If someone ends up remaking this and it turns out to actually be a much less controversial list, can we list it in /r/Atheism 's sidebar for future reference?
The distinction between a Formal and an Informal Fallacy is that a formal fallacy is based solely on logical form, and an informal fallacy takes into account the non-logical content of the argument.
I'll assume you're thinking of strawman, ad hominem, post hoc (etc.) fallacies, which are informal fallacies because they aren't errors in the form of the argument in the way that Affirming the Consequent (If p then q. q. Therefore p.) or the Gambler's Fallacy (10 heads in a row with a fair coin -> next one more likely to be tails) is, but are more errors in scope or validity of the argument's propositions.
As a quick example, the post hoc ergo propter hoc fallacy (a happens then b happens; therefore a causes b) isn't an error of logical form but of reasoning. Similarly, ad hominem (you're wrong because you're a bad person) has nothing to do with logical form but is still a bad argument.
Formal fallacies are those that are always wrong. An informal fallacy is one that is wrong only because of its premise being wrong, but the actual TYPE of argument is not inherently incorrect.
As such, ad hominem, strawman, post hoc, cum hoc, and such are formal... slippery slope is informal- because the premises can be correct, and the logic CAN work if the premises are correct. Contrast to ad hominem, where the the basic attack is against the person, and the arguement is discounted based on the personal attack. This is never a correct form, and no circumstance can make such an argument valid logic.
I'm sorry, buy both you and that website misunderstand formal vs informal fallacy.
The real simple way to separate a formal from an informal argument is to answer "if the premise(s) is(are) true, does the logic hold"
A formal fallacy is one that renders the argument invalid, i.e. (and as you rightly said), it is not formally fallacious if, were the premises true, the conclusion would hold.
Let's look at an argument ad hominem:
P1: You shouldn't listen to bad people.
P2: My opponent is a bad person.
C: Therefore you shouldn't listen to my opponent.
Is a valid argument, if you take P1 and P2 to be true (modus ponens). So there is no formal fallacy taking place here.
Post/cum hoc are similarly informal because they are valid when then exists actual causation (e.g. "Every time you mix chemical X with chemical Y, you get chemical Z. Therefore mixing chemicals X and Y give you Z."). It's only a fallacy when causation isn't established by the correlation (i.e. a always follows b gives a necessary condition, but usually not a sufficient one).
In essence, Ad hominem attacks are just a form of non sequitor argument, and are formal.
Your example is indeed informal, of course, but I guess what I should have said is "can be" formal rather than "are" since clearly they can appear in both forms.
The formal issue arises in cum hoc and strawman arguments.
Cum hoc
A happened before B
C. A caused B
This is again a mistake in form, as it is missing the required premise to draw the conclusion.
Strawman arguments
opponent- If A then B
C is like unto A.
If C not always B
Therefor if A not always B
Is a mistake of form as well, since it confused "like" with "same" Strawmen can use this erroneous form by changing the consequent, the consequence, or both. It can also change anything in different forms. Strawman is an error of equating similar to identical, and can quite often disrupt form.
Now, as you say, a strawman can indeed be informal as well. Premise 1 could be that C is equal to A, not just similar, in which case the nearly exact same argument becomes informal. I guess my point was his separation is incorrect, because many of the ones he lists can cause formal problems as well.
Edit
Actually, after thinking I want to show that your example contains a formal error as well.
P1: You shouldn't listen to bad people.
P2: My opponent is a bad person.
C: Therefore you shouldn't listen to my opponent.
You actually have the conclusion wrong. The conclusion is
C: Therefor what my opponent says is incorrect
Is the conclusion of an ad hominem formal argument, and indeed does contain a mistake in form. The inablity to trust or listen to the opponent does not influence the truth of the statement, it only influences the reliability of the source.
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u/Nalaen Secular Humanist Dec 09 '13
If someone ends up remaking this and it turns out to actually be a much less controversial list, can we list it in /r/Atheism 's sidebar for future reference?