Rationality is the ability to bring means into alignment with ends.
Let's suppose for a moment that the two dozen Republicans who rejected Speaker Boehner's Plan B are rational. What do their chosen means tell us about their desired ends?
Is the end: "Defeat the Obama tax increases"? No, that can't be it. The rejection of Plan B means that taxes will go up more than if Plan B had somehow become law.
Is the end a more modest: "Protect Republicans from blame for the Obama tax increases?" No, that's not it either. Protection of the party from blame was precisely John Boehner's goal in devising Plan B. Plan B was never intended as a serious budget proposal. Plan B was intended only as a PR insurance policy if the country went over the fiscal cliff. Republicans could then say: "See we were prepared to tax millionaires if need be."
The defeat of Plan B leaves Republicans - all Republicans utterly defenseless against the onslaught to come.
Here's what happens now.
1) President Obama goes on holiday.
2) December 31 arrives.
3) Taxes go up on everybody. Unemployment insurance ends. Sequester begins to go into effect.
4) Screams, howls.
5) President Obama returns to Washington, speaks to country from Oval Office or else summons special session, unveils the "Obama Tax Cut and Job Creation plan" featuring a whacking big tax cut for 98% of Americans.
6) Plan polls at 70% or better.
7) Republicans resist.
8) Republicans surrender.
9) Economy recovers, "morning in America," Obama basks in glory in 2nd term.
That's what those Republicans voted for when they rejected Plan B. Again, assuming they are not irrational, they must have had a motive. What?
For almost any given Republican, the best possible outcome of the Plan B negotiations would have been for Plan B to pass - but for that particular Republican to vote nay. The trouble was, that there were too many Republicans who wished to avail themselves of that outcome. They all rushed the exits together, and there was not enough room to accommodate the stampede. It's a classic "prisoner's dilemma" problem from political science, and the dilemma achieved its usual grim result.
The more haunting question is: why was this dilemma allowed to exist?
The prisoner's dilemma arises, remember, because the prisoners have no way to make binding agreements. But the whole point of a political party is to overcome that dilemma, to create structures that reward cooperation and punish defection.
The deepest moral of the Plan B debacle is that those structures have broken down inside the GOP. And that's a very scary moral indeed.