Jordan Ellenberg has a truly wonderful article in Slate. The
entities we study in science fall into two categories: those
which can be classified in a way a human can understand, and
those which are unclassifiably wild. Numbers are in the first
class—you would agree that although you cannot list all the
whole numbers, you have a good sense of what numbers are out
there. Platonic solids are another good example. There are just
five: the tetrahedron, the cube, the octahedron, the
dodecahedron, and the icosahedron. End of story—you know them
all. [...] In the second class are things like networks (in
mathematical lingo, graphs) and beetles. There doesn't appear to
be any nice, orderly structure on the set of all beetles, and
we've got no way to predict what kinds of novel species will
turn up. All we can do is observe some features that most
beetles seem to share, most of the time. But there's no periodic
table of beetles, and there probably couldn't be. Mathematicians
are much happier when a mathematical subjec