by Hanno
Lewis held that possible worlds exist exactly like this one. His view entails that there are uncountably many things that exist in "uncountable infinities of donkeys, protons and puddles, of planets very like earth... small wonder if you are reluctant to believe it." Actualist possible worlds tries to use the power of modal logic using actual objects to ground talk of possible worlds. Actualists reject Lewis's view precisely because it is unbelievable.
There are at least three versions of actualist possible worlds semantics. According to Stalnaker, possible worlds are uninstantiated properties. Like the property of being a unicorn, the property exists even if there are no examples of unicorns. In logic-speak, an instance of the property is its instantiation. So uninstantiated properties are properties that nothing has. But for many philosophers, like Plato, the property can exist even if there is no instance.
Alvin Plantinga holds that possible worlds are complete states of affairs. These exist as abstract entities and have properties. The state of affairs "Quine's being a philosopher" has the property of obtaining, while the state of affairs "Quine's being a politician" has the property of not obtaining. Some states of affairs have the property of being possible, and others do not (though Plantinga does not state whether he thinks the imposible states of affairs non-the-less exist). Plantinga has a technical way of defining complete, but basically, for any state of affairs S, a complete state of affairs either includes either S or not S.
According to Robert Adams, possible worlds are not states of affairs, nor abstract objects, but complete, consistent sets of propositions. Propositions are actual intensional abstract entities, and these propositions have the property of being either true or false. A proposition like "Aristotle is tall" contains both the individual Aristotle and the property of being tall. Consistency in complete sets of propositions is inherently modal: It is consistent if it is possible to be true together.
Other philosophers will use a logical notion of consistency, ie, it is consistent iff there is a model which satisfies the set of sentences which mean the propositions. In such a case, anything that is logically consistent is possible, and vise versa. Adams was trying to avoid that conclusion. But the more empiricist philosophers (Hume, Quine, Carnap, early Wittgenstein) all accept that kind of view.
Finally, there are the fictionalists: possible worlds are fictional entities. Just as there are truths about Superman, fiction can ground truth. Just as the fiction of the ideal gas law gives us important truths, so fictional truths can be important. Talk of possibility is grounded on these kinds of important fictions. David Armstrong (not the one at McNeese) holds this kind of view, and cites Wittgenstein as his inspiration.
So possible worlds are: complete consistent states of affairs, complete consistent propositions, complete consistent sets of sentences or made up entities, each version grounding the truth of our modal claims and justifying modal logic.
Showing posts with label actualism. Show all posts
Showing posts with label actualism. Show all posts
Monday, November 10, 2008
Monday, October 20, 2008
Possible Worlds
by Hanno
On the way out of last weeks meeting, Robert asked me about possible worlds. Todd, Lord Matt and I were using the notion of a possible world in the previous weeks discussion, but we did not say much about them. So here goes:
A possible world is a complete way the world might have been. Suppose you could have been a rock star. Then there is a possible world at which you are in fact a rock star. That part is easy, and gives us a way to think about what might have been. But what are possible worlds?
The first philosopher to use possible worlds was Leibniz, long ago. He used them in part to answer the problem of evil: This is "the best of all possible worlds" and hence a good God would naturally chose this one to being into existence. For Leibniz, possible worlds were ideas in the mind of God. Imagine Him going through each of the ways the world might have been. When he finally reaches the best of all of them, he chooses to make that one real. Philosophers call that "actualizing" or "instantiating" that world. So this is the only real world, but the others exist as ideas in the mind of God. When you say "I could have been a rock star" you are saying that there is an idea in the mind of God, and in that idea, you are a rock star. Unfortunately, that world was not the best of all possible worlds, and hence you are actually stuck with only dreams.
Possible worlds lay dormant as a philosophical tool until revived by Saul Kripke in the 1950's and 60's. In order to deal with a difficult problem in Modal Logic (defining validity, and hence providing a semantics, for those curious), he reintroduced the notion of a possible world. Pressed later on their metaphysical stauts, Kripke said that a possible world simply was a counterfactual situation. A counterfactual is a conditional (if-then claim) where the antecedent is false. Example: If Germany had won the war, blah blah blah. That stipulates a possible world where Germany did win the war. On his view, these do not exist as ideas in the mind of God, or any where else. However, if possible worlds do not exist, in what way do they make modal claims true?
Enter David Lewis. Lewis (and I'm not making this up) was reading a work of science fiction in which someone creates a new invention which allows people to travel to other possible worlds. Inspired by this, Lewis defends what he calls "Modal Realism," the view that other possible worlds exist exactly like this one, just in a different space-time. Real people, real situations. What happens in those other worlds makes our claim about what might have been true. Our world is the actual world for us, but our world is a possible world for them, and their world is their actual world. For anything that you think might have been, there is a world, which exists just like this one, where that actually happened.
Actualism is the view that the only thing that exists is the actual world, and actualists reject Lewis's views. More next week, if there is any interest.
On the way out of last weeks meeting, Robert asked me about possible worlds. Todd, Lord Matt and I were using the notion of a possible world in the previous weeks discussion, but we did not say much about them. So here goes:
A possible world is a complete way the world might have been. Suppose you could have been a rock star. Then there is a possible world at which you are in fact a rock star. That part is easy, and gives us a way to think about what might have been. But what are possible worlds?
The first philosopher to use possible worlds was Leibniz, long ago. He used them in part to answer the problem of evil: This is "the best of all possible worlds" and hence a good God would naturally chose this one to being into existence. For Leibniz, possible worlds were ideas in the mind of God. Imagine Him going through each of the ways the world might have been. When he finally reaches the best of all of them, he chooses to make that one real. Philosophers call that "actualizing" or "instantiating" that world. So this is the only real world, but the others exist as ideas in the mind of God. When you say "I could have been a rock star" you are saying that there is an idea in the mind of God, and in that idea, you are a rock star. Unfortunately, that world was not the best of all possible worlds, and hence you are actually stuck with only dreams.
Possible worlds lay dormant as a philosophical tool until revived by Saul Kripke in the 1950's and 60's. In order to deal with a difficult problem in Modal Logic (defining validity, and hence providing a semantics, for those curious), he reintroduced the notion of a possible world. Pressed later on their metaphysical stauts, Kripke said that a possible world simply was a counterfactual situation. A counterfactual is a conditional (if-then claim) where the antecedent is false. Example: If Germany had won the war, blah blah blah. That stipulates a possible world where Germany did win the war. On his view, these do not exist as ideas in the mind of God, or any where else. However, if possible worlds do not exist, in what way do they make modal claims true?
Enter David Lewis. Lewis (and I'm not making this up) was reading a work of science fiction in which someone creates a new invention which allows people to travel to other possible worlds. Inspired by this, Lewis defends what he calls "Modal Realism," the view that other possible worlds exist exactly like this one, just in a different space-time. Real people, real situations. What happens in those other worlds makes our claim about what might have been true. Our world is the actual world for us, but our world is a possible world for them, and their world is their actual world. For anything that you think might have been, there is a world, which exists just like this one, where that actually happened.
Actualism is the view that the only thing that exists is the actual world, and actualists reject Lewis's views. More next week, if there is any interest.
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