When does a logical fallacy not become a logical fallacy

[LausTibiChriste]

I put this here because it seems more philosophical in nature and I didn't know where else to put it.

I was talking to a friend today and we were discussing logical fallacies and it kind of got me thinking: at what point does a logical fallacy turn from a real thing to almost a fallacy itself, or does it ever?

To whit: If some neo-Nazi rejects every shred of evidence against Naziism, EVEN if they're first-hand documents, just because they were presented by a Jew - that's a genetic fallacy.

But if you have a prostitute tell you fornicating is wrong - I know she's right, but at the same time - well, I'm not taking that advice from her - I think you get the idea.


The other one that bothers me is the "No True Scotsman" - seems like any time someone screws up you could pull that card out of the bag.


This may be a basic question but I'm curious where or how to assess where to draw the line with some of these things.

[james03]

A logical fallacy is always wrong.  You're really giving examples of rhetoric, which generally have an embedded fallacy in them.

So for the prostitute, this would be an ad hominem argument (rhetoric).  The logical fallacy is that you can't construct a logical decision tree (syllogism) on whether her profession has anything to do with her statement, absolutely.  It would be something like: a (person) is b (prostitute), therefore c (fornication) is not d (sin).  Even though there is no connection between the first group (her profession) and the second group (sin).  The statement she is telling the truth may be true or may not be true, however you can't construct a syllogism to establish it.

The "No True Scotsman" argument is again rhetoric, in this case, on the defense.  However whether the statement you are defending is true or not can't be established.  It can be an actual exception to a general statement.  Or maybe an (see what I did there) Hindu moved to Scotland and is calling himself a Scotsman.  In which case the rhetorical argument is correct.

[Aethel]

I put this here because it seems more philosophical in nature and I didn't know where else to put it.

I was talking to a friend today and we were discussing logical fallacies and it kind of got me thinking: at what point does a logical fallacy turn from a real thing to almost a fallacy itself, or does it ever?

To whit: If some neo-Nazi rejects every shred of evidence against Naziism, EVEN if they're first-hand documents, just because they were presented by a Jew - that's a genetic fallacy.

But if you have a prostitute tell you fornicating is wrong - I know she's right, but at the same time - well, I'm not taking that advice from her - I think you get the idea.


The other one that bothers me is the "No True Scotsman" - seems like any time someone screws up you could pull that card out of the bag.


This may be a basic question but I'm curious where or how to assess where to draw the line with some of these things.

I'm not an academic in philosophy, but here's my understanding:

Logic can be split into two types of reasoning: deductive and inductive.

Deductive is air tight because it's a type of reasoning that follows from the premises. You go from the general to the specific.


If A, then B
If B, then C
A, therefore C


There's no way you can dispute the logic of the above proposition, assuming that the above statements are in fact true.


Where the line gets murky is "Inductive" reasoning, where one looks at the conclusions / the specific in order to get to the general, but the problem with this reasoning is that it isn't as airtight because different generals can lead to the same conclusion. It's still logical, and reason based, but it's not airtight.

Say you have A therefore C. You also know if B therefore C. Does that mean if A therefore B?

Possible, that is logically an explanation. But another explanation  could also be something like

If A therefore N,
If N therefore either Q or B, but not both.

And then something causes A and Not Q, leading to C.

It may lead to the same result, but this is not synonymous with If A then B.

You would have to look at the totality of what the other premises aren't to conclude what the missing general is, but this type of reasoning is often not 100% provable, because by its very nature you are using an aggregation of nots to prove something.

Famous example: a little girl was murdered with a knife. You were the only seen person there at that time with the girl. You just bought a knife of that same brand. There is almost perfect probability that you were responsible, but you can't air tightly prove that , because in theory, somebody else with very low probability could've snuck in with that same exact knife and killed her without anybody noticing.



Things are fallacious when the premises don't lead to the logical conclusion, or there's no way logically the conclusion can lead to the premises (usually with missing information).

As an example say I jump at dawn (J). The sun rises at dawn (S). Therefore if I jump at dawn the sun rises at dawn (J -> S). This is fallacious because J and S doesn't mean if J then S.

Likewise going backwards, say if it was true that when you jump at dawn the sun rises at dawn. That doesn't mean logically that you've jumped at dawn or the sun has risen at dawn.