A scientist then creates a theory about something he wants to look at in a little more detail. This will be based on a set of assumptions. This theory should be the best explanation of the observable facts. He could, for example, have a theory that gravity acts upwards. It would be obvious to anyone that this does not fit the observable facts. Such a claim would be ignored.
But sometimes we have no way of knowing whether the assumptions used in a theory are right. This is often the case when considering what happened in the past. We can't do experiments to test the theory and check the assumption
So it is important that we know what can be proved and what is only a theory that we can't prove.
We want to look quickly at a couple of examples where theories which lacked vital facts were trusted. The consequences were terrible.
Trusting unproven theories
In mid-nineteenth-century England, some public-health authorities believed the 'miasma' theory. This theory said that diseases were caused by a poisonous vapour known as 'miasma'.
In the same era, some doctors accepted the 'dyscrasia' theory, which said that disease was caused by an imbalance in the body's temperament.
People did their best with their limited knowledge. They were unaware that they lacked vital information about viruses and bacteria. They trusted theories that were not based on all the facts. They saw no problem in drinking water that had been contaminated with sewage. The result was that tens of thousands of people died in the four cholera epidemics that broke out in England between 1831 and 1854.
Nor did they see a problem in a doctor or nurse moving from a dead body to a living one without washing their hands in between. So, up to one in three women died in hospital giving birth to children. Many women preferred to give birth in the street rather than go to hospital.
Countless people died because these theories lacked vital facts. These two examples show that we need all the facts to come up with a correct theory. They also show us that we may not realise that we don't have all the facts.