In his eighth chapter, Harvey feels himself to be bringing forward considerations of so novel a character, that “I tremble,” he says, “lest I have mankind at large for my enemies, so much doth wont and custom, that become as another nature, and doctrine once sown and that hath struck deep root, and respect for antiquity influence all men: _still the die is cast, and my trust is in my love of truth, and the candour that inheres in cultivated minds_.” He found it impossible to account for the constant influx of blood into the arteries, and the return of blood to the heart, unless there was “a motion, as it were, in a circle.” And he shows by calculations of the quantity passing through the heart in an hour, that it is much more than the whole body contains, and that there is no way except by communications taking place from arteries to veins in every part of the body. Finally, he clearly shows how the valves in the veins promote the return of blood to the heart. Throughout the whole of this treatise considerations from comparative anatomy, from the phenomena of human diseases, and from natural philosophy, are thickly interspersed, and imagery of the most suggestive character is called into requisition; the whole forming a treatise that every scientific man might well read, and that no doctor should consider himself fully educated without having attentively perused. In a subsequent letter to John Riolan the younger, Professor of Anatomy in the University of Paris, Harvey lays down—in opposition to those who repudiate the circulation because they cannot see the efficient nor final cause of it, and who exclaim, _Cui bono?_—the fundamental scientific axiom, “Our first duty is to inquire whether the thing be or not, before asking wherefore it is.” Again, “He who truly desires to be informed of the question in hand, and whether the facts alleged be sensible, visible, or not, must be held bound, either to look for himself, or to take on trust the conclusions to which they have come who have looked; and indeed there is no higher method of attaining to assurance and certainty.”