The naturalistic fallacy or appeal to nature is a logical fallacy that is committed whenever an argument attempts to derive what is good from what is natural. Originally it was considered a type of equivocation, wherein the word "good" was used in the sense of "pleasant" or "effective" in the premises, and in the sense of "moral" or "ethical" in the conclusion. Now it refers to any case in which someone refers to something as morally necessary simply because it is more natural.
The converse argument, where one assumes that whatever is good must be part of the natural order, is known as the "moralistic fallacy".
A trivial example:
- Apples are good to eat (meaning they are delicious or have nutritional value).
- Therefore people who eat apples are better people (meaning more ethical).
A common Christian argument:
- Homosexuality is unnatural (meaning against the biological human drive to procreate or against the supposedly God-given purpose of sex).
- Therefore homosexuality is wrong (ethically).
- Note: the first premise here seems to be untrue, based on scientific investigation into the causes of homosexuality. The causes are not understood but natural biological processes are known to be factors in at least some cases.
- Natural selection works because the weak/stupid/disabled die and the rest survive to reproduce.
- The weak/stupid/disabled should therefore be allowed to die or be killed to keep the process going.
"Ought" vs. "Is"
- Main Article: Is-ought problem
As David Hume pointed out, knowing about the physical world never, by itself, tells you how to behave ethically. To behave ethically, you must not only know what is out there, but also have a value system that tells you what ought to be out there and what your place ought to be in making that happen. Knowing about the natural world is not enough to build a moral system.
"Natural" can mean many things, and none of them are the same as "moral"
"Natural" as opposed to "artificial" is not the same as moral, or else we would be obliged to give up sanitation, penicillin, modern agriculture, and other life-saving inventions as "immoral".
"Natural" as opposed to "supernatural" is not the same as moral. For one, many theists believe in both evil supernatural beings like demons or malevolent spirits and in good supernatural beings like gods or angels. For another, many people who believe in no supernatural phenomena at all have detailed moral codes.
"Natural" meaning "biological" as opposed to "cultural" is not the same as moral. Both altruism and psychopathy seem to have roots in human physiology. Similarly cultural influences can be either good or bad.