Saturday, 12 September 2009

Can a taxonomy be true?

In a previous posting I argued that what distinguishes knowledge from mere opinion is that the arguments and evidence in its favour make it seem more likely than not to be true. It’s not necessary, because it’s generally not possible, to be sure that they are true.

This is fairly clear for the simple propositions considered by philosophers and for scientific theories but I’ve argued, in Types of model, that knowledge includes other things, such as taxonomies. But what does it mean for a taxonomy to be true?

The need for clarity
The first requirement is that the taxonomy is clearly defined. Specifically, that the definitions of the categories ensure that the people using the taxonomy can fairly readily agree on how to categorise a typical thing to which the taxonomy is supposed to apply.

The need for similarity
The second requirement is that things allocated to one category should be more similar to one another than they are to things in other categories. The similarity can apply to any property of interest. If I classify things on the basis of some property of interest to you then you will be able to locate them subsequently. This works for filing systems, library classification systems, classification systems for businesses, etc.

The best taxonomies pass the similarity test for many properties of the things in question and often work because of some underlying property of those things. The Periodic Table of the Elements is an excellent example. Elements in the same group have many similar physical and chemical properties because of similarities in the orbits of their electrons.

Testing for falsifiability
As Karl Popper showed it is, outside logic and maths, generally impossible to prove truth but often possible, and generally more useful, to try to prove a claim false. So can a taxonomy be proved false?

Indeed it can.

Consider the system of diagnoses used by psychiatrists. There are some differences between psychiatrists but most divide mental illnesses into neuroses and psychoses:
  • Neuroses are further divided into anxiety, depression, obsession and various phobias.
  • Psychoses are further divided into schizophrenia, bipolar disorder, paranoia, etc.
Richard Bentall, a professor of clinical psychology, has examined the evidence for this diagnostic approach in Madness Explained. He's shown that these conditions are not met. Psychiatrists do not agree on diagnoses and diagnoses are poor guides to treatment or outcomes. Research also shows that doctors can get better guidance on treatment by considering symptoms alone and ignoring diagnostic labels.

These findings, which are not particularly new, have profound implications for psychiatry. It has been going down a blind alley for over 100 years!

It’s also significant for my model of knowledge since it shows that a taxonomy can be seen as an hypothesis that meets Popper’s test of falsifiability. So a taxonomy is a form of knowledge.