Three uses for truth tables are:
1) To check if a sentence is tautological, contradictory, or contingent by examining if it is always true, always false, or can be either given different truth value assignments.
2) To check if two sentences are logically equivalent by seeing if they have the same truth value across all rows.
3) To check if an argument is valid by seeing if there is any row where the premises are all true and conclusion is false, which would mean the argument is invalid.
2. Checking for Contingency
• A sentence that is true no matter the truth values of
the parts is tautological.
• A sentence that is false no matter the truth values of
the parts is contradictory.
• A sentence that might be true or might be false
depending on the truth values of its parts is contingent.
• Since a truth table is a convenient way of listing all the
possibilities for component sentences, it allows a rapid
determination of a sentence’s status as
tautological, contradictory, or contingent.
3. Truth-Table Test for Contingency
A B B (B A)
T T F T T
T F T T T
F T F T F
F F T T T
Since all the values under the main connective
(the left horseshoe) are T, this table shows
that the sentence B (B A) is a tautology.
4. Checking for Equivalence
• When two sentences have the same truth
value no matter what the values of the
component parts are, the sentences are
equivalent.
• Each of the rows on the truth table represents
a way the world could be. If there is no way
the world could be that gives two sentences
different truth values, then they are truth-
functionally the same.
5. Truth-Table Test for Equivalence
A B A B A B
T T T F T
T F F F F
F T T T T
F F T T T
This table shows that A B is equivalent to A B.
Since the column under the conditional and the
column under the disjunction are the same on
every row, it is not possible for the sentences to
differ in truth value.
6. Checking for Validity
• Remember that in a valid argument it is
impossible to have all true premises and a false
conclusion.
• Since a truth table is a convenient way of listing
all the possibilities, it can tell us at a glance
whether it is possible for an argument to have all
true premises and a false conclusion.
• If no row of the table has each premise true and
the conclusion false then the argument is valid. If
there is at least one row with all true premise and
a false conclusion then the argument is invalid.
7. Truth Table Test for Validity
A B A B B ∴ A
T T T F F
T F F T F
F T T F T
F F T T T
Since no row has all true premises and a false conclusion the
argument is valid. The only row with all true premises is the
bottom row and on that row the conclusion is true. The
important thing is not that there’s a row with all true
premises and a true conclusion but rather that there’s no
row with all true premises and a false conclusion.
8. Another Truth Table for Validity
A B A B A ∴ B
T T T T F
T F T T T
F T F F F
F F T F T
The top row of the table, where A is true and B is true,
is a row where the premises of the argument are all
true and the conclusion is false. So, it is possible for
the argument to have true premises and a false
conclusion. That means the argument is invalid.