Part of our job as informed citizens and voters is to sift through the political claims that we hear and arrive at our own sense of what’s true. I’ve been listening to such claims in the transit business, and sometimes making them, for almost 30 years now. It occurs to me that one of the most important tools for evaluating these claims is something you probably learned in high school math and forgot. (Yes, some of you remembered, but I’m really talking to the ones who forgot. To those of you who just don’t like math, don’t worry if you don’t follow this next bit; just skim ahead to the example. This IS really important.)
- The Converse, [B –> A] is not necessarily true.
- The Inverse [NOT A –> NOT B] is not necessarily true.
- The Contrapositive [NOT B –> NOT A] IS true.
You already know this. Consider the statement “John is very tall.” Your first job is to see the “if-then” formulation of the statement, in this case “If this person is John, then this person is very tall.”
So in this case:
- The Converse (“If this person is not John, then this person is not tall”) is obviously not true in this case.
- The Inverse (“If this person is tall, then this person is John”) is also obviously not true.
- The Contrapositive (“If this person is not tall, then this person is not John”) IS true. In fact, it’s logically equivalent to the first statement.
Duh. And yet, a huge percentage of our political rhetoric is designed to make you get this wrong. Political rhetoric is often just trying to create a vague feeling of intimacy between ideas A and B, and discourage you from asking what the logical relationship of the two ideas really is.
Consider this slogan from the Seattle Monorail Project (1996-2005)
If I’m on a monorail, then I’m not stuck in traffic.