You must be able to check assertions out. Inveterate skeptics must be given the chance to follow your reasoning, to duplicate your experiments and see if they get the same result. The reliance on carefully designed and controlled experiments is key, as I tried to stress earlier. We will not learn much from mere contemplation. We let experiment do it.
Control experiments are essential.
If, for example, a new medicine is alleged to cure a disease 2o percent of the time, we must make sure that a control population, taking a dummy sugar pill which as far as the subjects know might be the new drug, does not also experience spontaneous remission of the disease 20 percent of the time. Variables must be separated. Suppose you're seasick. You find the unpleasantness vanishes. You can tell only if you take the one without the other, next time you're seasick.