worldling

"Formal logic cannot prove the existence of external reality. Therefore you cannot use formal logic to examine Christianity."

It's obviously fallacious, but what is the formal name of the fallacy?

Thanks in advance.

Nowhere357

I wonder if maybe it's begging the question.

Neilium

My hunch: appeal to ignorance.

The statement is asking you to accept the truth of a proposition unless an opponent can prove otherwise.

Also, it smacks a bit of irrelevant conclusion.

Good luck. I assume this is the same person you were arguing with previously.

-Neil

worldling

Yes, it's the same argument with the same person. Frankly, I think he is simply jibbering here.

I am quite happy to accept his premise, but obviously his conclusion doesn't follow. Maybe it's a simple non sequitur?

Here's how I replied:

"A hammer may be a useless tool for fixing a computer, but it does a great job of hammering nails into wood. Similarly, formal logic may not be able to prove the existence of an external reality, but it does a fine job of proving that the Christian hypothesis is bunk."

(As far as his response to my objections to him making up his own definition of "rationality" - he simply deleted that part of my reply and pretended it never happened. Pathetic really.)

Bill Snedden

Really? You're "happy to accept" the premise that formal logic cannot prove an external reality?

I guess it depends upon what one means by "prove." Certainly you can construct valid deductive and inductive arguments that prove an external reality. Whether or not they will be seen as conclusive proofs will depend upon what evidence supports the premises & how one's opponent will evaluate them, but that's true for every argument, not just the ones attempting to prove an external reality.

IMNSHO, your opponent is invoking solipsism as an answer to your argument. As he himself appears not to be a solipsist, that would seem to render his argument self-refuting.

As for fallacies, I can think of a couple it fits. Argumentum ad Ignorantium is certainly one. It would also seem to fit ignorantio elenchi, or irrelevant conclusion.

Regards,

Bill Snedden

worldling

Thanks, Bill. Actually, he is trying to portray me as a "radical skeptic". Having agreed on our terms (rationality = logic, illogical = irrational), he claims this:

"I'll happily concede all the points you make as long as you agree that on that basis believing in an external world is irrational, believing I exist is irrational, believing that you are embodied is irrational and believing that we are actually having this discussion is irrational.
If you concur with all those points, then I happily agree Christianity is
irrational."

Basically, he thinks it is "inappropriate" to examine the Christian hypothesis using logic as a tool. I am endeavouring at the moment to get him to say why, but it's hard work!

matthias j.

It most certainly is. Just think about it: he's saying Christianity actually _is_ an external reality and he's trying to prove that by saying that formal logic can't prove its non-existence becaus it _does exist_ as an external reality.

In short he's saying:

1 A PREM
2 C PREM
3 B PREM
4 A does not imply C
5 B = C

That's total rubbish logical speaking

BTW: I don't think the quoted person is a solipsist and neither are his arguments, he's just a heuristically challenged sceptic

worldling

Thanks, M J. I suppose that really is what he is claiming - and it does beg the question, in a big way.

Bill S, I should have addressed your first point:

I am sure you are abolutely right here, Bill. But I didn't want to go down that road. I have been debating this guy for over 3 years, and we are only just getting round to the point. He has been throwing red herrings at me for years - quantum physics, cosmology, the philosophy of science - and I do not want to get diverted into trying to logically demonstrate external reality with him.

I am homing in on the crucifixion/salvation hypothesis. I have even got him to admit that it doesn't stand up to logical scrutiny. Now he is trying to backpedal, and I don't intend to let him.

So rather than argue with his premise (which would take the heat off his theology), I am willing to assume it is true. The argument from the premise is fallacious anyway, so I don't have to try to prove it wrong. I think it might be denying the antecedent

worldling

No, it's not denying the antecedent at all. It IS begging the question

DoubleDutchy

That 's true. Still formal logic can (and is) very well be used to examine (the concept of) external reality. If it turns out to be logically consistent nothing is proven. If it is found to be inconstent then there is probably something wrong with the concept.
This is a good old non sequitur.
You can use logic to examine Christianty. If it turns out to be logically consistent nothing is proven.
If it is found to be inconstent (as sometimes is stated) then there is probably something wrong with the concept.

This stratagem is very common in high school geometry-proofs:
Assume X is the case. ( f.i. a triangle exists where a + b < c).
Examine ( using logic) the consequences.
You run into contradictions.
Follows: X is not the case.