Which error is this?

[LadyForCamus]

Here's a well-founded version of the same argument so you can see that the logic actually works:

1) If water does not exist then fish cannot exist. (Well founded P1 because fish obviously require water to survive)
2) Fish exist.
3) Therefor water must exist.

I see it now, thank you! You guys rock. I wouldn’t have figured this out anywhere else.

[brewer]

LFC, what theist are you doing battle with?

[LadyForCamus]

LFC, what theist are you doing battle with?

Just some dippity do on FB. It’s already over. 😂

[Fake Messiah]

It's an unfounded premise.

1) If Thor does not exist then thunder does not exist. <--- Why assume that this is true?
2) Thunder exists.
3) Thor must exist.

That's entirely sound and a decent example of denying the consequent. If Thor is the only possible cause of thunder then the existence of thunder logically requires the existence of Thor.

But it depends entirely on the validity of P1. The instant that you introduce the possibility that thunder can exist without Thor then denying the consequent fails and the argument collapses.

So what is the difference between the unfounded premise and circular reasoning?

Is it that circular reasoning also contains unfounded premise, like

[GrandizerII]

It's an unfounded premise.

1) If Thor does not exist then thunder does not exist. <--- Why assume that this is true?
2) Thunder exists.
3) Thor must exist.

That's entirely sound and a decent example of denying the consequent. If Thor is the only possible cause of thunder then the existence of thunder logically requires the existence of Thor.

But it depends entirely on the validity of P1. The instant that you introduce the possibility that thunder can exist without Thor then denying the consequent fails and the argument collapses.

So what is the difference between the unfounded premise and circular reasoning?

Is it that circular reasoning also contains unfounded premise, like

The difference is that a non-circular argument can also have an unfounded premise. It is not exclusive to circular arguments.

And even if you want to argue that the argument stated in the OP is circular, there is still nothing wrong with the structure. A circular argument can still be logically valid.

[polymath257]

I wasn’t exactly sure which sub forum to put this in. 

Which formal structural error is this? Is it still considered Affirming the Consequent if they’re using negations of p and q in the second premise?

The argument form

!p -> !q
q
------
p

is valid. In fact, !p->!q is logically equivalent (contrapositive) to q->p.  Then, with q, we can, in fact, conclude p.

The problem, as others have pointed out, is the assumption that !p->!q. This is what makes the argument unsound.

[brewer]

LFC, what theist are you doing battle with?

Just some dippity do on FB. It’s already over. 😂

Dippity Do??? My guess is Mormon/JW/SDA.

[Paleophyte]

It's an unfounded premise.

1) If Thor does not exist then thunder does not exist. <--- Why assume that this is true?
2) Thunder exists.
3) Thor must exist.

That's entirely sound and a decent example of denying the consequent. If Thor is the only possible cause of thunder then the existence of thunder logically requires the existence of Thor.

But it depends entirely on the validity of P1. The instant that you introduce the possibility that thunder can exist without Thor then denying the consequent fails and the argument collapses.

1) If Thor does not exist then thunder might be caused by something else.
2) Thunder exists.
3) Thunder has an indeterminate cause and we can't say a bloody thing about Thor's involvement.

It's a lovely example of an argument that's logically sound but entirely unfounded. Which looks to have been the point of the person posting the obviously ridiculous Thor exists argument.

It's argument by assertion. Thor is the only possible source of thunder. God is the only possible source of morality. And that's arse backwards since they have to prove that god exists first before assigning him attributes.

I thought a formal argument was sound if and only if the structure is valid and all the premises are true? If we assume for the sake of the argument that P1 and P2 are true, then like you said, that doesn’t necessarily lead to a true conclusion. Doesn’t that mean, by definition, the argument is invalid? Not trying to be a pest here, just trying to learn.

Sorry, I've confused my terminology there. You're right. The argument is valid (the logic is internally consistent) but since the premises aren't all true it's not sound.

[LadyForCamus]

I wasn’t exactly sure which sub forum to put this in. 

Which formal structural error is this? Is it still considered Affirming the Consequent if they’re using negations of p and q in the second premise?

The argument form

!p -> !q
q
------
p

is valid. In fact, !p->!q is logically equivalent (contrapositive) to q->p.  Then, with q, we can, in fact, conclude p.

The problem, as others have pointed out, is the assumption that !p->!q. This is what makes the argument unsound.

Thank you, Poly. I was so sure the structure was invalid I didn’t bother to check it against the valid ones! Hehe.

[Rev. Rye]

It took me trying to check it against multiple formal fallacies for half an hour to figure out that the problem was less the form than the initial premise. I can remember taking some philosophy class and hearing the teacher talk about how sometimes, people can construct arguments that seem airtight until you realise "oh, wait, the initial premise actually doesn't work." I think this was during a unit on Kant, and I think that the OP's argument was an example in miniature.