Here we learn that Mr. Munroe apparently has a thing for math. Specifically, he finds math the "purest" field, though of course that brings up the question of how "purity of field" is measured. I suppose he means that personally biases are less likely to creep into math, but I'm not sure that's true. In any case, the whole thing is highly reminiscent of a joke which I have heard attributed to Steve Martin, that "When you really study science, you learn that everything in psychology is really just all about biology, and if you study biology it's all chemistry. Once you learn chemistry you learn that chemistry is really just physics, and when you get into it physics is really just math. Math is just philosophy, and philosophy is bullshit." Oh and the other thing it's highly reminiscent of is comic 263.
Oh also the characters in this comic are not being accurate. To choose one example, biology is not applied chemistry. At the molecular level it is indeed very much chemical, though it is of course the study of chemistry in living things. In any case, much of biology is about evolution, populations, etc, which is really not chemical at all. You know what is applied chemistry? Applied chemistry.
astoundingly late update: A sociologist reader retorts with this: