Harvey was very much influenced by the ideas of Greek philosopher Aristotle and the natural magic tradition of the Renaissance. His key analogy for the circulation of the blood was a macrocosm/microcosm analogy with the weather system. A macrocosm/microcosm analogy sees similarities between a small system and a large system. Thus, one might say that the solar system is a macrocosm and the atom is a microcosm. The Renaissance natural magic tradition was very keen on the idea of the human body as a microcosm. The macrocosm for Harvey was the Earth’s weather cycle. Water was changed into vapour by the action of the Sun, and the vapour rose, was cooled, and fell again as rain. The microcosm was the human body, where the action of the heart was supposed to heat and change the blood, which was cooled again in the extremities of the body. Harvey says (and compare the earlier quote concerning the king) that:

So the heart is the beginning of life, the Sun of the Microcosm, as proportionably the Sun deserves to be call’d the heart of the world, by whose vertue, and pulsation, the blood is mov’d, perfected, made vegetable, and is defended from corruption and mattering; and this familiar household-god doth his duty to the whole body, by nourishing, cherishing, and vegetating, being the foundation of life, and author of all.

This was critical to Harvey. How could arterial blood be rapidly, efficiently, and consistently converted into venous blood (and vice versa) within one system? This was a key question, which prompted Harvey to draw on his macrocosm/microcosm analogy. It also should be noted that much of his terminology for change was drawn from the alchemy of his time. Harvey was very much a man of the later Renaissance—not a man of the scientific revolution and its mechanical nature.

Studies of reproduction

Harvey spent much of the latter part of his career working on the nature of reproduction in animals. He worked on chickens as an example of oviparous reproduction, in which embryonic development occurs within eggs hatched outside the mother’s body, and on deer as an example of viviparous reproduction, in which embryonic development occurs within the mother’s body, resulting in the birth of live young. Harvey’s work in this area generated a wealth of observational detail. At the time, reproduction was poorly understood, and Harvey investigated issues of the role of sperm and menstrual blood in the formation of the embryo. His observations were excellent, but such matters could not be resolved properly without the use of the microscope.