Read this tutorial, which will introduce you to truth tables. Truth tables are an objective way of determining the validity of an argument as a whole when the argument is expressed symbolically.
Complete the exercises for this tutorial and check your answers.
We start with truthtables for the sentential connectives in sentential logic (SL). A truthtable shows how the truthvalue of a complex wellformed formula (WFF) depends on the truthvalues of its component wellformed formulas (WFFs).
So what are truthvalues?
In sentential logic (SL) there are only two truthvalues: T and F, which stands for truth and falsity. To say that a statement has truthvalue T is just to say it is true. To say that its truthvalue is F is to say that it is false. Notice that in sentential logic (SL) we assume the principle of bivalence: a wellformed formula (WFF) either has truthvalue T or F.
Some systems of logic, such as fuzzy logic, reject the principle of bivalence. Notice also that some logic or engineering textbooks use "1" and "0" in place of "T" and "F."
Let's now look at the truthtable of each of the sentential connectives.
§2. Negation
φ  ~ φ 

T 
F 
F 
T 
Consider the statement "whales are mammals." Let's use the sentenceletter "P" to translate this statement. (We can use any sentence letter we want.) This just means we are now taking the symbol "P" to have the same meaning as "whales are mammals."
But suppose you disagree with the statement. Then you might express your disagreement by saying things like:
In sentential logic, these three different sentences are all translated as "~P." Obviously, "P" and "~P" have opposite truthvalues – if one is true, then the other one must be false, and vice versa.
The truthtable displayed here is the truthtable for the negation sign. It shows that when you have a wellformed formula (WFF) and you add the negation sign in front of it to make a new wellformed formula (WFF), you end up with a wellformed formula (WFF) that has the opposite truthvalue.
Notice that we use the Greek symbol "φ" in the table to stand for any wellformed formula (WFF). So the table tells us that when "P" is true, "~P" is F, and when "(Q&~R)" is F, "~(Q&~R)" is T, etc.
§3. Conjunction
φ 




T  T  T  
T  F  F  
F  T  F  
F  F  F 
Now consider these two statements: "it is raining" and "it is hot." Each of them can be either true or false independently of the other, so in terms of their truthvalues there are four different possibilities:
Now what about the truthvalue of the complex statement "it is raining and it is hot"? What would be its truthvalue in each of the four situations? This question is easy to answer because we know that this statement is true only in the first situation, and false in the other three.
In sentential logic we can translate this complex conjunction using "&" to conjoin the two conjuncts. The truthtable on the left shows how the truthvalue of a conjunction depends on the truthvalues of the conjuncts, just as in the example we have looked at. The first row of truthvalues tells us that when the first and second conjuncts are true, the whole wellformed formula (WFF) is true. The other three rows tell us that the conjunction is false in all other situations.
φ  ψ  ( φ ∨ ψ ) 

T  T  T 
T  F  T 
F  T  T 
F  F  F 
The disjunction symbol "∨" is usually used to translate "or." The truthtable on the left tells us that a disjunction is false when both disjuncts are false. Otherwise, it is always true.
Notice that the first row tells us that the disjunction is true when both disjuncts are true. In other words, if "(P∨Q)" is used to translate "Either Peter will leave, or Amie will leave," and it turns out that they both leave, then the whole complex statement is still true.
There are two things to be said if you think this is counterintuitive. First, one might say that a better translation of "(P∨Q)" is "Either P or Q (or both)." Second, it is arguable that there are certain uses in ordinary language where we consider "either ___ or ___" to be false even when the disjuncts are considered to be true.
For example, when ordering from a set menu in a restaurant you may be told that you can either have the salad or you can have the soup, but you cannot have both! We could say, "either P or Q (but not both)."
These two senses of "or" are called inclusiveor and exclusiveor respectively. We should always understand the disjunctive sign "∨" in sentential logic (SL) in the inclusive sense. To express "P or Q" in the exclusive sense you might use the wellformed formula (WFF) "((P∨Q)&~(P&Q))" instead. This is an example of how formal logic can actually help us understand better the linguistic usages of natural languages.
In this web site, you should take "or" to mean inclusiveor, unless otherwise indicated.
§5. Biconditional
φ  ψ  ( φ ↔ ψ ) 

T  T  T 
T  F  F 
F  T  F 
F  F  T 
Suppose you are at a party and wonder whether your friend Jane is around. You ask another friend, and he replies, "Jane is at the party if and only if Matthew is at the party." If you accept this statement as true, what can you conclude? This
statement on its own does not tell you whether Jane is here or not. But it does tell you that if she is here, then Matthew is also here, and if one of them is not at the party, the other person is also absent.
This sense of "if and only if" (or "iff") is captured in the truthtable on the left.
φ  ψ  ( φ → ψ ) 

T  T  T 
T  F  F 
F  T  T 
F  F  T 
The arrow sign is often translated as "If... then..." Its truthtable is probably the most difficult one to understand among the ones we have studied so far. Perhaps it will be easier to remember the truthtable the following way. Suppose
you make a conditional statement such as "If I have lots of money then I shall be happy."
Under what condition will this statement be false? Obviously, your statement is false if you have a lot of money but still you are not happy. In other words, when the antecedent is true and the consequent is false, the whole conditional statement is false. This is exactly what the second row of the truthtable says. Just remember that the conditional is true in all other situations.
This explanation is not quite the full and correct account of why the truthtable of "→" should look the way it does. We shall provide the full explanation in a separate tutorial in a different section.
Consider this diagram:
Source: Joe Lau and Jonathan Chan, https://philosophy.hku.hk/think/sl/connectives.php
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