Rescinded, see below.

My hypothesis is that it's a language issue. Somewhere back in time he or she mentioned he or she was not a native speaker.
I tried reading Philosophy in Korean which is my native language, but it was actually more difficult to understand. I think problem is the translation.
Why he or she didn't mention this when it was pointed out how 'therefore' was being interpreted i this thread, I have no idea. I realize that the parallel error is happening in the symbolic logic, but perhaps it is inspired by not really getting 'therefore' (which can be used a few ways), as somewhat tricky word in a second language.

You have this wrong. The logically entailed negation of 'I think, therefore I exist' is 'I don't exist, therefore I don't think' not 'I don't think therefore I don't exist'. — Janus

Could you forward your full explanation why it is?

It's a rookie mistake you're making. — Janus

You obviously don't seem know what had been tried there for the proof. Do you even understand what logical proofing means?

I guess that's a possible explanation. But the meaning seems clear. To put it another way:

'If I am thinking I must exist'
It follows that
'If I don't exist I am not thinking'.
It doesn't follow that
'If I not thiing I don't exist'

My hypothesis is that it's a language issue. Somewhere back in time he or she mentioned he or she was not a native speaker. — Bylaw

Your hypothesis make no sense. Do you reject the standard meaning of "therefore" from the dictionaries? Symbolic logic works for all the languages in the world.

'If I am thinking I must exist'
It follows that
'If I don't exist I am not thinking'.
It doesn't follow that
'If I not thiing I don't exist' — Janus

You seem to be just citing what is on the internet or textbook for symbolic logic truth table.
For proof process, you must apply your own reasoning to the statement you want to prove or disprove.

Anyhow it was the last attempt to make the dualist understand the core problems. As Banno put it correctly, Cogito is not logically provable. It is an intuition. It is a subjective psychological solipsistic statement.

I guess that's a possible explanation. But the meaning seems clear. To put it another way: — Janus

Please note. In the Internet truth table, P and Q has the truth values, which were given as either T or F. Hence they can make axiomatic assumptions. It is still assumptions based on the truth values given to P and Q.

But here, we are not assuming any truth values at all to P and Q. Hence we can make most realistic assumptions and assertions against the original assumption based on the reasonable inference. I hope you see my point in the proof process.

You were citing something you saw on the internet truth table, and citing that as if all proof process must follow that, or it is wrong was your claim, which was really wrong and silly.

They look very much like arguments to me. — Banno

They do to me too. Convincing or not, they are edifying. And much more than I know has likely been built upon, or because of, them.

Are you suggesting that the arguments in the Second Meditation are metaphors? — Banno

I recognize this may be excrutiating to some, maybe you. They can also be satisfying as poetry; read as metaphor. I believe you might have been rhetorical, so rather than offend you, I'll withhold any elaboration. But when you read the meditation, think of Descartes as an existing human being, grappling with a profound personal struggle. For my part, I defy you not to see the poetry.

Anyway, I respect where you're coming from and I won't trouble you with anything further on the topic.

Obviously, reading metaphysics strictly for its logic and reasoning is the orthodox approach.

No I wasn't citing anything, just applying ordinary logic.
IF I must exist in order to think (or do anything else for that matter) then it follows that there can be no thinking or anything else done by me if I don't exist.
It doesn't follow that if am not thinking or doing any other particular thing, that I don't exist.

Whether or not 'I think therefore I am' can be logically proven is irrelevant. It cannot be disproven by the spurious entailment you adduced.

Counting the seconds until corvus once again says "Read my posts over and over again until you accept that denying the Antecedent isn't a Fallacy", because apparently that's the extent of his skills at explaining his reasoning.

Did you notice his bizarre understanding of the word "therefore" already as well? It's going to make it hard to reason with him, if he doesn't have a solid grasp of the basic English words we use to talk about reason and logic.

It seems he will try anything to avoid facing the fact he is mistaken. He is either not acting in good faith or he is more obtuse than I can imagine

I think he is acting in good faith, good-ish anyway, but that his grasp of English in certain areas isn't as good as he'd like it to be and he's too proud to consider that he might be wrong, and his intuition for logic is clearly entirely compromised which inherently makes it very hard to reach him with logic. How can you explain to an illogical person that their reasoning is illogical? If they don't accept logic, then it's really really hard!

The word “therefore” can be a source of confusion, especially when translating philosophical or logical statements.
Causal Interpretation:
In English, “therefore” is a logical connective that indicates a conclusion drawn from preceding premises.
However, in Korean, the equivalent word “그러므로” (geureom-eoro) can sometimes be interpreted more causally or chronologically.
Korean speakers might associate it with a cause-and-effect relationship, even though the intended meaning is more about logical inference.
Context Matters:
The context in which “therefore” appears plays a crucial role.
Philosophical discussions often involve nuanced reasoning, and the precise meaning depends on the overall context and philosophical background.
Alternative Translations:
To emphasize the logical aspect, one could use alternative translations like “결론적으로” (gyeollonjeog-eulo), which directly means “conclusively.”
Using “그래서” (geulaeseo) is another option, which is less causal and more focused on the logical connection.

Or it could just be a coincidence that the native speakers here are pointing out thattherefore is being taken by Corvus as causal and chronological, when in fact this is not the case, AND this is real possibility for native speakers of Korean to take the word in those incorrect ways.

I don't know...his writing in English seems good enough for poor comprehension not to be a plausible explanation for his confusion. He doesn't seem to want to even consider the good explanations for why he is mistaken he has elicited.

we know his first language isn't English, and we know he thinks "a therefore b" can only mean "a happened and then later b happened". He has more confusions than that, yes, but this language hiccup is a big thing stopping him from understanding.

Do you think he can understand 'if a is the case then b must be the case'?

I was going to agree, but...

Editing post now I have time to take a look...

t=I think
e=I exist

P = I think, therefore I exist — Corvus

t→e

Q = I don't think, therefore I don't exist. — Corvus

¬t→¬e

And the syllogism is...

P - > Q — Corvus

(t→e)→(¬t→¬e)

Not Q — Corvus

¬(¬t→¬e)

therefore Not P (P is FALSE) — Corvus

⊢~(t→e)

Giving
(((t→e)→(¬t→¬e))∧¬(¬t→¬e))→¬(t→e)

Which is valid.
Check my working.

Also,

The logically entailed negation of 'I think, therefore I exist' is 'I don't exist, therefore I don't think' not 'I don't think therefore I don't exist'. — Janus

t→e negated is ¬(t→e))


but

¬(t→e) ↔ (¬e→¬t) is invalid, and
¬(t→e) ↔ (¬t→¬e) is invalid.


¬(t→e) ↔ (¬e∧t)?
(fixed link)

I can't follow it set out formally like that. I think an equivalent of 'I think therefore I am' is 'If I am thinking, then I must exist". 'If I am not thinking then I must not exist' does not follow, but 'if I don't exist, then I must not be thinking' does follow, as far as I can tell.

As I read it @Corvus purports to prove that 'I think therefore I am/ is false, but I think his purported proof is invalid. It doesn't seem appropriate to talk about 'I think therefore I exist' as being valid or invalid, because it is not really an argument, but a premise.

You could put it as

P 1: If I am thinking then I must exist
P2: I am thinking
C: Therefore I exist.

That seems valid but it may not be sound I suppose, although it is hard to see what is wrong with it. Perhaps Corvus misinterprets the argument as claiming that thinking is not only sufficient, but necessary for existence. I think that is a different argument. That would be 'If I am existing, then I must be thinking'.

no, he would get caught up on the word "then" as a time signifier.

your proof is treating (t→e)→(¬t→¬e) as a premise.

He thinks it's ALWAYS true. He thinks for all statements t implies e, it's always true that not t implies not e.

He even called that idea "modus ponens", that's how I know he thinks you can always do that for all implication statements.

(t→e)→(¬t→¬e) CAN be a premise in a valid proof, and it's synonymous with (t <-> e). But it hasn't been used that way

this is (completely understandably) misunderstanding what he meant by negation there. Negation isn't the word he should use, I'm not even sure if there is a word he should use.

He's talking about modus Tollens though.

P 1: If I am thinking then I must exist
P2: I am thinking
C: Therefore I exist.

That seems valid but it may not be sound I suppose, although it is hard to see what is wrong with it. — Janus

One problem I note is that "I" is not well defined. Does "I" refer to some immaterial thing which interacts with the pineal gland?

Of course we all have some conception(s) associated with "I", but how accurate is that conception?

I think an equivalent of 'I think therefore I am' is 'If I am thinking, then I must exist". 'If I am not thinking then I must not exist' does not follow, but 'if I don't exist, then I must not be thinking' does follow, as far as I can tell. — Janus

Ok, so

(t→e) ↔ (¬t→¬e) is invalid.
"I think therefore I am" is not equivalent to "I don't think therefore I am not".

And
(t→e)↔(¬e→¬t)
"I think therefore I am is equivalent to "I'm not, therefore I don't think"

But this is not the argument @Corvus presented in the quote.

your proof is treating (t→e)→(¬t→¬e) as a premise. — flannel jesus

Here's the argument quotes:

P = I think, therefore I exist.
Q = I don't think, therefore I don't exist.

P - > Q
Not Q (Q is FALSE)
therefore Not P (P is FALSE) — Corvus

(t→e)→(¬t→¬e) is a premise. it's P->Q.

(t→e)→(¬t→¬e) is a premise. it's P->Q. — Banno

It's not easy to see what you're saying here. It looks like you're saying

(t→e)→(¬t→¬e)

Is equivalent to saying

(t→e)

Or in other words, whenever you have

(t→e)

You must also have

(¬t→¬e)

Is that what you're saying?

It's not easy to see what you're saying here. It looks like you're saying

(t→e)→(¬t→¬e)

Is equivalent to saying

(t→e) — flannel jesus

Only if you misread what is writ.

I certainly did not write

(t→e)→(¬t→¬e) ↔ (t→e)

That's invalid.

Indeed, I am not saying anything of that sort, but pointing out that the argument Corvus uses appears valid.

I did say it wasn't easy - I was acknowledging the strong possibility that I misread. Can you please explain more what you mean by

(t→e)→(¬t→¬e) is a premise. it's P->Q. — Banno

Oh, you know what, I get it now.

He wasn't always formulating his argument like that, he did that mid conversation. That is of course a VALID argument, but the question is, where does that premise come from?

We know where Corvus gets it from - he gets it from a missapplication of modus ponens, where he denies the Antecedent.

I don't know how to do that any further than I already have.

P = I think, therefore I exist.
Q = I don't think, therefore I don't exist. — Corvus

P= (t→e)
Q= (¬t→¬e)

The first assumption:

P - > Q — Corvus

(t→e)→(¬t→¬e)

I can't see how to make that any clearer.

it's clear now, thank you. I did misread. See my last comment.