This forum has been quiet for a while, and I'd like to stir the pot
with a controversial issue. (01)
Two widely known rigid classifications established a paradigm,
which some people mistakenly consider the norm: the periodic
table in chemistry and the Linneaen taxonomy of living things.
But the rigid boundaries of those categories are the result of
underlying laws of nature that explain why intermediate cases are
impossible (periodic table) and rare (taxonomy of species). (02)
For physics and chemistry, quantum mechanics implies discrete steps,
which create discrete classifications of elementary particles.
At the next level up, it also implies discrete combinations of
such particles -- combinations of quarks to form baryons (protons
and neutrons) and combinations of atoms to form molecules. (03)
For biology, discrete molecular operations support the stable
molecules needed for life and the mechanisms for replicating the
huge molecules needed for DNA. But those mechanisms are only
weakly stable -- that leads to random mutations. (04)
For the next level up, natural selection creates fuzzy boundaries
among interbreeding populations, but crisp boundaries between
isolated populations. For example, look at the sharp distinction
between foxes and wolves, but fuzzy boundaries among dogs. When
humans allow dogs to "do their own thing", the breeds quickly
revert to a generic ur-dog -- which is usually healthier and
more robust than many breeds. (05)
Some biological classifications are not based on DNA. Examples
are trees and berries. For example, the family Rosaceae includes
rose bushes, apple trees, and raspberries. Biologically, all
berries are fruit, but apples are more likely to be grouped with
oranges as "typical" fruit than with raspberries. (06)
By height and woodiness, an apple tree is more likely to be
classified with a remotely related spruce tree than with
a rose bush. And many evergreens become bushes or trees
at the whim of some human with a pair of shears. (07)
There is even a debate in India whether bamboo should be
classified as grass or tree: "Recently there was a controversy
when the union ministry of environment and forests asked states
across India to recognise bamboo as a minor forest produce." (08)
See http://www.bbc.co.uk/nature/life/Bamboo (09)
In general, what makes any classification rigid is some *law*,
which could be a law of nature or some human rule. Since it's
a lot easier to change human laws, such classifications are
likely to change with culture, technology, or fads. (010)
Summary: Fuzzy boundaries are the norm in most classifications.
Whenever a boundary seems to be sharp, look for some axiom, law,
principle, or convention that creates the distinction. Those
laws are more fundamental than any grouping by "similarity". (011)
And a warning: Unless you can find an immutable law of nature
that creates a classification, don't expect it to be a solid
foundation for a "standard ontology". (012)
John Sowa (013)
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