Health is "a state of complete physical, mental and social well-being and not merely the absence of disease or infirmity," states the preamble to the constitution of the World Health Organisation, adopted in 1946. But what is the source of health? What is well-being? And is illness always an impediment to well-being?
"Illness and health aren't opposite poles, they're two dimensions," says Joachim Kugler, a professor of health sciences and public health at Dresden University of Technology.
"You can be physically ill, but still feel very healthy," he said, citing the example of Austrian writer Thomas Bernhard (1931-1989), who he said had reached the peak of his artistic powers after being diagnosed with sarcoidosis and initially seeing it as an existential threat.
Then there are people with nothing physically wrong with them who regard themselves as not healthy - hypochondriacs, for example-Kugler said.
So health is a matter of attitude? Yes, basically, according to Dierk Petzold, a German physician and author who says that health resulted from the pursuit of a harmonious life.
For Petzold, that means the ability to learn from your mistakes and to seek things that do you good, be it the sun and warmth or people you get along well with. The widespread "positive thinking" approach falls short, though, he said.
"You should by all means take problems, risks and dissonances seriously and not sugarcoat them as in positive thinking," Petzold advised.
A study by the German public health insurance company Barmer GEK and the Competence Centre for Health Management and Public Health at the University of Wuppertal found that "health competence" - ie the ability to overcome unusual challenges or stress, such as illnesses - had a very strong influence on health and well-being.
People with high health competence had markedly better health knowledge and overall health compared with those whose competence was low.
"The more often that people experience the ability to overcome illnesses successfully, the more motivated they are to invest time in health-related activities," the study's authors wrote. For someone with low health competence, negative mental states outweigh positive ones.
"People should be more strongly encouraged to find out how and why they stay healthy," the researchers noted.
This conclusion coincides with Petzold's thinking. "Scientific medicine today deals chiefly with the causes of disease and doesn't look at a person's subjective history and well-being," he says.
Petzold is the author of Praxisbuch Salutogenese (Salutogenesis Practice Book). The term "salutogenesis," which comes from the Latin "salus" (health) and Greek "genesis" (origin), was coined by Aaron Antonovsky, an American/Israeli medical sociologist who found in the 1970s that many women who had survived Nazi concentration camps were apparently healthy later in life despite their traumatic experiences.
The women's "sense of coherence," Antonovsky said, helped them to emerge from the horrors with their health intact.
Kugler described this coherence as "the feeling of being the director in your life," of being able to stay healthy despite having had a traumatic experience or receiving a diagnosis of a terminal illness.
Petzold said that Antonovsky's concept could also be characterized as basic trust, but he preferred to call it "harmonious connectedness." To achieve it, he said, you should frequently ask yourself questions like, "What does me good?" and "What are my strengths?"
A person who thinks about such questions does a lot for his or her health and gains health competence. This competence, in Kugler's view, includes the ability to say no in certain situations, not to be in thrall to messages such as those propagated in food advertisements, and to have a stable network of good friends to fall back on-not only in emergencies, but in day-to-day life as well.