In a passage on the treatment of pneumonia, Osler quoted van Helmont’s remark that ‘a bloody Moloch presides in the chairs of medicine’. This paper explores Helmont’s use of ‘Moloch’ as a term of abuse against the bloodletting of traditional Galenist physicians and his vigorous opposition to the use of bleeding in treating diseases. The possible reasons for Helmont’s opposition to the practice of bloodletting are discussed, leading to the conclusion that it arose from his theories of the origin of diseases, reinforced by some observations of its malign effects on a patient who had been bled excessively. The question of whether Helmont knew of Harvey’s discovery of the circulation of the blood is explored and the conclusion reached that, if he did, he ignored it completely. Helmont believed that ‘pure’ and ‘impure’ blood coexisted in disease and could be affected selectively by bloodletting which, in effect, concentrated the ‘impure’ at the expense of the ‘pure’ blood to the great detriment of the patient.
