Sun, 10 Aug 2008: Indefinite article
Juan es filosofo
Juan es un filosofo.
In the first, John is a philosopher by profession; we are just identifying him as a member of a class by giving him that label.
In the second, he is just being philosophical; this is not a standard label for John but is something exceptional.
Compare: Juan es atleta / Juan es un atleta importante
In the first, he is a professional athlete but in the second, while athletes are a class, "important athlete" individualizes John.