Underlying any sustained scientific investigation is a sense of the background conditions: nature. In the ancient Greek bio-medical world nature was understood in at least three ways: (a) as a material force that brings about material results; (b) as material that is proximately in the control of Divine Force(s); and (c) as a part of material forces that are essentially inscrutable so that skepticism had to be attached to the outcome. Sometimes these three background conditions overlapped. It is important to make these distinctions to better understand the causal nature of medical explanation. After exploring these background conditions a few examples focusing upon the role of blood will be examined.