But we must look less cursorily at the growth of the idea of bacteria causing disease. More than two hundred years ago Robert Boyle (1627-91), the philosopher, who did so much towards the foundation of the present Royal Society, wrote a learned treatise on _The Pathological Part of Physic_. He was one of the earliest scientists to declare that a relationship existed between fermentation and disease. When more accurate knowledge was attained respecting fermentation, great advance was consequently made in the etiology of disease. The preliminary discoveries of Fuchs and others between 1840 and 1850 had relation to the existence in diseased tissues of a large number of bacteria. But this was no proof that such germs _caused_ such diseases. It was not till Davaine had inoculated healthy animals with bacilli from the blood of an anthrax carcass, and had thus produced the disease, that reliance could be placed upon that bacillus as the _vera causa_ of anthrax. Too much emphasis cannot be laid upon this idea, that unless a certain organism _produces_ in healthy tissues the disease in question, it cannot be considered as proven that the particular organism is related to the disease as cause to effect. In order to secure a standard by which all investigators should test their results, Koch introduced four postulates. Until each of the four has been fulfilled, the final conclusion respecting the causal agent must be considered _sub judice_. The postulates are as follows: (_a_) The organism must be demonstrated in the circulation or tissues of the diseased animal. (_b_) The organism thus demonstrated must be cultivated in artificial media outside the body, and successive generations of a _pure culture_ of that organism must be obtained. (_c_) Such pure cultures must, when introduced into a healthy and susceptible animal, produce the specific disease. (_d_) The organism must be found and isolated from the circulation or tissues of the inoculated animal.