The doctrine of correct reasoning was developed in the Western World as logic. This is an activity of the intellect that apparently began with Zeno of Elea, being formalized by Aristotle, and which received its name from the Stoic philosopher Chrysippus. It corresponds to the structure or anatomy of thought. Logic empiricism introduced systematic use of the logistic language into epistemology. The latter discipline designates the philosophy of science, i.e., the critical foundation of its principles, hypotheses, methods, and results. Strictly speaking, it does not constitute an analysis of the scientific method, which is rather the object of methodology, nor anticipation or synthesis of scientific results. It can be considered that, concerning science, epistemology constitutes the second step with a primary activity. In other words, it is a reflection on science, considering the latter as an element to be respected and not as a domain to be ruled. Dr. Hermann Boerhaave was the first physician to challenge problems of an epistemologic character in a coherent and systematic manner (XVIII Century). Others followed him in this direction during the subsequent centuries. In the light of Popper's critical rationalism, construction of a medical instrument, conception of a therapeutic procedure, development of a useful model in biology or medicine could also be considered as epistemologic problems. The corresponding examples that follow are worthwhile mentioning: Riva-Rocci's sphygmomanometer; metabolic therapeutics for ischemic heart disease, and elaboration of theoretical models. In turn, epistemology suggests that assessment of a fact, perceivable by the senses, is generally more difficult that elaboration of a hypothesis.