Analogical Reasoning in the Harveian Model

Harvey prefaced his De motu cordis with a plea for analogical reasoning. Indeed, in developing his theory in De motu cordis and De circulatione sanguinis, he discussed about a dozen analogies, using most of them to support his own theory. Some analogies he borrowed from the Greeks, some from political theory. Some were biological, and some were mechanical.

Harvey indulged in lazy or macrocosmic analogy only twice. In one case he made the commonplace observation that circularity is a natural phenomenon, citing Aristotle's view that "the air and rain emulate the circular movement of the heavenly bodies," that the condensation and evaporation cycle is a kind of meteorological circularity, and that the sun's circular motions cause storms.^[30] Harvey's purpose here was to justify a loose use of the terms "circle," "circularity," and "circulation." Moving from the truly circular revolutions of heavenly bodies — Harvey was no Keplerian — to the figurative circularity of the condensation-evaporation cycle, Harvey suggested that in this figurative sense, repetition constitutes circularity, which is thus akin to rejuvenation.

The second instance of macrocosmic analogy evolved into a biological analogy. Harvey wanted to show that "blood permeates from the right ventricle of the heart through the parenchyma of the lungs into the vein-like artery and the left ventricle." To show that such a passage is possible in nature, Harvey reminded the reader that water seeping through the earth "gives rise to streams and springs." Two other examples — sweat passing "through the skin" and urine "through the parenchyma of the kidneys" — show that Harvey employed a lazy analogy to demonstrate the general possibility of seepage in nature. But he chose a biological comparison with sweat and urine to illustrate seepage in the body.^[31]

A clearer instance of causal biological analogy exists in Harvey's explanation of the two motions of the heart, which "occur successively but so harmoniously and rhythmically that both [appear to] happen together and only one movement can be seen."^[32] He cited three analogies. Two were technological (involving comparisons with geared machinery and flint-lock firearms). The third and most extended comparison was with swallowing, and Harvey used this biological analogy to make his causal point.

Harvey anticipated later seventeenth-century biologists in taking "as a model of the living thing the living thing itself."^[33] He did not hesitate, however, to use mechanical models — such as the pump, the glove, and the filling of leather bottles — to draw causal inferences, starting from the premise that similar motions have similar causes.^[34] Thus, in careful hands even technological models could serve as causal analogies for the biological sciences.

Harvey's analogies reveal various causal assumptions and traits of argument. First, he explained biological processes chemically. Second, several models, such as the image of the "carefully planned and ingenious arrangement of ropes on a ship," were solely didactic. Third, in scrutinizing the analogies of his opponents, Harvey demanded rigorously close comparisons, pushing the analogies of others to their limits and ridiculing inept comparisons (like the notion that blood flows like water in the seas) by reductio ad absurdum.^[35]

In summary, Harvey used analogies in three ways. He proved that a process (such as permeation) was possible in one structure because it was already known in another. He taught by likening a phenomenon to a more familiar sight (such as a gun, a machine, or the ropes on a ship) whose workings were either well known or obvious to the observer. Finally, he justified the loose use of the word "circulate." Reasoning from macrocosmic models was relatively insignificant for his argument. While Harvey demanded that his opponents' analogies be accurate with respect to both behavior and causation, his own models were principally a source of general inspiration or a means of teaching. Whether heuristic or explanatory, they came mostly from outside the biological sciences. His causal analogies (the comparisons with swallowing, the leather bottle, the glove, or the fermentation of wine) simply assumed that similar phenomena resembled one another because they had similar causes. Finally, Harvey's analogical reasoning was not systematic but rather ad hoc or episodic. Thus, analogy rarely inspired him to apply either the methods or theories of another discipline to his own; it was unusual that likening the heart to a

pump led him to measure the flow of blood through the body or that chemical comparisons led him to draw theoretical inferences.