BASIC SYMBOLIZATION: THE OPERATORS AND, OR, and NOT
Sorry, it's been a while - I've been quite busy. We're not that far from constructing arguments using propositional logic; but before we can do that, we have to be able to symbolize these propositions. We can also connect propositions together using logical operators, which we'll see.
In logic, a capital letter, like 'P' would stand for an actual proposition. When you symbolize an argument from natural language to logic, you want to include a dictionary so people will know what you're talking about. Here's the basic idea.
Dictionary:
S: Socrates is a man.
A: All men are mortal.
M: Socrates is mortal.
The argument could then go like this:
1. S
2. A
/ M
It's not a very interesting argument (and, it turns out, it's not one you can prove in this logical system), but it's there. And it's symbolized. Now, I could have used any letters I wanted to symbolize those propositions - I just picked those because they made it easier to remember. Just note that you can use the same letter twice - that would get confusing real quick.
LOGICAL OPERATORS
With our propositions symbolized, we can now connect them using our logical operators. To do this kind of logic, you actually only need 2 logical operators. But most texts use 5; I'm going to cover the first 3.
NOT:
When we symbolize a proposition, we want to make sure it's atomic. That means that it's a basic as it can get - we don't want any logical operators contained in there.
So here's a sentence:
I am not at home.
The way we would symbolize this statement is ~H (read: not H). That little squiggle is called a tilde (TIL-duh) and in logic it just means "not." It says of whatever it's connected to that whatever it says is false.
So if C = The Cubs will win the World Series, then ~C = The Cubs won't win the World Series.
It important to take any negations (nots) out of your sentences and just use the ~ to symbolize the negation.
There are some exceptions to this rule, but they're very minor. Let's take the sentence:
Dan thinks that this won't work.
If we took out the not and symbolized it like ~D, then someone might read it as "Dan doesn't think this will work" or "It's not the case that Dan thinks this will work." But those aren't quite right - if you don't see why, just trust me on this one :)
But there's no need to worry because we don't care what Dan thinks. These kinds of sentences aren't even dealt with logically. You tell a logician that Dan thinks this won't work and the logician will ask you, so what?
AND
The logical operator and, which we use a ^ for, works just like an 'and' in grammar - it connects two sentences together.
So,
B: Bill went to the park. J: Jim went to the park.
We can put these together like this:
B ^ J : Bill and Jim went to the park.
And if you're trying to symbolize a statement with an 'and' in it, then you're probably going to have to represent it logically. It's important to note here that the logical 'and' just connects two sentences together - it says that they're both true.
So the sentence:
Bill went to the park but Jim didn't.
Would be:
B ^ ~J : Bill went to the park and Jim didn't go to the park.
Notice we represented 'but' just like an 'and'. But if you understand what the sentence is saying, it makes sense why we do this. Logically, they're the same thing. Just like 'Bill went to the park although Jim didn't'. As long as the sentence is saying that both parts are true, you use 'and'.
OR
In logic, the 'or' operator says that either one thing is true or the other is true or that they're both true. So the only time an 'or' statement in logic is false (at least this logic) is when both sides (disjuncts) are false.
So if our statement was B v J then it would be true is Bob or Jim went to the park - even if they both did. It would only be false if neither of them went to the park.
You can do a lot of symbolizing with just these operators. Look around and find stuff to symbolize if you want some practice. And if you guys want me to post a few for you to work out, just let me know. I just figured there are already plenty of sentences out there :)