1. Metaphysics Seminar 8: Modality (1)
Henry Taylor

2. Recap
We’ve looked at personal identity: metaphysics as applied to the self.
We’ve looked at ontology: metaphysics as applied to the world as a whole.
We’re now going to look at modality: metaphysics as applied to other possible stuff.

3. Modality
Modality is the study of possibility, actuality and necessity.
We make all sorts of claims about how the world is:
‘The Eiffel Tower is in Paris’
’Die Hard is the best Christmas film of all time’
Etc.

4. Modality
We also make claims about what might be the case, what the world could be like.
‘I could go to the seminar today, or I could go to the pub’
This is a statement about two alternative possibilities.
‘If I hadn’t come to the seminar today, I would have gone to the pub’
This is also a statement about possibilities.
It’s a special one: a counterfactual.

5. Modality
Counterfactuals are cases where we make claims about what would have happened if something that didn’t actually happen had in fact happened.
‘If Britain had lost the battle of Britain, Germany would have won the second world war.’
You have a claim about something that didn’t actually happen.
Followed by a claim about what the world would have been like if that thing had in fact happened.

6. Modality We also make claims about what is impossible.
‘I could have gone to the pub today, but I couldn’t have flown to the moon’
‘It is possible that Germany could have won world war II, but it’s not possible for 1+1 to equal 5’
And so on…

7. Modality We also can make sense of the idea of a necessary truth.
This is a truth where it is not possible for it to be false.
So ‘1+1=2’ is necessarily true
‘The second largest empire in the world was the Mongul empire’ is true, but not necessarily true (the Mongul empire might have never existed, or the Roman one could have grown bigger, or something).

8. Modality
Claims about the possible and necessary really seem to be part of both normal discourse and science:
‘If the speed of light were slower…’
‘If Planck’s constant were different…’

9. Modality
Possibility and necessity interact with each other very closely.
‘Necessarily P’ just means ‘It is not possible that not-P’
‘Necessarily not-P’ just means ‘it is not possible that P’
‘Possibly P’ just means ‘it is not necessary that not P’
‘Possibly not-P’ just means ‘it is not necessary that P’
So claims about necessity and possibility can always be translated into each other.

10. Modality Everything that is necessary is possible:
‘1+1=2’ is both necessary and possible.
Not everything possible is necessary
‘Clinton could have won the presidency’ expresses a possibility, but it’s clearly not necessary.

11. Modality We also have actuality: stuff that is actually the case.
Everything that is actual is possible.
Some things that are actual are necessary.
Some things are possible but not actual.
Nothing that is necessary is not actual.

12. Modality
Warning: these are the dominant ways of thinking about actuality, possibility and necessity.
Not everyone agrees with all of those claims.
And if you wanted to look at whether they’re true, that would make a great essay…

13. Questions/comments?

14. Modality
A puzzle: how do we account for the apparent truth of modal claims?
If we make a claim about actuality ‘the cat is sitting on the mat’, it’s clear what could make that true and false: it’s true if the cat really is sitting on the mat, false if it’s not.
But what about ‘I could have gone to the pub rather than coming to the seminar’, what makes that true?
What about ‘if Britain had lost the Battle of Britain, then Germany would have won WWII’? What makes that true or false?

15. Modality
What do we think? What makes modal claims true, if anything?

16. Modality: Lewis David Kellog Lewis. 1941-2001. American philosopher
Most famous books: On the Plurality of Worlds and Counterfactuals.

17. Modality: Lewis Lewis was one of the greatest philosophers in history.
He had a huge impact on metaphysics, logic, philosophy of mind, philosophy of science, ethics, epistemology, philosophy of language….
He was a system builder: he had a grand vision of everything, and used this system to solve philosophical problems. This makes him like Kant, Hume, Aristotle, Wittgenstein or Plato.
On the Plurality of Worlds (1986) is a masterpiece on a level with any work in all of philosophy: anything by Kant, Hume, Aristotle, etc.

18. Modality: Lewis
He famously developed modal realism: the belief in other ‘possible worlds’.
This world is just one world in a vast sea of possible worlds.
In this world, Britain won the battle of Britain.
In another one, Britain lost.
In this world you chose to come to the seminar.
In another possible world you went to the pub.
And so on….

19. Modality: modal realism
Once you believe this, it’s easy to account for possibility and necessity.
Something is possible if and only if it occurs at some possible world.
Something is necessary if and only if it occurs at all possible worlds.
And then you can define everything else in terms of those things.

20. Modality: modal realism
Actuality can be accounted for too.
‘Actual’ is different depending on the world.
So, for something to be actual at this world means that it really is the case in this world.
For something to be actual at some other world, X, means that it really is the case in that world.
When philosophers say ‘actual’, they almost always mean ‘actual at this world’

21. Modality: modal realism
These worlds are (for Lewis), not useful fictions, they’re not ways of talking, or models, or thoughts, or anything like that.
They are real: as real as a chair, a cucumber or a horse.
That’s modal realism.

22. Modality: modal realism
The worlds are all spatiotemporally isolated from one another.
They have no spatial or temporal connections to each other at all.
And nothing in one world has any spatiotemporal relation to anything in another world at all either.

23. Modality: modal realism
They have some relations: closeness.
Closeness is dictated by similarity: more similar worlds are closer.
So, imagine you have 100,000 hairs on your head in this world. A possible world that is otherwise just like ours, but that you have 100,001 hairs is pretty close to our world.
And a world where the universe exploded a million years after it came into being, and the Earth was never formed, is obviously not very ‘close’ to our world, that’s pretty far away.

24. Modality: modal realism
Questions/comments?

25. Modal realism and properties.
Remember from seminar 4, we looked at universals and properties in general.
We looked at ostrich nominalism and realism about universals.
One of the best reasons to believe in something in metaphysics is if it can explain something that metaphysicians (or indeed anyone) needs to explain.
Lewis gives us tonnes of reasons to believe in modal realism based on this: one of them is that modal realism can give us a good account of properties.

26. Modal realism and properties.
How does this work?
Well, imagine that you wanted to be a nominalist, but you didn’t want to be an ostrich nominalist like Quine or Devitt.
But you still don’t want to believe in universals or tropes.
Well, you can say this: ’properties do exist, but they aren’t universals or tropes, they are sets of objects’

27. Modal realism and properties.
So, the property ‘red’ is the set of all red objects.

28. Modal realism and properties.
So, two objects resemble each other if they fall into the same set.
They are both red if they both fall into the set of red things.
Simple as that: problem of resemblance solved!

29. Modal realism and properties.
The problem of coextensive properties.
Some exciting words for you: Renate means ‘creature with a kidney’ and cordate means ‘creature with a heart’.
The set of ‘renates’ has the same members as the set ‘cordates’.
If two sets have the same members, they are the same set.
So (assuming that the set is the property) then the property of being a renate is the property of being a cordate.
But clearly they’re different properties.

30. Modal realism and properties.
Here is where modal realism comes in.
Properties are identical with sets of actual and possible entities
So in some possible world there is an animal with a heart and no kidney.
So the sets don’t have the same members at all.

31. Questions/Comments?

32. Modal realism and properties.
But this doesn’t work in all cases.
Consider triangularity and trilinearity: having three internal angles, and being bounded by three lines.
The set of triangular objects and trilinear objects is the same in all possible worlds.
So even Lewis’ theory predicts they’re the same property.
But clearly they’re not!

33. Modal realism and properties.
Lewis’ reply:
Look, we can use the word ‘property’ in all sorts of ways.
On some meanings, ‘triangularity’ and ‘trilinearity’ would be the same property.
And Lewis’ theory gives you a perfect account of this usage.
There may be other uses, but that’s beyond the theory.

34. Discussion What do you think of Lewis’ modal realist view?
What do you think of Lewis’ ‘indexical theory of actuality’, is it convincing?
Are there any objections to modal realism that you can think of?
Are you convinced by how Lewis applies the theory to the problem of properties?
What other advantages of the theory can you think of?