Medicine, at least textbook medicine, was rationalistic. Textbooks started with definitions and assertions regarding the fundamentals of health. This we see particularly in a Galenic writer such as Riverius. Medicine, he said, "stands upon the basis of its own principles, axioms and demonstrations, repeated by the demonstration of nature."[51] In his text, Riverius first expounded a groundwork concerning the elements, temperaments and humors, spirits and innate heat, the faculties and functions; then the nature of the diseases which resulted from disturbances of these; and finally the signs of disease and the treatment that was appropriate. All were beautifully interdigitated in a logical fashion, and for any recommended therapy a good reason could be found. There was, however, a serious difficulty. If anyone were so bold as to ask, _But how do you know?_ only a rather lame answer would come forth. The exposition rested in large part on authority or else largely on reasoning from accepted premises--a "just" reasoning. And while much keen observation was duly recorded and a considerable mass of fact underlay the theoretical superstructure, the idea of empirical proof was not current. Riverius chopped logic vigorously and drew conclusions from unsupported assertions in a way that strikes us as reckless. For a body of knowledge to be a science, it must indicate a logical connection between first principles, which were "universal," and the particular case. The well-educated physician could always give a logical reason for what he did. The empiric, however, was one who carried out his remedies or procedures without being able to tell _why_. That is, he could not trace out the logical connection between first principles and the particular case. Galenism suffered especially from logical systematization, and the system of van Helmont, while far less orderly, also had its own basic principles on which all else depended. Boyle, however, practiced medicine on a thoroughly different basis. He did not depend on system or logic. In the words that Hunter used to Jenner over a hundred years later, other physicians would _think_ the answers to their problems. Boyle, however, preferred to _try the experiment_. He wanted _facts_. But this attitude, which sounds so modern, so praiseworthy and enlightened, had one serious flaw. What _was_ a fact? And how did you know? This important problem, so significant for the growth of scientific medicine, we can study quite readily in the works of Robert Boyle.