Charles Darwin just turned 200 years old, and encomia are flowing. Of the many cultural implications of Darwinism one of the least recognized is its conceptual core, which is this: There is no essence to anything.

This view (called anti-essentialism by philosophers) has important relevance to psychiatry. What it means is that there is no such thing as an "essence" to mental illness; we should give up all take of saying that schizophrenia is "essentially" X, or that a patient does not have bipolar disorder because he is not having symptom Y.

There is no essence to anything; this is a profound biological fact, with deep psychological consequences. Darwin was interested in species; species vary inherently in many features, he argued; they have no essence; thus they can evolve. This is the key notion behind the whole concept of evolution.
So go species, so go all biological entities, including illnesses. Many have criticized psychiatry, especially as in DSM definitions, as not having any definitive criteria for mental illnesses. Patients could have differing symptoms of depression, and still have depression, for instance. Critics, unconsciously perhaps enacting postmodernist ideas, have seen this approach as social, relativist, and nonbiological. Yet, if so, then Darwin was nonbiological. Many criticize psychiatric diagnoses for not "carving nature at its joints," but Darwin showed us that there are no clean joints to cut.