This heat which is innate in all living animals was styled by Aristotle not only "innate" but "natural," "vital,"[25] and "physical,"[26] it being indispensable to life and to the working of the soul. He held the continued existence of the innate heat to depend upon conditions similar to those under which a fire is kept alive, viz.: protection both from burning out and from extinction due to external forces. Yet the true nature of combustion was not settled till more than a century after Harvey's death. The fact that air is necessary to fire must always have been a matter of common knowledge. Therefore, the views of the relations of air to fire maintained by Aristotle nearly twenty-one centuries before the discovery of oxygen did not seem naïve to Harvey, whatever they may seem to us. Aristotle held that air exerts upon fire a cooling influence which saves it from burning out too fast; and that the same influence is exerted upon the vital innate heat of animals by the air which they breathe in, or the water which bathes their gills.[27] Moreover, Aristotle says:-- "Why those animals breathe most which have lungs containing blood, is plain from this: that the warmer an animal is, the greater need it has of cooling, while at the same time the breath passes easily toward the source of warmth within the heart. But the way in which the heart is pierced through toward the lung must be studied from dissections and from the history of animals which I have written. In general terms, then, it is the nature of animals to need cooling on account of the firing of the soul within the heart."[28] In the treatise styled the "History of Animals," to which he refers us, Aristotle says:-- "There are also channels from the heart which lead into the lung and divide in the same way as the windpipe, and they accompany the channels from the windpipe throughout the entire lung. The channels from the heart lie uppermost; but no common channel exists, for it is by contact[29] that they receive the breath and transmit it to the heart."[30] The collection of ancient Greek commonly called the "Works of Hippocrates" is judged to be of the fifth and fourth centuries B.C. There is included in this collection a brief treatise on the heart; and in this occurs the earliest known account of the structure and use of the semilunar valves, which together with the rest of the cardiac valves were unknown to Aristotle. In the same Hippocratic treatise the doctrine is adhered to of the entrance of air into the heart for cooling purposes, both the right and the left ventricle being specified as receiving it. The author says:-- "The vessel which leads out of the right ventricle ... closes toward the heart, but closes imperfectly, in order that air may enter, though not very much."[31]