The Most Dangerous Conditional Rule on the LSAT

    LSAT Conditional Reasoning

    Don't have a nuclear meltdown on the LSAT!On our LSAT Discussion Forum recently, I've been running into a recurrent question about a conditional reasoning rule. These questions revolve around a really tricky phrasing, one that has devastated test takers when it has appeared on previous LSATs. But if you can learn the idea, it takes something the test makers expect to be very difficult and turns it into something fairly easy. Plus, it's not that tough to learn! So what is this mysterious but critically important concept?

    On the LSAT you are frequently presented with conditional reasoning. And, in many instances, the statements are constructed so that one of the conditions is positive and the other is negative. For example, you might see a Logic Game rule such as "If F is selected, then G cannot be selected." Here, the sufficient condition is positive and the necessary condition is negative:

    conditional_1 Readers of the LSAT Bibles or students in one of the PowerScore LSAT courses will recognize that we typically turn the above statement into the following super-statement: 

     

    f_doublenot_g

     

    This representation captures the full meaning of the initial diagram and its contrapositive (and we discuss that elsewhere in great detail, so I'll skip that breakdown for now). What is interesting here is what is possible and impossible under this scenario. There are three possible outcomes and one impossible outcome:


    1. F occurs, G does not occur
    2. G occurs, F does not occur
    3. Neither F nor G occurs

    What is impossible is for both F and G to occur.

    Thus far, everything looks fairly normal, and the vast majority of "one positive condition and one negative condition" rules that one encounters on Logic Games come in the above form. However, there is a similar rule that the test makers throw in on rare occasion, but when they do, it causes all sorts of problems (for example, see the famous December 2000 Birds in the Forest game). This tricky rule is worded in the following form: "If F is not selected, then G must be selected." Here, the sufficient condition is negative and the necessary condition is positive:

    conditional_2 This rule looks so similar to the previous one that it is easy to see why it would cause confusion. Both rules contain F and G and a single arrow, and there is one negative and one positive term. But, when the negative is moved over to the sufficient condition, it completely changes the meaning of the rule. Under this rule, there are also three possible outcomes and one impossible outcome: 


    1. F occurs, G does not occur
    2. G occurs, F does not occur
    3. Both F and G occur

    What is impossible is for neither F nor G to occur. 

    The effective result of the change is that the two rules have inverse outcomes. In the first rule (F → G), you can never have both variables present, but in the second rule (FG) you can never have both variables absent. So, when you are setting up a Logic Game, the first rule means that one of F and G (and possibly both) is always out; the second rule means that one of F and G (and possibly both) is always in. That is a huge difference in meaning in a game, and one that—if you did not get it right—would cause you to miss multiple questions (if you have the LSAT Logic Games Bible, check out the game on page 340 (the photograph game). If you mis-analyze the fourth rule in this game, you are dead). 

    The next LSAT isn't that far away. I don't know for sure that this rule will appear, but it has been used more in recent years than it had been used previously. If you can commit this rule to memory and understand how it works, then if it appears, it just might save your score.

    Have any questions or comments? Please post them below!

     

    Photo: "Central Nuclear Trillo" courtesy of Rodrigo Gómez Sanz.