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Chapter 9 Analogical Reasoning: The Model
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Chapter 9
Analogical Reasoning:
The Model

In the late seventeenth century botanical thinking derived its inspiration from four sources: natural phenomena; the more or less traditional ideas that defined and constituted the field itself; ideas borrowed from other disciplines; and newly invented scientific apparatus.

The stimulus provided by natural phenomena was particularly dramatic in the early modern period because, with the discovery of the new world and the more assiduous exploration of the old, the number of plant species known to Europeans quadrupled. As a result, fifteenth-, sixteenth-, and seventeenth-century botanical literature reflects a delight in the superabundance of nature. Many botanists focused on the natural resources of the discipline, trying to record and describe all known types. Academicians contributed to this enterprise by preparing their natural history of plants.

Traditional botany provided part of the conceptual framework for students of plants, who distinguished flora from other living things in an Aristotelian manner and modeled their treatises in style and content after those of distinguished predecessors. But by the middle of the seventeenth century there was a marked shift in botanical writing away from herbals and toward specialized treatises on classification, regional flora, plant anatomy and physiology, and exotic specimens. When academicians looked backwards, it was often to disprove survivals from earlier literature that they regarded as superstitions. Insofar as they were influenced by botanical literature it was by natural histories of the recent past rather than by physiological treatises by contemporaries such as Grew or Malpighi.[1]


In the late seventeenth century savants challenged and expanded the traditional ideas. They did so through cross-disciplinary borrowings of ideas and instruments, which provided new interpretations and phenomena for contemplation. Theories adapted from other disciplines, especially from animal anatomy and physiology, stimulated botanists to reinterpret the structure and behavior of plants. Instruments such as the microscope and air pump helped them see plants in more detail and from new perspectives. As a result of such inspirations, academicians and their contemporaries were swept up in a compelling new explanatory momentum.

Both descriptive and explanatory botanical writing drew from all four sources — the plant, the field of botany, borrowed ideas, and new apparatus — to some extent. As a result, by the eighteenth century the concepts of the plant and of botany were transformed. Natural history focused primarily on naming, describing, and classifying plants, while natural philosophy studied the processes related to the plant's life cycle, drawing heavily on theories and equipment developed in other contexts.

At the Academy the natural history was inspired primarily by natural phenomena and by ideas from traditional botany. When academicians drew on ideas from other sources, they relied on chemistry. Oddly, the natural history of plants excluded anatomy, even though this was the mainstay of the Academy's natural history of animals. Instead academicians applied any anatomical study of vegetables to their physiological theorizing. Their natural philosophy, in contrast to their natural history, depended for its inspiration principally on other disciplines, especially zoology, and on new instruments. In the end it altered the very idea of the plant itself.

Academicians focused their natural philosophical research into plants on three questions: does sap circulate in plants the way blood circulates in animals; which accounts better for plant physiology, chemical or mechanical explanation; and do the air pump and microscope clarify how plants reproduce and grow? The present chapter describes analogical reasoning and the Harveian model that caused academicians to ask the first question; the following chapters address their answers to all three questions.

The Nature and functions of Analogical Reasoning

Analogical reasoning has played an important role in the development of the sciences. As a means of explaining the unfamiliar in terms of the


familiar or of subsuming one field under the laws governing another, it has didactic and heuristic value; fertile analogical theories may enrich both the borrowing and the lending disciplines. The stimuli to reasoning from analogy may be both general and specific. In the seventeenth century, Galileo, Descartes, Newton, and others had predisposed savants to "a unitary conception of natural forces," and discoveries in many individual scientific disciplines were so impressive as to become paradigmatic for other fields as well. Thus botanists were beguiled in particular by advances in animal physiology to essay zoological methods and theories in their own domain.[2]

Two characteristics distinguish late seventeenth-century analogical reasoning about plants and animals from earlier comparative theories. First, unlike the traditional theories of sympathies and antipathies, the new analogies provoked further tests; insofar as the model itself was experimental, so was the analogical hypothesis. Second, savants spurned the technological models and mathematical standards that many had previously favored and chose instead models and standards from the life sciences themselves. That was possible principally because the Harveian theory explaining the motions of the heart and blood offered a seductive model. Consequently, much analogical reasoning in botany reveals a double trend: toward increased experimentalism in botany and zoology and toward greater self-reliance within the biological sciences.

The most ambitious and elusive of botanical analogical leaps in the seventeenth century was the hypothesis that sap circulates in plants as blood does in animals. First propounded in 1660 by Johann Daniel Major, only one generation after William Harvey published his De motu cordis, the idea caught on quickly in England, France, and Italy. Although Nehemiah Grew and Marcello Malpighi are probably the best known adherents of the theory, they were not alone in exploring it systematically. Members of the Academy were the first to push the analogy to its limits. Claude Perrault and Edme Mariotte defended the idea at meetings of the Academy during the summer of 1668, with Nicolas Marchant demonstrating Mariotte's experiments and Samuel Cottereau Duclos opposing the hypothesis. In 1679 and 1680, Mariotte, Perrault, and Duclos published revised statements on the subject, and later in the 1680s other academicians, especially La Hire, tried to repair the analogy.[3] Their efforts shed light on how the Academy fostered the biological sciences, on the dramatic changes in botanical research in the late seventeenth century, and on the merits of analogical reasoning.

Before examining the theory of the circulation of the sap as developed in


the Academy, a double foundation must be laid by establishing the nature of analogical reasoning and by describing the features of the Harveian model that inspired academicians. Only then can academicians' elaboration of the hypothesis be assessed.

Analogy is a means of comparing two things. By identifying similar traits in both objects, scientists infer the existence of a causal mechanism affecting both. Several types of analogy used by scientific savants have been identified by historian-philosophers of science. Claire Salomon-Bayet distinguishes "lazy-universal" from "experimental or observational" analogies; Georges Canguilhem differentiates mathematical analogies from explanation by reduction; and Mary Hesse compares formal and material analogies.

Salomon-Bayet's distinction is addressed specifically to the early modern period. Taking Paracelsianism as the exemplar, she defines lazy analogy as a mental habit of ancient origin that simply assumes untested the sympathy or antipathy of all parts of the universe. By contrast, experimental or observational analogy is open to verification and correction. Experimental analogy subsumes particular objects or phenomena under general theories either by applying the laws or theories of one discipline to another or by employing a model; the first method is more fertile than the latter, which has didactic but not explanatory power.[4]

Canguilhem focuses on analogical reasoning in biology, contrasting deduction (or the use of mathematical models) and explanation by reduction (or the use of mechanical analogies or analogical models). The former is less naive but also less useful than the latter for biology, which is not always susceptible to expression in mathematical language.[5] Canguilhem's discussion also clarifies Salomon-Bayet's distinction between analogy from theories and analogy from models. Two examples suggest the two kinds — structural and functional — of analogy: the stirrup and anvil after which the bones in the ear are named, and the ancient irrigation system which inspired the Greek concept of the motion of blood. Unlike Salomon-Bayet, who insists that a fertile analogy must prompt experiment or observation, Canguilhem allows that in biology analogy from models can be an alternative to experiment, because models permit "the comparison of entities which resist analysis.[6] Both Canguilhem and Salomon-Bayet agree that models may aid the quest for laws.

Mary Hesse is interested in both the physical and the biological sciences and, unlike Canguilhem and Salomon-Bayet, systematically analyzes analogical reasoning as a logical tool appropriate in all sciences. Hesse distinguishes between material analogy and formal analogy. Material analogy


is both substantive and predictive, whereas formal analogy is neither, since it is simply a one-to-one correspondence between different interpretations of the same formal theory. Thus Hesse's analysis of material analogy is relevant to seventeenth-century botany.

Material analogy is a means of comparing different organisms or phenomena by pairing and comparing their individual traits. The model is the organism or phenomenon that is already understood; the explicandum is the organism or phenomenon that needs to be explained. A systematic comparison of their traits will determine whether the explicandum can be understood in terms of the model. Thus, if the traits in the model resemble those in the explicandum, and if the traits in the model constitute a causal mechanism, then the same causal mechanism may be inferred in the explicandum.

Hesse provides a schema for such comparisons, listing the paired traits in two columns, one for the model and one for the explicandum. Paired traits are subject to "horizontal" comparison, while the theory linking a set of traits as the causal mechanism in the model provides a "vertical" connection. The closer and more numerous the horizontal similarities between pairs, the more likely that the vertical causal chain of the model may be inferred to exist in the explicandum.[7]

Material analogies may have explanatory power if three conditions are met. First, the model and explicandum must have something in common beyond the analogy in question, that is, there must be "pretheoretic" similarities between them. Second, their horizontal similarities must be substantial. Third, there must be a causal connection among the traits in the model.[8]

Hesse's analysis clarifies Canguilhem's and Salomon-Bayet's distinctions. Thus the most fruitful dichotomy is not between models and the application of one discipline to another, as Salomon-Bayet argues, nor between mathematical models and explanation by reduction, as Canguilhem would have it. Rather, there are empty and productive analogies, and the latter may be either mathematical or material; but only material analogies with pretheoretic similarities may have predictive power.

Analogical reasoning has various advantages and shortcomings. As a form of induction, it is prey to all the shortcomings of the inductive method. But analogical argument enjoys a special role when observation or experiment are inadequate.[9] In such a case, however, it is inconclusive not only for all the usual inductive reasons but also because it rests on an incomplete identification of similarities. Nevertheless, analogy offers a means of selecting


a hypothesis, because it draws attention to comparable properties that may betray causal similarities.[10]

As differentiated from experiment, analogy permits comparisons between traits or phenomena that cannot be analyzed.[11] Just as an experimenter uses theory to suggest predictions that do not proceed from tests and observations, so the savant uses analogy to suggest hitherto unsuspected causes or theoretical entities.[12]

Analogy resembles theory, because it offers a way of subsuming a pattern of behavior under a set of laws, and because it holds out the promise of generating explanations and predictions. Canguilhem explains how analogies differ from experiment and are like theories:

What validates a theory is the possibility of extrapolation and prediction which it permits in directions which the experiment, keeping to its own level, would not have indicated. Similarly, models are judged and tested one against another by the completeness of the accounts they give of the properties to which they direct attention, in the object of study, and also by their aptitude for revealing properties hitherto unnoticed. The model, one could say, predicts.[13]

In explaining this similarity of analogical reasoning to theory, Max Black's study of metaphor is useful. A metaphor is a comparison whose thrust is indefinite. So long as the scientist does not know "how far the comparison extends" and tries to push the analogy to its extreme, unexpected theoretical implications may emerge, for "it is precisely in its extension that the fruitfulness of the model may lie."[14]

Good analogies, therefore, supplement and encourage experiments and, like theories, suggest what, unobserved, might have remained unsuspected. It sometimes happens that an analogy, like a metaphor, changes the way both the model and the thing being explained are viewed. This may be due to the impoverishment of the model,[15] or to changes in the meanings of the concepts associated with the model and explicandum; sometimes "the two systems are seen as more like each other."[16]

Analogical thought has influenced biological thought in both positive and negative ways, depending on how the model is selected. For example, when models are more appropriate than experiments, analogy can stimulate alternative observations or anticipate evidence that is inaccessible given experimental capabilities. But overreliance on mechanical and technological models has injured biological analysis, by emphasizing structure at the expense of function. Thus, Greek and Latin anatomical nomenclature suggests the appearance but not the function or causal mechanism of the anatomical part. Such analogies cannot "show the identity of the


general laws of the two fields of phenomena which are brought together" and thus are causally insignificant.[17]

The risks and rewards entailed in the choice of model are illustrated in two theories of the motion of the blood. Both Harvey and the ancients explained the motion of the blood analogically. The ancients compared it to the unidirectional supply of water to irrigation channels and hence argued that blood was absorbed by the body and had to be continually replenished; their theory was conceptual, not experimental. Harvey replaced the notion of irrigation with the idea of circulation within a closed circuit, an idea that was compatible with his experimental findings.[18] Models chosen from technology on the basis of structural resemblance are likely to lead to an explanatory cul-de-sac when applied to the biological sciences. But in the seventeenth and eighteenth centuries savants began to use biological models more often, with positive results for the life sciences and especially for plant and animal physiology.

Analogical reasoning can serve as the prelude to comparative method. This is particularly important in analogies between plants and animals, which often resemble one another functionally, but have different structures. Either similarities or differences may be emphasized, but pretheoretic resemblances determine whether the comparisons are valuable.[19] The important point for comparative method is to test, not assume, any similarities. As will be seen in the following chapter, some academicians used analogy in precisely this fashion; starting with a comparison between plants and animals, they tested the analogy experimentally, admitting structural dissimilarities between plants and animals and investigating how plants accomplished equivalent functions in the absence of equivalent organs. In such cases, analogy identifies crucial dissimilarities and becomes a preliminary step to understanding causal differences.

Analogies are full of pitfalls for the unwary. Even scientists who rigorously examine the traits of two organisms for dissimilarities need to guard against claiming too much for an analogy. In the eighteenth century, Albrecht von Haller complained that analogy had become a substitute for experiment. This was the flaw that had weakened medicine, he believed, because "the great source of error in physics" was due to "physicians, at least a great part of them, making few or no experiments, and substituting analogy instead of them."[20] Analogies should inspire experiment, and although they may extend it, they should not be a substitute for it. The overenthusiastic savant runs the risk of conferring "a representational value on a model" and of letting the model become axiomatic instead of being


only the lender of a mechanism.[21] Analogies are frail inductive tools. When the identification of similarities or dissimilarities is incomplete, then the analogy is inconclusive.

In summary, analogies have more than a didactic value to scientists. They offer a means of selecting hypotheses. As a weak form of induction, they can be useful when only a few instances are known or when only sparse observational data exist. They should inspire, may supplement, and will sometimes replace experiments. They resemble theories in suggesting new experiments, subsuming the behavior of an object under a general rule, predicting, and explaining. They are fertile because, like metaphors, their extension is indefinite; when scientists push analogies to their extremes, they may discover what would otherwise have eluded them. The best analogies pay compound interest on what they have borrowed, by changing the way both the borrowing and the lending disciplines are perceived. Finally, failed analogies have a particular use: by drawing attention to crucial dissimilarities in the things being compared, they stimulate a search for the causal mechanism. Failed analogies, therefore, are the beginning of comparative method and show where further research will be necessary.

Improved analogical reasoning helped advance the biological sciences. The choice of model was important. What was needed was a broadly conceived, well developed theory, one with adequate observational and experimental supports to win adherents, and sufficient specificity to avoid sectarian splits. Chemistry was appealing, and was regarded by Paracelsus, Helmont, Boyle, and others as a source of knowledge about the basic and unchanging components of the universe, but it was beset by doctrinal divisions.

In the absence of an approved general explanatory theory, savants had recourse to smaller-scale theories with a more limited explanatory range. William Harvey's writings on the movements of the heart and blood offered just such a theory, and as his hypothesis became acceptable scientists in other fields borrowed it. In the Academy and elsewhere, botanists formulated the hypothesis that sap circulates in plants. Their analogical leap is an important case study for several reasons. It is an example of borrowing from within the biological sciences and illumines the relative importance of structural and functional analogies in botany. It indicates whether savants tested analogy experimentally and how they responded to its limitations. Finally, it reveals that when the model failed, academicians used analogy as a stimulus to comparative method and thus came to ask an important botanical question.


The Circulation of the Blood

Three theories of circulation competed in France after 1628: the hypotheses of William Harvey, Jean Riolan, and René Descartes. Harvey claimed that blood made a complete circuit of the body, that the heart pumped it into the arteries, that blood then passed to the veins and returned to the heart, and that blood nourished and heated the body. He believed that the heart, veins, and arteries were "constructed for that purpose with extreme foresight and wonderful skill," and thus that their structures revealed their functions.[22] Descartes and Riolan accepted the idea of circulation. But they disagreed with Harvey about important details, challenging, for example, his estimate of the speed with which blood circulated. As a result, they proposed alternative explanations of the motions of the heart, and they retained certain elements from ancient theories about the motion of the blood. French botanists who wished to formulate an analogical theory of the circulation of sap had, therefore, three models from which to choose. None, however, was perfectly compatible with vegetable anatomy and physiology. These models must be clear if the theories of the circulation of the sap are to be understood.

Harvey's theory was the most important and was widely accepted in scholarly circles by the late 1660s. It was novel in several respects: it unified the venous and arterial systems, described the pulse as a mechanical effect of the heartbeat, calculated the quantity of the blood, and characterized the circulation as a closed system in which all blood returned to the heart without being consumed. Harvey challenged standard notions about the hierarchy of bodily organs. He maintained that the blood was more important than the heart because it preceded the heart in the development of a fetus. He also claimed that the heart was formed prior to the brain and liver and was thus more important to life than either of those organs.

Harvey also retained certain traditional views. He believed, for example, that the purpose of circulation was to nourish and warm the body by generating heat and spirits necessary for life.[23] This made a circulatory motion necessary, in his view, not only so that all the parts of the body "may be nourished, warmed, and activated by the hotter, perfect, vaporous, spirituous and, so to speak, nutritious blood," but also in order to repair the blood which "may be cooled, coagulated, and be figuratively worn out" in its travels. The heart held a special, beneficial position in the body, for it was the "source or the centre of the body's economy" and could restore blood "to its erstwhile state of perfection. Therein, by the natural, powerful, fiery heat, a sort of store of life, it is re-liquefied and becomes impregnated


with spirits and (if I may so style it) sweetness."[24] The Harveian model was experimental and mechanistic, but as such passages reveal, it was also teleological and vitalist.[25]

To prove his theory, Harvey tested and confirmed three assumptions. First, "the blood is continuously and uninterruptedly transmitted by the beat of the heart … into the arteries" in large quantities that cannot be made up by intake of food. Second, the pulse of the arteries drives the blood into every part of the body, in greater quantities than necessary for nutrition, and in such amounts that a rapid circular motion must be assumed. Finally, "the veins themselves are constantly returning this blood from each and every member to the region of the heart." As proof, Harvey cited his measurement of the amount of blood passing through the body in a half-hour; his experiments with ligatures of blood vessels; and his description of the structure and function of valves in the veins. Once the three suppositions were confirmed, Harvey could state "that the blood goes round and is returned, is driven forward and flows back, from the heart to the extremities, and thence back again to the heart, and so executes a sort of circular movement."[26]

Although Peiresc and others in France defended Harvey's theory from the beginning, Riolan and Descartes both proposed alternative theories of the circulation of the blood. Riolan, a respected anatomist and member of the medical faculty of Paris, did not object to the idea of circulation in itself. But he found Harvey's formulation distasteful because it challenged Galen and undermined some of the theoretical bases of traditional medical practice. Riolan also mistrusted Harvey's assumption that the anatomy of animals may resemble that of humans.

Riolan argued that the blood traveled through the arteries and veins to the extremities of the body and returned to the heart two or three times a day. Not all blood returned to the heart, however, because some of it was assimilated into the body. Although the normal route of the blood was away from the heart in the arteries and to the heart in the veins, when the veins of the arms and legs threatened to become empty the blood in the veins of the trunk could flow backwards to prevent a void. Thus Riolan maintained conventionally that blood ebbed and flowed in the veins and that it was consumed as nutriment by the parts of the body.

Riolan also calculated the amount of blood in the heart and the entire body and the quantity of blood that passed through the body in one hour. But he disagreed with Harvey. Riolan did not believe that the heart propelled the blood, as Harvey had shown. Instead he claimed that the blood kept the heart in motion, as a stream moves the wheel of a water mill. In


Riolan's view, blood prevented the heart from drying out, while the heart reheated the blood and replenished it with spirits. Although Riolan agreed with Harvey about that function of the heart, he contradicted him in insisting on the primacy of the liver.[27]

Descartes's theory was closer than Riolan's to Harvey's. Thus Descartes accepted the full circulation of the blood through the body, but he rejected Harvey's theory of the motion of the heart. Arguing that physiological phenomena resulted from chemical processes, Descartes claimed that when the wet blood reached the hot heart it vaporized and expanded. This stretched the heart. As the blood cooled, it condensed, and the heart contracted. This alternate vaporizing and condensation accounted, in Descartes's system, for the heartbeat and pulse.

Both Descartes and Riolan agreed with Harvey that there were anastomoses connecting the arteries to the veins. Descartes incorrectly gave Harvey credit for discovering them, although Harvey had simply assumed they existed. Descartes accurately summarized three of Harvey's proofs for the circulation, namely the argument from ligation, the argument from the function of valves in the veins, and the fact that all blood in the body can exit from one cut artery. But he did not stress Harvey's estimate of the amount of blood that passes through the heart in an hour. Like Harvey and Riolan, Descartes believed that circulating blood carried heat and nutrition to the body. Like Harvey, he argued that blood was not itself a nutriment but carried food. In order to explain how the body obtained this food, Descartes drew on an analogy with sieves, which permit small particles to pass but retain larger ones. Descartes believed the heart repaired and renewed the blood. To explain the motion of the heart, Harvey and Riolan cited mechanical models — a pump and a mill — while Descartes, the mechanist, derived his explanation from chemical processes.[28]

Harvey first published his theory in De motu cordis in 1628. It quickly found defenders and detractors in France. Although it was banned from the Parisian medical school, lecturers at the Jardin royal disseminated the theory, and by the 1660s a Harveian school had established itself in France, counting among its members Claude Tardy, Jean Pecquet, Jacques Mentel, Pierre Guiffart, Jean Martet, Jacques Chaillou, and Pierre Betbeder. Several defended the Harveian theory of circulation in vernacular treatises about such topics as the lacteal veins, the lymphatic vessels, chyle, and the preparation of blood. Their affiliations reveal that medical faculties had become receptive to the theory of circulation and that even physicians educated by faculties hostile to the theory might adopt it. Tardy was physician to the duke of Orléans and doctor regent at the Parisian medical


faculty. Chaillou practiced medicine at Angers. Martet was a master surgeon and royal anatomist in the faculty of medicine at Montpellier. Mentel had been educated in medicine at Paris in the late 1620s and early 1630s. Pecquet corresponded with Harvey and became one of the original members of the Academy. Most of Harvey's defenders published in the vernacular, perhaps hoping, as Guiffart put it, to reach a less dogmatic audience. Harvey's French proponents stressed his quantitative findings (although they gave different figures) and his experiments with ligatures, but they relied far less on analogies to explain the theory than had Harvey.[29]

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.


Harvey's theory of circulation unified the heart, veins, and arteries in a single system. He believed that his theory had utilitarian implications and could explain some "events fundamental in practical medicine," such as "the suppression or cause of hemorrhage, sloughing and gangrene, the assistance derived from ligature in castration or the removal of tumours."[36] The explanatory power of the theory encouraged Harvey and his proponents to use it also to improve general knowledge of physiology, both animal and vegetable. His work offered more than a theory to botanists; it was also a model of experimental method and analogical reasoning.

By the 1660s the idea that blood circulates was well established in France, save in a few ultra-conservative circles. Although Harvey's theory had triumphed, it remained controversial and some savants were loyal to the competing views of Riolan and Descartes. Circulatory theory depended principally on observation, quantitative analysis, and an assumption that structure and function were closely related. But it was also indebted for its inspiration and exposition to analogical reasoning. It was only reasonable, therefore, for natural philosophers to apply circulation theory to plant physiology. While German and English savants were the first to raise the possibility, the French academicians soon surpassed them by testing systematically how well the model applied to the vegetable kingdom.


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