Had me going for a minute...

I didn't learn about informal logical fallacies until I had a speech class. Well, actually I did, but ... well ... informally.

It's interesting to see the uses to which these fallacies are put. Sometimes they'll be introduced into an argument, and sometimes people will accuse others of using them in order to score points for their side of an argument.

Sometimes, I'll see someone claim an informal fallacy is being used, when in fact it's not – it merely looks like one. (Most amusingly, sometimes the person seems to have gone to a web page and looked up a whole bunch of fallacies, and applied the labels to any statements that bear any slight resemblance to the examples cited on that page.)

[This is a variant on the "fallacy fallacy" – an argument is fallacious, therefore its conclusion is false. It's a "variant" because the user in this case doesn't even make the effort to make sure that the argument actually is fallacious. (updated Feb. 16 2005)

On the other hand, this argument is invalid, because of a formal fallacy:

The reason informal fallacies are called "informal" is that they are fallacies (or not) because of the content of the statement, not the form. Formal fallacies, on the other hand, are those which depend on the form of an argument, independent of its content. For example, here's a valid formal argument:

1) If A then B 2) A 3) therefore B.

It doesn't matter what A and B are. If statements 1 and 2 are true, statement 3 can't possibly be false. Statement 3 follows from statements 1 and 2, and the rules of formal logic.

1) If A then B 2) B 3) therefore A.

Statement 3 may be true, it may not. There's no way to tell from statements 1 and 2. The argument is invalid.

A concrete example: let A stand for "Johnny set the house on fire", and B stand for "The house is on fire".

The first (valid) argument is then, "If Johnny set the house on fire, the house is on fire.", "Johnny set the house on fire", therefore, "The house is on fire".

The second (invalid) argument is "If Johnny set the house on fire, the house is on fire.", "The house is on fire", therefore "Johnny set the house on fire".

The conclusion does not follow. Someone else could have set the fire. There could have been a short in the wiring. The point is, with an invalid form, true premises can lead to a false conclusion.

Informal fallacies are even harder to deal with. You have to do more than just look at the form of an argument.

Now, the statement that led to this lecture:

Overlooking the difference between selection and evolution might be the cause of some serious confusions. Selection gives rise to the idea of "random mutation", which seems to me an appeal to ignorance, i.e. "we don't know why mutation happens"*. When it is assumed a priori that the mutation is random, we can only expect a ritard in the search for the natural, internal causes of specific change.

OK, we have a problem. An "appeal to ignorance" (argumentum ad ignorantiam) is an argument in the form of "there is no evidence to prove A is true, therefore A must be false". Alternatively, "there is no evidence to prove A is false, therefore A must be true."

There is a difference this and the statement above about random mutations.

An appeal to ignorance would be:

1) There is no evidence that mutations have specific causes. 2) Therefore mutations have no specific causes.

The actual argument is:

1) We know of no evidence that mutations have specific causes. 2) Therefore, we know of no known causes for mutations.

It's not a statement that because we can't prove A is true, A is therefore false. It's a statement that because we don't know that A is true, we can't claim A is true.

And indeed, from the fallacy taxonomy,

Another type of reasoning is called "auto-epistemic" ("self-knowing") because it involves reasoning from premisses about what one knows and what one would know if something were true. The form of such reasoning is:

If p were true, then I would know that p. I don't know that p. Therefore, p is false.

For instance, one might reason:

If I were adopted, then I would know about it by now. I don't know that I'm adopted. Therefore, I wasn't adopted.

Similarly, when extensive investigation has been undertaken, it is often reasonable to infer that something is false based upon a lack of positive evidence for it. For instance, if a drug has been subjected to lengthy testing for harmful effects and none has been discovered, it is then reasonable to conclude that it is safe.

And likewise, if mutations have been studied for a long time by people trying to find non-randomnesses and none have been discovered, then it's reasonable to conclude that they are, in fact, random.