[Those who cannot remember the past are condemned to repeat it].

To the casual observer, the history of medicine seems one of continuous progress. In reality, for every therapeutic accomplishment that has withstood the test of time there have been several ineffective or even harmful treatments that are now forgotten; yet they were based on theories regarded as plausible at the time. Examples abound, also in the archives of the Dutch Journal of Medicine: treatments for poliomyelitis and sciatica provide striking illustrations. Even in the current era of controlled trials and meta-analyses, physicians need to be reminded that a logical theory in itself does not justify treatment decisions unless it is supported by experimental evidence. The essential difference between mainstream medicine and complementary medicine is not the plausibility of the underlying theories but the rigour in testing hypotheses. On the other hand, many decisions in medicine must be made without sufficient evidence. Logic is indispensable but should be part of an empirical cycle in which new thoughts lead to new experiments and vice versa.