A * simple statement* is one that does not contain any other statement as a part. We will use the lower-case letters, p, q, r, ..., as symbols for simple statements.

A * compound statement* is one with two or more simple statements as parts or what we will call

An * operator* (or

The symbols for statements and for operators comprise our notation or symbolic language. Parentheses serve as punctuation.

**Simple statements**

p | "p is true" | assertion |

~p | "p is false" | negation |

**Compounds and connectives**

p q | "either p is true, or q is true, or both" | disjunction |

p · q | "both p and q are true" | conjunction |

p q | "if p is true, then q is true" | implication |

p q | "p and q are either both true or both false" | equivalence |

Implication statements (p q) are sometimes called * conditionals*, and equivalence statements (p q) are sometimes called

To indicate that a compound is to be taken as a whole or single statement, we put it in parentheses. For example, p q is a compound. Its negation is ~(p q). When the negation sign is outside the parentheses, it affects the entire compound, not just the first component, p. Its conjunction with the compound, q r, would be expressed, (p q)·(q r). If we wanted to say that the whole latter statement was false, we would write, ~[(p q)·(q r)]. If we wanted to say that either the whole latter statement was true, or that s t was true, we would write, ~[(p q)·(q r)] (s t). And so on.

The * truth value* of a statement is its truth or falsity. All meaningful statements have truth values, whether they are simple or compound, asserted or negated. That is, p is either true or false, ~p is either true or false, p q is either true or false, and so on.

A compound statement is * truth-functional* if its truth value as a whole can be figured out solely on the basis of the truth values of its parts or components. A connective is truth- functional if it makes only compounds that are truth-functional. For example, if we knew the truth values of p and of q, then we could figure out the truth value of the compound, p q. Therefore the compound, p q, is a truth-functional compound and disjunction is a truth-functional connective.

All four of the connectives we are studying (disjunction, conjunction, implication, and equivalence) are truth-functional. Negation is a truth-functional operator. With these four connectives and negation we can express * all* the truth-functional relations among statements. (Can you imagine how we would prove this?)

A * truth table* is a complete list of the possible truth values of a statement. We use "T" to mean "true", and "F" to mean "false" (though it may be clearer and quicker to use "1" and "0" respectively).

For example, p is either true or false. So its truth table has just 2 rows:

p |

T |

F |

But the compound, p q, has 2 components, each of which can be true or false. So there are 4 possible combinations of truth values. The disjunction of p with q will be true as a compound whenever p is true, or q is true, or both:

p | q | p q |

T | T | T |

T | F | T |

F | T | T |

F | F | F |

If a compound has * n* distinct simple components, then it will have 2

The truth table columns that define the basic connectives are as follows:

p | q | ~p | ~q | p q | p · q | p q | p q |

T | T | F | F | T | T | T | T |

T | F | F | T | T | F | F | F |

F | T | T | F | T | F | T | F |

F | F | T | T | F | F | T | T |

Most statements will have some combination of T's and F's in their truth table columns; they are called * contingencies*. Some statements will have nothing but T's; they are called

In the conditional, p q, the first statement or "if- clause" (here p) is called the * antecedent* and the second statement or "then-clause" (here q) is called the

Any * argument* can be expressed as a

An argument is * valid* if and only if its corresponding conditional is a tautology. There are other tests for validity using truth tables. The chief alternative test searches for a

Two statements are * consistent* if and only if their conjunction is not a contradiction.

Two statements are * logically equivalent* if and only if their truth table columns are identical --if and only if the statement of their equivalence using "" is a tautology.

Obviously truth tables are adequate to test validity, tautology, contradiction, contingency, consistency, and equivalence. This is important because truth tables require no ingenuity or insight, just patience and the mechanical application of rules. No matter how dumb we are, truth tables correctly constructed will always give us the right answer.

Can you see why a truth table can be constructed for every truth-functional statement, and only for truth-functional statements?

This file is an electronic hand-out for the course, Symbolic Logic.

Most of the logic symbols in this file are GIFs. See my Notes on Logic Notation on the Web.

Peter Suber,
Department of Philosophy,
Earlham College, Richmond, Indiana, 47374, U.S.A.

*peters@earlham.edu*. Copyright © 1997, Peter Suber.