Article: Negation and disjunction (Dr. Van Cleave)

By Matthew Van Cleave

In this section we will introduce the second and third truth-functional connectives: negation and disjunction.  We will start with negation, since it is the easier of the two to grasp.

Negation

Negation is the truth-functional operator that switches the truth value of a proposition from false to true or from true to false.  For example, if the statement “dogs are mammals” is true (which it is), then we can make that statement false by adding a negation.  In English, the negation is most naturally added just before the noun phrase that follows the linking verb like this:

But another way of adding the negation is with the phrase, “it is not the case that” like this:

Either of these English sentences expresses the same proposition, which is simply the negation of the atomic proposition, “dogs are mammals.”  Of course, that proposition is false since it is true that dogs are mammals.  Just as we can make a true statement false by negating it, we can also make a false statement true by adding a negation.  For example, the statement, “Cincinnati is the capital of Ohio” is false.  But we can make that statement true by adding a negation:

There are many different ways of expressing negations in English.  Here are a few ways of expressing the previous proposition in different ways in English:

Each of these English sentences express the same true proposition, which is simply the negation of the atomic proposition, “Cincinnati is the capital of Ohio.”  Since that statement is false, its negation is true.

There is one respect in which negation differs from the other three truth-functional connectives that we will introduce in this chapter.  Unlike the other three, negation does not connect two different propositions.  Nonetheless, we call it a truth-functional connective because although it doesn’t actually connect two different propositions, it does change the truth value of propositions in a truth-functional way.  That is, if we know the truth value of the proposition we are negating, then we know the truth value of the resulting negated proposition.  We can represent this information in the truth table for negation.  In the following table, the symbol we will use to represent negation is called the “tilde” (~).  (You can find the tilde on the upper left-hand side of your keyboard.)

p	~p
T	F
F	T

 

This truth table represents the meaning of the truth-functional connective, negation, which is represented by the tilde in our symbolic language.  The header row of the table represents some proposition p (which could be any proposition) and the negation of that proposition, ~p.  What the table says is simply that if a proposition is true, then the negation of that proposition is false (as in the first row of the table); and if a proposition is false, then the negation of that proposition is true (as in the second row of the table).

As we have seen, it is easy to form sentences in our symbolic language using the tilde.  All we have to do is add a tilde to left-hand side of an existing sentence.  For example, we could represent the statement “Cincinnati is the capital of Ohio” using the capital letter C, which is called a constant.  In propositional logic, a constant is a capital letter that represents an atomic proposition.  In that case, we could represent the statement “Cincinnati is not the capital of Ohio” like this:

Likewise, we could represent the statement “Toledo is the capital of Ohio” using the constant T.  In that case, we could represent the statement “Toledo is not the capital of Ohio” like this:

We could also create a sentence that is a conjunction of these two negations, like this:

Can you figure out what this complex proposition says?  (Think about it; you should be able to figure it out given your understanding of the truth-functional connectives, negation and conjunction.)  The propositions says (literally): “Cincinnati is not the capital of Ohio and Toledo is not the capital of Ohio.”  In later sections we will learn how to form complex propositions using various combinations of each of the four truth-functional connectives.  Before we can do that, however, we need to introduce our next truth-functional connective, disjunction.

Disjunction

The English word that most commonly functions as disjunction is the word “or.”  It is also common that the “or” is preceded by an “either” earlier in the sentence, like this:

What this sentence asserts is that one or the other (and possibly both) of these individuals tracked mud through the house.  Thus, it is composed out of the following two atomic propositions:

If the fact is that Charlie tracked mud through the house, the statement is true.  If the fact is that Violet tracked mud through the house, the statement is also true.  This statement is only false if in fact neither Charlie nor Violet tracked mud through the house.  This statement would also be true even if it was both Charlie and Violet who tracked mud through the house.  Another example of a disjunction that has this same pattern can be seen in the “click it or ticket” campaign of the National Highway Traffic Safety Administration.  Think about what the slogan means.  What the campaign slogan is saying is:

This is a kind of warning: buckle your seatbelt or you’ll get a ticket.  Think about the conditions under which this statement would be true.  There are only four different scenarios:

Your seatbelt is buckled	You do not get a ticket	True
Your seatbelt is not buckled	You get a ticket	True
Your seatbelt is buckled	You get a ticket	True
Your seatbelt is not buckled	You do not get a ticket	False

The first and second scenarios (rows 1 and 2) are pretty straightforwardly true, according to the “click it or ticket” statement.  But suppose that your seatbelt is buckled, is it still possible to get a ticket (as in the third scenario—row 3)?  Of course it is!  That is, the statement allows that it could both be true that your seatbelt is buckled and true that you get a ticket.  How so?  (Think about it for a second and you’ll probably realize the answer.)  Suppose that your seatbelt is buckled but your are speeding, or your tail light is out, or you are driving under the influence of alcohol.  In any of those cases, you would get a ticket even if you were wearing your seatbelt.  So the disjunction, click it or ticket, clearly allows the statement to be true even when both of the disjuncts (the statements that form the disjunction) are true.  The only way the disjunction would be shown to be false is if (when pulled over) you were not wearing your seatbelt and yet did not get a ticket.  Thus, the only way for the disjunction to be false is when both of the disjuncts are false. 

These examples reveal a pattern: a disjunction is a truth-functional statement that is true in every instance except where both of the disjuncts are false.  In our symbolic language, the symbol we will use to represent a disjunction is called a “wedge” (v).  (You can simply use a lowercase “v” to write the wedge.)  Here is the truth table for disjunction:

p	q	p v q
T	T	T
T	F	T
F	T	T
F	F	F

 

 

 

 

 

As before, the header of this truth table represents two propositions (first two columns) and their disjunction (last column).  The following four rows represent the conditions under which the disjunction is true.  As we have seen, the disjunction is true when at least one of its disjuncts is true, including when they are both true (the first three rows).  A disjunction is false only if both disjuncts are false (last row).

As we have defined it, the wedge (v) is what is called an “inclusive or.”  An inclusive or is a disjunction that is true even when both disjuncts are true.  However, sometimes a disjunction clearly implies that the statement is true only if either one or the other of the disjuncts is true, but not both.  For example, suppose that you know that Bob placed either first or second in the race because you remember seeing a picture of him in the paper where he was standing on a podium (and you know that only the top two runners in the race get to stand on the podium).  Although you can’t remember which place he was, you know that:

This is a disjunction that is built out of two different atomic propositions:

Although it sounds awkward to write it this way in English, we could simply connect each atomic statement with an “or”:

That sentence makes explicit the fact that this statement is a disjunction of two separate statements.  However, it is also clear that in this case the disjunction would not be true if all the disjuncts were true, because it is not possible for all the disjuncts to be true, since Bob cannot have placed both first and second.  Thus, it is clear in a case such as this, that the “or” is meant as what is called an “exclusive or.”  An exclusive or is a disjunction that is true only if one or the other, but not both, of its disjuncts is true.  When you believe the best interpretation of a disjunction is as an exclusive or, there are ways to represent that using a combination of the disjunction, conjunction and negation.  The reason we interpret the wedge as an inclusive or rather than an exclusive or is that while we can build an exclusive or out of a combination of an inclusive or and other truth-functional connectives (as I’ve just pointed out), there is no way to build an inclusive or out of the exclusive or and other truth-functional connectives.  We will see how to represent an exclusive or in an upcoming section.

Do Quiz 7b before moving further in the course.