It's the distinction creationists try to manufacture between "historical" and "observational" science. The point being made is that if a person wasn't there to actually observe something, we are all equally clueless about what happened. (There would be much fewer criminal convictions if this were true, since many crimes don't have eyewitnesses, but are prosecuted based on other evidence).
Physicist Lawrence Krauss explains why that distinction is bogus.
A similarly manufactured distinction in health physics is that below radiation doses where we can "observe" an increase in cancer risk, we are clueless as to whether there is an increased cancer risk or not. No, we're not clueless. We have plenty of other evidence which allows us to conclude that at radiation doses below which we don't have sufficient epidemiological statistical power, we still have an increased cancer risk with increasing dose.