cousin_it comments on Self-Congratulatory Rationalism - Less Wrong

51 Post author: ChrisHallquist 01 March 2014 08:52AM

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Comment author: cousin_it 01 March 2014 09:29:25AM *  0 points [-]

Just curious, how does Plantinga's argument prove that pigs fly? I only know how it proves that the perfect cheeseburger exists...

Comment author: Alejandro1 01 March 2014 04:06:06PM 3 points [-]

Copying the description of the argument from the Stanford Encyclopedia of Philosophy, with just one bolded replacement of a definition irrelevant to the formal validity of the argument:

Say that an entity possesses “maximal excellence” if and only if it is a flying pig. Say, further, that an entity possesses “maximal greatness” if and only if it possesses maximal excellence in every possible world—that is, if and only if it is necessarily existent and necessarily maximally excellent. Then consider the following argument:

  • There is a possible world in which there is an entity which possesses maximal greatness.

  • (Hence) There is an entity which possesses maximal greatness.

Comment author: TheOtherDave 01 March 2014 04:49:27PM 2 points [-]

This argument proves that at least one pig can fly. I understand "pigs fly" to mean something more like "for all X, if X is a typical pig, X can fly."

Comment author: Alejandro1 01 March 2014 05:28:02PM 4 points [-]

You are right. Perhaps the argument could be modified by replacing "is a flying pig" by "is a typical pig in all respects, and flies"?

Comment author: TheOtherDave 01 March 2014 09:24:25PM 1 point [-]

Perhaps. It's not clear to me that this is irrelevant to the formal validity of the argument, since "is a typical pig in all respects, and flies" seems to be a contradiction, and replacing a term in an argument with a contradiction isn't necessarily truth-preserving. But perhaps it is, I don't know... common sense would reject it, but we're clearly not operating in the realms of common sense here.

Comment author: ChrisHallquist 03 March 2014 08:05:06AM 3 points [-]

Plantinga's argument defines God as a necessary being, and assumes it's possible that God exists. From this, and the S5 axioms of modal logic, it folllws that God exists. But you can just as well argue, "It's possible the Goldbach Conjecture is true, and mathematical truths are if true necessarily true, therefore the Goldbach Conjecture is true." Or even "Possibly it's a necessary truth that pigs fly, therefore pigs fly."

(This is as much as I can explain without trying to give a lesson in modal logic, which I'm not confident in my ability to do.)

Comment author: cousin_it 03 March 2014 10:18:20AM 1 point [-]

"Possibly it's a necessary truth that pigs fly, therefore pigs fly."

That's nice, thanks!

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