Q&A: Modal Logic
Modal Logic
Question
Hello Rabbi,
I saw a YouTube video about two philosophical theses, necessity and possibility (I don’t know if that’s a good translation). Possibilism is what sounds reasonable to me, meaning that there are entities whose existence is possible. Necessitism says that the existence of all entities is necessary, except that their actual realization in reality is possible rather than necessary. From the middle of the video onward, he presents a formal modal-logic argument that necessitism is correct. I’d be glad to hear the Rabbi’s opinion about his argument. The argument is based on the following formula (from Wikipedia), where they also mention this argument.
https://en.m.wikipedia.org/wiki/Barcan_formula
I think the solution is to deny the validity of the above formula, and I’d be glad to hear your opinion.
Best regards,
Gilad
Answer
I don’t answer questions that send me off to do homework. If you want to ask, then please do me the courtesy of presenting the argument and then formulating a question about it.
As for fallacies and analysis of modal-logic arguments, there are several columns on the site that deal with this. See columns 580, 160, 301, and others.
At first glance, it seems that he is describing a known argument that comes up in connection with the Barcan formula. I may address it in the future.
Discussion on Answer
A detailed answer is here: https://mikyab.net/posts/85953/#comment-81310
Thanks for the answer. I thought my own formulation would be inaccurate, and I think this argument might interest you anyway.