Implication

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==Unless==
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The statement "P '''unless''' Q" can be translated using implication as "If not Q, then P."
  
 
==Only if==
 
==Only if==

Revision as of 14:01, 7 December 2009

Implication is a logical operation on two statements, typically represented by the variables P and Q. "P implies Q" is written symbolically as "P → Q". Equivalent statements include "if P then Q", "P is sufficient for Q", and "Q is necessary for P".

Contents

Definition

Implication, the result of "P → Q" is defined by the following table:

P Q P→Q
True True True
True False False
False True True
False False True

Note that if P is true, then Q must also be true for the implication to hold (be true). However, if P is false, Q may or may not be true and the implication still holds.

To illustrate the latter fact, consider a teacher who tells her class that any student who gets 100% on the final exam will pass the class. In other words:

P: A student gets 100% on the final.
Q: That student passes the class.
P → Q: If a student gets 100% on the final, then that student passes the class.

Now consider the case in which two students do poorly on the final exam; one of them did well enough on the other exams to pass the course, but the other did not. Did the teacher lie?

No. She said nothing about students who do not get 100% on the final (i.e., the case where P is false). Unless there is a student who both got 100% on the final and did not pass the course, the teacher told the truth.

For this reason, "P → Q" can be restated as "¬(P ∧ ¬ Q)" or "not (P and not Q)", or "it is not the case that P is true and Q is false".

In summary, the following statements are equivalent:

In symbols In English
P → Q P implies Q; if P then Q; P sufficient for Q; Q necessary for P
¬(P ∧ ¬Q) not(P and not Q)
¬P ∨ Q (not P) or Q   [equivalent by De Morgan's laws]
¬Q → ¬P not Q implies not P; (not P) unless Q

Unless

The statement "P unless Q" can be translated using implication as "If not Q, then P."

Only if

The statement "P only if Q" is equivalent to "if P then Q" (P → Q), not "if Q then P".

To see why this is so, consider the statement:

I will pass the course only if I study hard.

What this means is:

If I don't study hard, I won't pass the course.

Symbolically:

P: I will pass the course.
Q: I study hard.
¬ Q → ¬ P: If I don't study hard, I won't pass the course.

But this is equivalent to:

P → Q: If I will pass the course then I study hard.

Or, rephrased to make more sense:

If I end up passing the course then I must have studied hard.

If and only if

The phrase "if and only if" means "is logically equivalent to", or "is a necessary and sufficient condition for". It is often abbreviated iff.

P if and only if Q

means the same thing as:

P implies Q, and Q implies P.
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