Heh, I can't tell whether this is irony or not. I didn't see that the thread was old before I responded to it. ETA: I see that you thanked me, so I think it was not irony!

Besides, it's better late than never. Especially when dealing with newbies such as the OP.

Do you know what the step from ?P→?Q to P→Q is called? I don't know. It's not convere (conversation?) that would be from P→Q to Q→P.

If this main operator signifies a material condition, then there is no rule of inference or replacement that allows a person to infer P→Q from ?P→?Q. Otherwise, denying the antecedent wouldn't be a formal fallacy, but it is.

I'm unsure what a conversation is but there is a word within logic called converse. Yet that has does not allow you to move from P→Q to Q→P. The converse, in this instance, is just redirecting the order of the letters. From the converse of P→Q is Q→P which logically identical to P→Q's inverse, ~P→~Q

You are quite right about that, it is better late than never. The best thing that can help out our fellow members is accuracy and reliability, especially on threads that do not generate as much traffic as they once did. As to the what the typo-ed inference is called, I don't have the slightest idea. Never seen it before. However, human ingenuity knows no bounds, so I will venture to conjure one up. Replacement rule... I dub thee "ignoring the obvious," or Obviare Veritas. Only by ignoring the obvious can we infer opposite truth values at our beckon whim within a closed system.

"An illegitimate step"? Why should it have a name?

I think the word you are looking for is "conversion".

I speculated that it might be called conversation because the word "converse" (I accidentally misspelled it before) is sometimes used.

Yes, I know that it is not a valid inference form.

Is "inverse" a synonym for "converse" or is there some difference?

I still don't know what the move from "P→Q" to "?P→?Q" is called.

I use "→" to mean material implication. I use "⇒" to mean logical implication. See this resource for symbols.

---------- Post added 11-10-2009 at 03:31 PM ----------



Heh.

Technically it's not "opposite truth values" since we did not even consider truth values here. There is a difference between "P is false" and "?P".

---------- Post added 11-10-2009 at 03:33 PM ----------



Because people use it. Just as when we give names to other invalid inferences also known as formal fallacies.

And yes, that's the name. Do you know a resource on these kind of inferences and their names?

Heh.

Technically it's not "opposite truth values" since we did not even consider truth values here. There is a difference between "P is false" and "?P".

Is "inverse" a synonym for "converse" or is there some difference?

I still don't know what the move from "P→Q" to "?P→?Q" is called.

Let's take this: P→Q. The converse of it is Q→P and the inverse is ~P→~Q. The inverse and converse, while logically equivalent, are not synonyms.