We’re a philosopher and a computer scientist trying to understand the world and the choices we make.

About ‘Contraposition’:  There’s an aphorism in philosophical circles that goes “One man’s modus ponens is another man’s modus tollens.”  The idea is that two people can agree on a conditional statement `If p then q‘, but use it to make different inferences.  Someone who endorses p will conclude that q (that’s modus ponens).  Someone who denies q will deny p (that’s modus tollens).  This double aspect of the conditional shows up in the fact that, in classical logic, ‘p -> q‘ is equivalent to ‘~q -> ~p‘.  Converting from one to the other is called contraposition.

We’ve noticed that much of the sustainability/green/environmental community likes to reason in only one direction:

(1) If we’re going to maintain the current economy (p), then we’ll need alternative fuels that meet current energy needs (q). 

But why not contrapose?

(2) If we can’t get alternative fuels to meet current energy needs (~q), then we won’t be able to maintain the current economy (~p). 

If you emphasize (1), the lesson is: let’s get to work on making alternative fuels meet current (extravagant) energy needs.  But if you emphasize (2), the lesson is: let’s start thinking about what a different economy would look like.  We think it’s vital to start thinking about both directions, and not neglect the contrapositive.