The following things are demonstrably true:

1. Evolution through natural selection is a scientific fact.
2. Approximately six million European Jews were killed during the Second World War.
3. Between 1969 and 1972, 12 American astronauts walked on the Moon.
4. Everyone should get their child vaccinated; vaccines save countless lives.
5. Human activity is raising the temperature of the planet.

That some people refuse to believe these demonstrably true things is both sad and frustrating. Sometimes it’s because they’ve been convinced by people whose interests are served by replacing a truth with a lie (promoting religious doctrine or a quack cure for autism, economic self-interest, rehabilitating Nazism), but sometimes nutbaggery has no agenda.

Either way, the end result is considerable effort spent defending, proving and reproving what is demonstrably true. What a waste.