The nature philosophers of the Ionian days did not contribute much to medicine proper, but their spirit and their outlook upon nature influenced its students profoundly. Their bold generalizations on the nature of matter and of the elements are still the wonder of chemists. We may trace to one of them, Anaximenes, who regarded air as the primary principle, the doctrine of the "pneuma," or the breath of life--the psychic force which animates the body and leaves it at death--"Our soul being air, holds us together." Of another, the famous Heraclitus, possibly a physician, the existing fragments do not relate specially to medicine; but to the philosopher of fire may be traced the doctrine of heat and moisture, and their antitheses, which influenced practice for many centuries. There is evidence in the Hippocratic treatise peri sarkwn of an attempt to apply this doctrine to the human body. The famous expression, panta rhei,--"all things are flowing,"--expresses the incessant flux in which he believed and in which we know all matter exists. No one has said a ruder thing of the profession, for an extant fragment reads: ". . . physicians, who cut, burn, stab, and rack the sick, then complain that they do not get any adequate recompense for it."(4) (4) J. Burnet: Early Greek Philosophy, 1892, p. 137, Bywater's no. LVIII. The South Italian nature philosophers contributed much more to the science of medicine, and in certain of the colonial towns there were medical schools as early as the fifth century B.C. The most famous of these physician philosophers was Pythagoras, whose life and work had an extraordinary influence upon medicine, particularly in connection with his theory of numbers, and the importance of critical days. His discovery of the dependence of the pitch of sound on the length of the vibrating chord is one of the most fundamental in acoustics. Among the members of the school which he founded at Crotona were many physicians. who carried his views far and wide throughout Magna Graecia. Nothing in his teaching dominated medicine so much as the doctrine of numbers, the sacredness of which seems to have had an enduring fascination for the medical mind. Many of the common diseases, such as malaria, or typhus, terminating abruptly on special days, favored this belief. How dominant it became and how persistent you may judge from the literature upon critical days, which is rich to the middle of the eighteenth century.