I get it!

Scalzi wrote:

...particularly among the dim-witted who cannot hold two thoughts in their brain at the same time.

...in the course of some other remarks worth reading. I just stopped and had an epiphany (good thing I was stopped, first). Why can't these people hold two thoughts in their brain at the same time?

Then it hit me, like a neutrino through 16 zillion cubic centimeters of anxious water. Duh.

It's a Pauli Exclusion thing at work here. Makes perfect sense. Two fermion-like thoughts cannot exist in the same brain at the same time!

Ha! One more of life's conundrums solved.