by Corry Shores
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[The following is summary of Priest’s text, which is already written with maximum efficiency. Bracketed commentary and boldface are my own, unless otherwise noted. I do not have specialized training in this field, so please trust the original text over my summarization. I apologize for my typos and other unfortunate mistakes, because I have not finished proofreading, and I also have not finished learning all the basics of these logics.]
Summary of
Graham Priest
An Introduction to Non-Classical Logic: From If to Is
Part I
Propositional Logic
2.
Basic Modal Logic
2.6.
Modal Realism
Brief summary:
(2.6.1) Modal realism is the view that possible worlds are real worlds that exist at different times and/or places. (2.6.2) The fact that modal realism is mind-boggling should not be a problem, because we allow modern physics to boggle our minds. (2.6.3) An objection to modal realism is that other possible worlds that are real would have to be physically related to our real world and thus be extensions of our world or co-partitions of one ultimate real world. The reply is that the fact that the other possible worlds are not spatially, temporally, or causally connected to our means they have no physical connection and thus cannot be extensions or co-partitions with our world. (2.6.4) Still, the objector can give the example of black holes where it is conceivable that there is a part of this world that is not spatially, temporally, or causally related to the rest of our world. (2.6.5) It can also be objected that we should not define possibility itself in terms of alternate reality or physically disconnected actuality, because intuitively we do not think that some actuality in our present world demonstrates its possibility in other times or places of our world.
[Modal Realism: Possible Worlds as Real but not Temporally and/or Spatially Convergent with Ours]
[Modal Realism’s Mind-Bogglingness as Acceptable]
[Objection to Modal Realism: Not Really Separate Worlds. Reply: Not Physically Connected, Because No Spatial, Temporal, or Causal Connection.]
[Objection Continued: Black Hole as Possible Counter-Example]
[Possibility as Not Being Physically-Alternate Actuality]
Summary
[Modal Realism: Possible Worlds as Real but not Temporally and/or Spatially Convergent with Ours]
[Modal realism is the view that possible worlds are real worlds that exist at different times and/or places.]
[Recall from section 2.5 some of the following ideas. Modal logic uses the intuitive notion of a possible world, but as we saw, it is formulated using mathematical machinery where it is not obvious what any of it has to do with the metaphysics of possible worlds. The assumption is that the mathematics somehow represents “something or other which underlies the correctness of the notion of validity” (p.28, section 2.5). For example:
no one supposes that truth is simply the number 1. But that number, and the way that it behaves in truth-functional semantics, are able to represent truth, because the structure of their machinations corresponds to the structure of truth’s own machinations. This explains why truth-functional validity works (when it does).
(p.28, section 2.5)
So Priest ended by asking, “what exactly, in reality, does the mathematical machinery of possible worlds represent? Possible worlds, of course (what else?). But what are they?” (p.28, section 2.5).] One explanation for what the mathematical entities for possible worlds represent are simply other real worlds that exist in different times or places:
The simplest suggestion (usually termed ‘modal realism’) is that possible worlds are things exactly like the actual world. They are composed of physical objects like people, chairs and stars (if any exist in those worlds), in their own space and time (if there are such things in those worlds). These objects exist just as much as you and I do, just in a different place/time – though not ones in this world.
(28)
[Modal Realism’s Mind-Bogglingness as Acceptable]
[The fact that modal realism is mind-boggling should not be a problem, because we allow modern physics to boggle our minds.]
As we said above in section 2.6.1, modal realism says that possible worlds are real worlds existing at different times/places than ours. Priest now notes that this can boggle our minds. But he adds that a lot of what is said in modern physics about reality can also boggle our minds. [Perhaps it can even make similar claims about other universes parallel to ours.] Priest then says that if modern physics is allowed to boggle our minds, why cannot metaphysics do that too (28)? [In fact, should metaphysics not even have more of a right, so to allow it greater freedom of conception that is useful in philosophical thinking?]
[Objection to Modal Realism: Not Really Separate Worlds. Reply: Not Physically Connected, Because No Spatial, Temporal, or Causal Connection.]
[An objection to modal realism is that other possible worlds that are real would have to be physically related to our real world and thus be extensions of our world or co-partitions of one ultimate real world. The reply is that the fact that the other possible worlds are not spatially, temporally, or causally connected to our means they have no physical connection and thus cannot be extensions or co-partitions with our world.]
Priest now notes a specific objection to modal realism. [It seems to be that we are confusing matters by calling such a world a different world. Supposing it to be real, it can never be more than a physical addition to or an extension to our world. Thus really we have one world of which ours is a part, and the other so-called possible worlds are other parts. The reply is then that these other possible worlds should really be thought of as different worlds, because they are unconnected by space, time, and causation. So, two worlds that have no spatial, temporal, and causal connection have no physical connection at all and thus cannot be connected as parts of a larger world. Let me quote, because I went a little beyond the text in my summary:]
Many arguments may be put both for and against this proposal – as they may be for all the views that I will mention. Here is one argument against. What makes such a world a different possible world, and not simply part of this one? The natural answer is that the space, time and causation of that world are unconnected with the space, time and causation of this world. One cannot travel from here to there in space or time; nor can causal processes from here reach there, or vice versa.
(29)
[Objection Continued: Black Hole as Possible Counter-Example]
[Still, the objector can give the example of black holes where it is conceivable that there is a part of this world that is not spatially, temporally, or causally related to the rest of our world.]
Priest now continues that line of objection, even in the face of the reply (for both, see section 2.6.3). He gives an example where there could conceivably be a part of our world which is not temporally, spatially, or causally connected with the rest of our world, namely, a black hole situation:
But why should that make it a different world? Suppose that because of the spatial geometry of the inside of a black hole, one could travel thence down a worm hole into a part of the cosmos with its own space and time; and suppose, then, that the worm hole closed up.We would not think of that region, now causally isolated from the rest, as a different possible world: merely an inaccessible part of this one.
(29)
[Possibility as Not Being Physically-Alternate Actuality]
[It can also be objected that we should not define possibility itself in terms of alternate reality or physically disconnected actuality, because intuitively we do not think that some actuality in our present world demonstrates its possibility in other times or places of our world.]
[Priest makes the point now in a different way, but I have trouble connecting it with the above objections. He now says we might object that we should not understand possibility in this world in terms of it actually happening at another place or time. So maybe the problem here is no longer with the notion of defining a possible world in terms of its alternate reality but now also with defining possibility itself in terms of physically alternate reality/actuality. Let me quote:]
The point may be put in a different way. Why should we think that something is possible in this world merely because it is actually happening at another place/time? I do not, after all, think that it is possible to see kangaroos in Antarctica merely because they are seen in Australia.
(29)
From:
Priest, Graham. 2008 [2001]. An Introduction to Non-Classical Logic: From If to Is, 2nd edn. Cambridge: Cambridge University.
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