What is humour?

From the LRB:

But it is also hard to avoid the thought that Blair is himself shirking the real question. He faced two serious and determined enemies during his time in Downing Street: al-Qaida and Gordon Brown. One, he concluded, represented a force so strong and rooted that it had to be uprooted and destroyed, since confrontation was inevitable; the only question was when and how. The other had to be contained, because stepping over the line would have been crazy and made war inevitable. But why on earth did he think that al-Qaida was an example of the first, and Gordon Brown of the second, rather than the other way round?