.
War is often said to be bad because it's a human invention, which isn't really true (chimpanzees and ants).
- Animals fight to gain territory, food, authority and reputation and to attract mates which by the way is a large part of what war is about. Not running is perhaps unique to humans because animals tend to Know When to Fold'Em.
- Also not human inventions: Agriculture (ants and termites, among others), division of labor (multiple species), language (disputed- multiple species), ownership (disputed- multiple species), tool use (apes, certain birds, and others), or... well, actually, we didn't invent a lot. We mostly just do a lot of things other species do, but do it on a grander scale. What makes humans, or perhaps even just certain cultures, unique is the method in which we adapt and pass information on, forming increasingly complex societies that have greater ecological impacts.
  - We didn't even invent paper. Wasps did that.
Both sides of the LGBT issue are guilty of this, claiming either that it is unnatural, and therefore wrong, or that it is perfectly natural, and therefore acceptable. This is particularly jarring since nobody really seems to have any idea what they mean by "natural" in this context.
This is a point in arguments against preservation of endangered species. Extinction is a natural event that occurs when a species is no longer fit to survive in its environment. Attempting to repopulate a Dying Race works against the natural order in both the target species as well as those that share a niche.
A mode of thought that pops up with many above topics such as war and the LGBT issue is the idea that if a human practice has a parallel among some other species, then it is acceptable. Proponents of this idea tend to forget that animals also have plenty of habits that pretty much everyone would consider reprehensible if practiced by humans. This is what gave rise to the rhyme: "monkeys throw their poo, should we do so too?"

Looks like this fallacy but isn't

Natural Law Theory, in which the nature appealed to refers to the essence of something, not the wild and woolly outdoors.
The speaker is explicitly an adherent of a religion that emphasizes veneration of nature. In this case it is closer to Appeal to Authority but may not be an argument at all depending on how it is phrased. In any case such an argument would only be relevant to a fellow adherent of the same or similar hypothetical religion.
Claiming nature as evidence of practicality. If a contractor claims it more "natural" to build a road around a mountain than to dig through he is probably only claiming that conforming to terrain features is less expensive.