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Conditionals and Biconditionals: → and ↔
Chapter 7 adds two new connectives to our collection of treasures: → and ↔. We can read "P → Q" as "if P, then Q" and we can read "P ↔ Q" as "P if and only if Q." (Sometimes philosophers abbreviate "if and only if" to "iff.") A sentence of the form "P → Q" is called a conditional. The antecedent of the conditional is P, and its consequent is Q.
As with our other connectives, we can give the semantics of the conditional and biconditional by using truth tables:
|
P |
Q |
P → Q |
P ↔ Q |
| T | T | T | T |
| T | F | F | F |
| F | T | T | F |
| F | F | T | T |
There are a few tricky points to keep in mind in doing translations involving these connectives. Let's consider a few of the many English phrases that can be translated using conditionals. Some of the following sentences are frequently stumbling blocks for students doing translations, so if you have trouble doing the translation exercises, you might want to look over this table again!
| English Sentence | Translation into Symbolic Notation | Comments |
| If P then Q P is sufficient for Q |
P → Q | |
| P only if Q | P → Q | note: "P only if Q" means the same thing as "if P then Q"! |
| P is necessary for Q | Q → P | Q doesn't occur without P also occurring |
| P if Q | Q → P | except in the case of "only if," whatever follows the "if" is usually the antecedent of the conditional |
| P unless Q | ¬Q → P or P ⋁ Q | an easy way to remember how to translate "unless" is to think of it as "if not" |
| P if and only if Q P just in case Q |
P ↔ Q |