John Snow set examples of health and illness research that critical realism (CR) supports. He used three methods. Induction: when he observed many cases and began to form theories about the cause of cholera. Deduction: when he formed his hypothesis that cholera is waterborne and set out to test it through counting households and mapping people's daily movements and habits. Thirdly, he used retroduction: this involves:

• • searching beyond evidence for the unseen cause only seen in its effects;

• • working out the simplest explanation;

• • imagining new possibilities;

• • taking seriously a new idea (that most doctors dismissed);

• • asking ‘what must the world be like for this to occur?’ (that disease perhaps spreads through water); and

• • rethinking and potentially rejecting older beliefs (about the actual nature of clean water and cross-infection).

Retroduction, a major CR concept, is ‘democratic’ in that scientists and everyone else in everyday life use it. Snow put his research idea into practice. By having the pump handle removed, he changed the world through helping to reduce and prevent disease and eventually to change medical knowledge and practice. He promoted equality by assisting disadvantaged groups, who had to rely on public water pumps.