区分充分与必要条件

Sufficient and necessary conditions

Conditional reasoning is a fundamental component of both the Logical Reasoning and Logic Games section of the LSAT.

A sufficient condition can be defined as an event or circumstance whose occurrence indicates that a necessary condition must also occur.

A necessary condition can be defined as an event or circumstance whose occurrence is required in order for a sufficient condition to occur. In other words, if a sufficient condition occurs, you automatically know that the necessary condition also occurs.

Consider the following statement:

         To look sexy, you must smile.

We represent this statement as follows:

             Sufficient                  Necessary

            Look sexy                     Smile

One of the confusing elements in recognizing conditional statements is that so many different terms can be used to introduce a sufficient or necessary condition. Consider the following statements:

If you want to look sexy, you must smile.

Smiling is necessary to look sexy.

When someone looks sexy, she must be smiling.

Only someone who is smiling can look sexy.

Unless you smile, you cannot look sexy.

You will look sexy only if you smile.

All the above would be diagrammed the same way:

            Sufficient            Necessary

            Look sexy             Smile

The following list may be helpful to you.

Sufficient Condition Indicators     Necessary Condition Indicators

        If                                                        Then

        When                                                  only

        Whenever                                          only if

        Every                                                  must

        All                                                       required

        Any                                                     unless

        eople who                                        except

        In order to                                          until

                                                                   Without

You should be able to memorize the indicator words on each list, which could assist you in recognizing the types of situations faster.