## Thursday, May 08, 2014

### Elements of Modal Logic II

Part I

Let's do a little more carefully what we did in the previous post. To do this, I will add another kind of table, which I will call the Reference Table. It's where we'll find our modal information. (Depending on what you are doing, the Reference Table is sometimes set apart, and sometimes is just one table among man. For instance, you could be doing something with times, and want to make the Reference Table 'now' -- then the Reference Table is one of the tables of times. We could also have things set up so that any table can be the Reference Table for other tables. In the example that follows, the Reference Table is just a different sort of thing from the tables it describes.) The Reference Table can describe different things, depending on what we are doing. One of the most important things in modal logic is often to know what your Reference Table is; a lot of mistakes in modal logic are caused by switching Reference Tables, or not having a clear idea of what the Reference Table is.

The single most important thing in modal logic, however, is to know exactly what all your other tables are supposed to describe. Suppose you want to compare the Tolkien books on the top shelf of your bookcase; and, as it happens, you have three: The Hobbit, The Lord of the Rings, and The Silmarillion. We will therefore have one table for each of these. We can make a table for each of these, and put down some of the things we know about the characters in those books.

TABLE 1: The Hobbit
Bilbo Baggins is mentioned.
Gandalf is mentioned.
Elrond is mentioned.

TABLE 2: The Lord of the Rings
Bilbo Baggins is mentioned.
Gandalf is mentioned.
Elrond is mentioned.
Frodo Baggins is mentioned.

TABLE 3: The Silmarillion
Elrond is mentioned.
Gandalf is mentioned.
Frodo Baggins is mentioned.

Now, if this is all we know about the characters, we still can put some information on our Reference Table. The rules will be this: (1) If we would find a statement on any table there might be, we can list it in our Reference Table as Box (□); (2) If there's a statement definitely on some table somewhere, we can list it as Diamond (◇). (Note that rule 1 is tentative in a way rule 2 is not, since it doesn't actually tell us that there are any other tables besides the Reference Table, while rule 2 always gives us a table; the reason for this difference is complicated, and somewhat arbitrary, but it's the way many systems are set up, so we'll go with it for now.) The Box tells us that, for the things we are talking about, something 'is always true' or 'is true everywhere' or 'must be true'; the Diamond tells us that something is true 'sometimes' or 'somewhere', or else that it 'can be true'. How we translate it depends on what we're talking about. Here Box tells us about every Tolkien book on the top shelf and Diamond tells us about some Tolkien book or other on the top shelf.

REFERENCE TABLE: Tolkien Books on the Top Shelf
□(Elrond is mentioned)
□(Gandalf is mentioned)
◇(Bilbo Baggins is mentioned)
◇(Frodo Baggins is mentioned)

This is not an exhaustive list of statements we could make, since there are lots of others, like ◇(Elrond is mentioned and Gandalf is mentioned and Bilbo Baggins is mentioned and Galadriel is mentioned and Frodo Baggins is mentioned), which is a description of Table 2, but this will do for our purposes, since we're keeping things simple for now. (Note, too, why our Reference Table is separate from the other tables: 'Tolkien Books on the Top Shelf' is not a Tolkien book on the top shelf.)

So now we've taken our original tables, where there are no modal statements, and created a table of modal statements out of them. So far, this is just a way to do what we did last time. But in many cases of modal reasoning, we aren't just summing up our information with modal statements; we are working backwards from modal statements to try to see what they tell us.

So let's forget our first three tables for a moment, and suppose that someone else did all the work to get the Reference Table, and we are trying to reconstruct the information in the other tables just from what the Reference Table tells us, without knowing anything about what the books are. Because our Reference Table does not describe the other tables in exact terms, we will not be able to reconstruct them exactly. Let's see how close we can get, though.

REFERENCE TABLE: Tolkien Books on the Top Shelf
□(Elrond is mentioned)
□(Gandalf is mentioned)
◇(Bilbo Baggins is mentioned)
◇(Frodo Baggins is mentioned)

We'll do the Diamond statements first, because each one tells us something about some table somewhere. But, they don't tell us which tables. The statements might be true for the same table, but they might not be. And since we have three Diamond statements, we get three tables, which might be different or might not, and which might be all of the original tables or only some of them:

SOME TABLE OR OTHER
Bilbo Baggins is mentioned.

SOME TABLE OR OTHER
Frodo Baggins is mentioned.

SOME TABLE OR OTHER

But our Box statements still need to be added. By rule 1, they tell us that any tables the Diamond statements give us have the other statements, too. So now our tables look like:

SOME TABLE OR OTHER
Bilbo Baggins is mentioned.
Elrond is mentioned.
Gandalf is mentioned.

SOME TABLE OR OTHER
Frodo Baggins is mentioned.
Elrond is mentioned.
Gandalf is mentioned

SOME TABLE OR OTHER
Elrond is mentioned.
Gandalf is mentioned.

And remember, just from what our Reference Table tells us, we don't know if these are all the same table, or if two of them are the same, or if none of them are the same; likewise, we don't know whether these are all the tables or just some of them. The information in our Reference Table was very incomplete and not very precise. But it still gave us enough information to reconstruct something about the Tolkien books on the top shelf.

Most reasoning in modal logic is like this last example: we have a Reference Table and are trying to see what its implications are. You can think of it like a puzzle, in which the Reference Table is a list of clues that someone else gave you, and you are trying to see what those clues tell you.

In the next post we'll look at how some slightly more complicated cases work. What you'll find, though, is that we've pretty much covered all the essentials of basic modal reasoning -- it all works exactly like this, and more complicated cases are only more complicated because they give us more information to use.

to be continued

1. Future Student of You and Me Both: "Mr.-Blah-Blah-I-Don't-Take-Notes, are the things we write on tables just things that are TRUE about the books? Or anything at all we could say about the books?"

"And what's a Silly Mellon?" (ok, more likely to happen to me than to you)

2. branemrys4:45 PM

Fortunately the first question is easy enough to answer. They are things that are true about the books when you're interested in what's true about the books; but they can be something else when you're interested in something else.

I wouldn't be surprised if I got the second question, though. I've had students who couldn't name any book written by Lewis Carroll and who had never heard of Uncle Tom's Cabin.

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