Fallacy index

Recurring mistakes in reasoning, collected from every module — including the ones made by people who agree with this site.

Mistake

'It's natural, so it's good' (and its mirror image)

Shape: X is natural, therefore X is good. Or: X is unnatural, therefore X is wrong.
Why it fails: A fact about nature never gets you to a conclusion about right and wrong without a hidden moral step. Watch out: 'animals do it too' is the exact same mistake, pointed the other way. It disproves a false claim about nature. It does not prove anything is good.
Example: Cholera is natural. Chemotherapy is not. Deadly nightshade is organic. Nothing follows from any of that.

Mistake

Mistaking the wound for the disease

Shape: Group G has more mental illness, therefore what defines G is a disorder.
Why it fails: It ignores what is being done to the group. The stigma explanation makes a prediction — remove the stigma and the gap should shrink. It does. The 'disorder' explanation predicts nothing at all, which is a bad sign for a theory.
Example: Applied honestly, this reasoning makes bullied children, refugees and widows all mentally disordered.

Mistake

'We've always done it this way'

Shape: It has always been like this, therefore it should stay like this.
Why it fails: Age is not a justification. There is a serious version — long-standing institutions may encode wisdom we cannot see, so change carefully. But that argues for caution, not for never.
Example: Slavery was traditional. Denying women the vote was traditional. So was a husband's legal right to rape his wife.

Mistake

A slippery slope with no actual slope

Shape: Allow A, and B and C must follow.
Why it fails: A slope needs a reason things slide. Here there is a brake, and it is already fitted: consent. It separates A from B and C, and it is the same brake used everywhere else in law and morality.
Example: Letting adults drink alcohol has not led to letting eight-year-olds drink alcohol. The line held, because the line was principled.

Mistake

Treating disgust as evidence

Shape: I find X repulsive, therefore X is wrong.
Why it fails: Disgust is information about the person feeling it, not about the thing.
Example: Disgust has historically been triggered by foreign food, foreign accents, other ethnic groups, and disabled people. It tracks unfamiliarity at least as reliably as it tracks harm — which makes it a poor moral instrument.

These are the ones worth learning first. An argument that relies on a bad premise is a liability, whichever direction it points.

Mistake · our own side

Making "born this way" the foundation

Shape: They were born this way, therefore it's fine.
Why it fails: It hands the moral case to scientists. If the evidence changed, would the ethics change? Of course not — which shows the science was never the reason. It also asks for pity, not equality: religion is chosen and fully protected. Build on A2 — it harms nobody. Innateness is a shield, not a foundation.