The Trustworthiness of Reason - A Case Study

THE TRUSTWORTHINESS OF REASON: DESCARTES, HARVEY

AND THE HEART - A CASE STUDY ON THE NEED FOR CAUTION

Copyright 1999 by Emerson Thomas McMullen

ABSTRACT

During the Middle Ages, scientists and philosophers debated the trustworthiness of human reasoning ability. In the Reformation, the limits of human reason and will appeared in the controversy over predestination. This debate emerged in a slightly changed form in the seventeenth century. An example is the competing explanations over the motion of the heart of Rene' Descartes (1596-1650) and William Harvey (1578-1657).

Descartes and Harvey represent two different approaches to conducting scientific research. Descartes founded his method on reason, with some observation. Harvey based his approach on reason guided by varied and careful observations and by thoughtful experiments. Descartes' scientific method shows the dangers of primarily relying on reason. A most telling example is his explanation of the function of the human heart, which he showcased for his mechanistic anatomy and physiology.

INTRODUCTION

When Greek science, primarily in the form of Aristotle's teaching, entered the curriculum of the medieval universities, a far-reaching debate involving reason and truth ensued. The immediate question involved Christian faith versus pagan thought. This debate is illustrated in the picture on the right. In it, Aristotle, the height of human reasoning, willingly submits to the sensuous Phyllis, a well-known beauty from Greek legend. It is based on a thirteenth-century sermon arguing that reason unaided by revelation is suspect.

The debate over reason received a different twist during the Reformation. Luther and Calvin maintained that humans are incapable of choosing God on their own due to the Fall and resultant sin nature. Therefore God had to choose who would be saved. Erasmus and other opposed this view - they thought humans had free will and are capable of making a proper choice. The long-term question, "How trustworthy is human reasoning?" is still with us. A case study for this debate is Descartes' mostly reason-based explanation of the human heart's motion, compared to Harvey's mostly observational and experimental approach to determining how the heart works (1).

Harvey's Explanation of Cardiac Motion

In Chapter V of his famous book, An anatomical disputation concerning The Movement of the Heart and Blood in living creatures (1628), Harvey described the heart's primary movement as a contracting muscle:

" First of all, the auricle contracts, and in the course of its contraction throws the blood . . . into the ventricle, which being filled, the heart raises itself straightway, makes all its fibres tense, contracts the ventricles, and performs a beat, by which beat it immediately sends the blood supplied to it by the auricle into the arteries. . . ."(2).

This explanation lacks the quantitative details of a modern one, but, unlike Descartes', it is a correct statement of the heart's motion (3).

Our knowledge of the heart's movement is such that Harvey's description seems the most natural thing in the world. It is hard to realize how much he was breaking with explanations from Aristotle, Galen, and Vesalius. Ideas such as an attractive power in the heart had to be refuted. From his extensive observations and experiments, Harvey concluded that only Realdo Columbo (c. 1510-1559) had given the best explanation in his De re anatomica (1559). Harvey was the first to recognize this and argue for it. To Columbo's observations, Harvey added his own, and then went even further by providing experimental data.

From his studies of chick embryo development, Harvey reasoned out the heart's genesis and the very beginning of life. After four or five days of an egg's incubation, Harvey observed a "point of blood" in the midst of the "little cloud" that would become the chick. This blood began to beat, but the amount was so small that it disappeared from sight during contraction. On relaxation it appeared again as a red dot as tiny as the point of a needle or pin.

[This blood dot was such]"that betwixt the visible and invisible, betwixt being and not being, as it were, it gave by its pulses a kind of representation of the commencement of life"(2).

The dot of blood next became the auricle, and then later, the heart. But what made the blood beat in the first place? Vitalist Harvey speculated that a spirit with "an obscure papilation" may have been the cause. So, while Harvey reasoned and even speculated, he was guided by observation and experiment. He never let reason override observation. We will see that this was not the case with Descartes' methodology - he did let reason override observation.

Descartes' Explanation

Descartes thought of the heart, not as a muscle, but as a source of heat sufficient to rarify the blood. He had read Harvey's explanation, but rejected it. More in line with traditional Aristotelian thinking, Descartes thought that two large drops of blood cause each expansion of the heart. The heat of the ventricles rarefies each drop of blood, Descartes imagined, and thus it is the rarefaction of the blood, and this alone, that causes an expansion that moves the heart (4). This is obviously wrong to the modern reader, but the faults were not as clear to Descartes' contemporaries. One person did offer criticisms, however.

Plemp's Objections

Vopiscus Fortunatus Plemp, a Dutch physician, offered several criticisms to Descartes' explanation. One was that when the heart is removed from the lungs, it continues to beat long after the blood supply has been cut off. Descartes replied that residual blood in the atria kept the heart beating. Another objection was that a fish's heart is not very hot. Descartes answered that some liquids have low boiling points (5). Finally Plemp noted that Aristotle had already had this idea. A biographer of Descartes, William R. Shea, remarked how this last charge "galled Descartes who brooded over it for years" (6). Finally Descartes addressed this criticism a decade later in his Description of the Human Body:

"[I]t is the rarefaction of the blood, and this alone, that is the cause of the heart's movement. It may seem that Aristotle thought of this when he remarked, in Chapter Twenty of his book On Respiration, `this movement is similar, to the action of a liquid boiled by heat' . . . it is clearly only by chance that he happened to say something approaching the truth . . . "(7).

Descartes implies that he was the originator of this idea on heart movement, and if Aristotle seemed to have thought of it first, it was just a lucky guess.

Further Objections

There are other criticisms that contemporaries could have made, but did not. For instance, in Descartes' scheme, the rarifaction of the blood occurs only in the ventricles. This immediately raises the question of why the ventricles are so much hotter than the atria, even though both are made of the same tissue and are adjacent to each other. Descartes avoided this issue by generally referring only to the heat of the heart and not to that of the ventricles. Yet his explanation falls apart if all of the heart is hot enough to cause the blood to expand. He must have only the ventricles hot enough - otherwise the heated blood expanding in the atria would push back into the veins since there are no one-way valves there. Such a push back into the arteries would stop the blood from circulating and defeat the whole purpose of the heart's function. Descartes needed to supply some rationale explaining why the ventricles, made out of the same tissue as the auricles and connected to them, are so much hotter.

Another weakness in Descartes' explanation is that the two large drops of blood have to enter the ventricles simultaneously. But he supplies nothing to make the drops enter each ventricle at the same time in the first place. And there is nothing to ensure that the drops then continue to enter at the same time.

A more likely outcome in Descartes' "rarification" mechanism is that an expansion in the right ventricle would cause blood to flow into the left ventricle where it would expand in turn. The result would be a sequential expansion where a rarifaction in the right ventricle feeds the left side of the heart, which then undergoes its own, separate rarifaction. This is the more likely mode, instead of a one-step simultaneous expansion in both ventricles. Even if the two drops started out entering the ventricles simultaneously, any sudden irregularity in the beat, or just the heart's speeding up or slowing down, could cause it to slip into the sequential mode. A controlling mechanism of some kind is needed in Descartes' cardiac system to keep his simultaneous action from slipping into a sequential one, but he did not provide for one.

A final objection is that the asymmetry of the human heart causes problems for the symmetry of Descartes' simultaneous drops and expansions. Based on the asymmetry of the heart, Aristotle argued in Book III of On the Parts of Animals that the right side of the heart was hotter than the left. Yet Descartes, whose explanation followed Aristotle's thinking, suddenly broke with it. Descartes argued for a symmetrical arrangement, with equal times of blood entry, and seemingly equal heat in both left and right ventricles. He forced a basically symmetrical explanation onto the motion of an asymmetrical heart. Admittedly, this criticism is one that would have more appeal to a contemporary's way of thinking than that of a modern. However, the hotter right ventricle could also be a factor that could throw the simultaneous expansions out of synchronization.

Harvey versus Descartes

Both Harvey and Descartes based their arguments concerning the heart's motion on appeals to reason, observation, and experience. Both made extended use of vivisection and reported their observations. Harvey watched the slowed-down heartbeats of dying mammals and studied the simpler hearts of cold-blooded animals. Descartes reported putting a finger into a vivisected dog's heart in order to feel the motion, and spoke of observing the beating of a rabbit's heart (8). Concerning experimentation, Harvey made a deep incision in a beating heart and saw the blood spurt out on contraction.

Both investigators claimed wide experience. In a 1639 letter to Marin Mersenne, Descartes asserted:

"I have spent much time on dissection during the last eleven years, and I doubt whether there is any doctor who has made such detailed observations as I"(9).

Harvey recounted to the College of Physicians that:

"Having observed [the motion of the heart] for whole hours at a time, I was unable to discern [these things] easily by sight or touch. . . . See how arduous and difficult [it is] to discern either by sight or by touch, dilation and contraction. . . . "(10).

The high speed of the mammalian heartbeat makes it difficult to observe cardiac movements accurately. This observational difficulty was something Harvey was much more aware of than Descartes and explains why Harvey studied the slowed-down heartbeats of dying animals - something Descartes did not do.

Descartes

Regardless of his claims to do so, Descartes never balanced his commitment to a rationalistic methodology with careful observation and experiment. In his writings on the heart, he sounds as if he has this balance because he asks any reader unversed in anatomy to observe the dissection of a heart. Among other things, he invites his readers to:heart than in any other place in the body, and finally that this heat is capable of causing a drop of blood to swell and expand as soon as it enters a cavity of the

"heart, just as liquids generally do when they are poured drop by drop into some vessel which is very hot"(11).

Did Descartes really see drops of blood expand as he implies in his explanation? He saw the movement of the heart, its chambers, and the arrangement of the valves. He saw the difference between venous and arterial blood, but did he actually see blood expand due to heat? The answer is no - yet he calls on his readers to "observe . . . that this heat is capable of causing a drop of blood to swell and expand . . . "

In spite of not observing the blood's expansion, Descartes argues for his version of the heart's movement in the strongest possible terms:

"Now those who are ignorant of the force of mathematical demonstrations and unaccustomed to distinguishing true reasons from probable may be tempted to reject this explanation without examining it. To prevent this, I would advise them that the movement I have just explained follows from the mere arrangement of the parts of the heart (which can be seen with the naked eye), from the heat in the heart (which can be felt with the fingers), and from the nature of the blood (which can be known through observation). This movement follows just as necessarily as the movement of a clock follows from the force, position, and shape of its counter- weights and wheels"(12).

The parentheses in the above passage are Descartes', so he is claiming again that his mechanism is observable. He also invokes the force of a mathematical demonstration in his argument and implies that his critics are inexperienced in discerning true reasons from probable ones. Finally, he alleges that his true (not probable) explanation of the heart's movement necessarily occurs, just as the movement of a clock is mechanically driven by its various parts (13). Shea thinks these claims constitute "one of his boldest exercises in propaganda" (14).

Observation and Experiment

Descartes accepted Harvey's discovery of the blood's circulation, so how could Harvey be correct on the circulation, but seemingly incorrect on the operation of the heart? Descartes insisted that "all that this proves is that observations may often lead us astray when we do not examine their possible causes with sufficient care." He agreed that Harvey's claim that the heart contracts could explain the observations and the experiments made on the heart. "But all this does not alter the fact," Descartes stated, "that the same effects could also proceed from a different cause, namely the expansion of the blood which I described" (15). Descartes has let an imagined cause, instead of careful observation, guide his science. Why? Despite appearances, Descartes' appeals to observation and experiment are obviously not a part of his methodology for scientific advance. His dictum that "observations may often lead us astray" follows the thinking of Parmenides and Plato, who also did not trust sense data over reason. As already noted, Descartes never saw the blood expand because of the heart's heat (16). Yet he declares to us that this phenomenon is observable and tries to convince his readers that he did observe it. It is astounding that he could be so sure of himself and at the same time be so wrong.

CONCLUSIONS

Descartes' methodology leaves us pondering the true role of observation and experiment in his science. It appears that he used observations and experiments in two ways. The first way was to gain an initial understanding of the phenomena. The second way was to justify and support what he had intuitively concluded, based on his guiding idea of a mechanical universe (17). The method he used to break away from the old science was by reasoning "from first principles" along mechanical lines of thought. Unfortunately, for him and his followers, this did put his science in some fanciful positions. He is an example of the error of trusting too much in reason. The way forward for science was not just accepting Harvey's explanations over Descartes', but using Harvey's methods over Descartes' (18).

The value of experimentation and the testing of theories cannot be overemphasized. Explanation is important, but the example of Harvey and Descartes offering fundamentally different explanations for the same observed motions of the heart, is a cautionary warning to all future scientists. Speculation must be constantly curbed and guided by all known data, and must be tested by as many ways as possible.

REFERENCES

(1)Toellner T: The Controversy between Descartes and Harvey Regarding the Nature of Cardiac Motions. "Science, Medicine and Society in the Renaissance, Essays to honor Walter Pagel," Vol. 2. Ed. Debus AG. New York: Science History Publications, 1972, p82. Details about Descartes in this article have been developed from part of my book, "William Harvey and the Use of Purpose in the Scientific Revolution." Lanham, MD: University Press of America, 1998. See it for more details on Harvey.

(2)Harvey W: "The Works of William Harvey." Trans. Willis R. Philadelphia: University of Pennsylvania Press, 1989, p31.

(3)For those who need a refresher, the following is how our hearts work: Basically, the human heart is muscle arranged in two upper chambers, the atria (or auricles) and two lower chambers, the ventricles. These chambers move the blood by contraction and by the configuration of the one-way cardiac valves. The resting heart contracts about seventy-five times a minute, in which case each cardiac cycle (one heartbeat) takes 0.8 of a second. During the first 0.4 second of this cycle, the heart is relaxed and blood is flowing from the atria down through the open atrioventricular (tricuspid and bicuspid) valves into the ventricles. The semilunar (pulmonary and aortic) valves are closed and prevent blood from flowing back into the ventricles from the arteries. Then a muscular contraction wave begins at the top of the heart. During the next 0.1 second, the atria contract and push more blood into the ventricles, completely filling them. The contraction of the ventricles occurs in the next 0.3 second. During this period the atrioventricular valves close and the contracting ventricles force the blood out of the heart through the semilunar valves and into the arteries. The cycle then repeats.

The heart is asymmetrical - the left ventricle has much more muscle than the right. The extra muscle is needed to propel the blood throughout the arterial system. During contraction, the blood pressure in the right ventricle rises to about 30 millimeters (mm) of mercury (Hg). However, the blood pressure in the left ventricle climbs to about 120 mm Hg. See Tortora GT and Grabowski, SR: "Principles of Anatomy and Physiology," 7th Ed. New York: HarperCollins College Publishers, 1993, pp605-607.

(4)Descartes R: "The Philosophical Writings of Descartes," Vol. 1. Trans. Cottingham J, Stoothoff R and Murdoch D. London: University of Cambridge Press, 1985, pp318-319. Pierre Gassendi did not accept Harvey's explanation for the circulation of the blood, but for a different reason. He thought he had once seen a demonstration of the presence of the pores in the septum (as per the received view). He reasoned that, if the pores were there, they had to be there for a purpose. Thus, Harvey was in error concerning the heart. Debus AG: "Man and Nature in the renaissance." Cambridge University Press, 1978, reprinted 1983, p70.

(5)Shea WR: "The Magic of Numbers and Motion, The Scientific Career of Rene^' Descartes." Canton, MA: Science History Publications, 1991, p307.

(6)Ibid, p308.

(7)Descartes R: "Writings," Vol. 1, p319.

(8)Ibid, pp317-318.

(9)Descartes R: "Philosophical Letters." Trans. Kenny A. Minneapolis: University of Minnesota Press, 1981, p64.

(10)Harvey W: "Lectures on the Whole of Anatomy." Trans. O'Malley CD, Poynter FNL, and Russell KF. Berkeley: University of California Press, 1961, pp185-186.

(11)Descartes R: "Writings," Vol. 1, p135.

(12)Ibid, p136.

(13)This passage runs counter to Larry Laudan's argument that Descartes thought of science as "necessarily hypothetical and probabilistic," and that "We can never get inside nature's clock to see if the mechanisms are what we think them to be." See Laudan's "Science and Hypothesis." Boston: D. Reidel Publishing Co., 1981, pp27-58.

(14)Shea, WR: "The Magic of Numbers," p307.

(15)Descartes R: "Writings," Vol. 1, p317.

(16)Another explanation for seeing phenomena that does not occur is "theory-laden" observation. See Hanson NR: "Patterns of Discovery." New York: Cambridge University Press, 1958/1965.

(17)This is one answer as to why Descartes collected empirical data but did not test his intuitions. Spyros Sakellariadis, based primarily on an analysis of Descartes' physics, thinks the answer is due to Descartes' need for a general theory to show that the data in question are relevant. See Descartes' Use of Empirical Data to Test Hypotheses. Isis 73:68-76, 1982. However, my interpretation better explains a letter which Descartes had written to Mersenne in 1639. In it, Descartes stated that he explained everything differently than Harvey did, but if what he had written was false, he would concede that the rest of his philosophy likewise was in error. Descartes R: "Philosophical Letters," p62.

(18)For a more comprehensive discussion of how science changes, see my Scientific Discoveries, Change, and Advancement: A New Framework and Analysis. GA J Sci 55:161-173, 1997.

ACKNOWLEDGEMENTS

"Aristotle and Phyllis" by Urs Graf is courtesy of Anhaltische Behrdenbibliothek, Dessau, Germany. I thank the reviewers of a similar article to this that was published in the Georgia Journal of Science, Vol. 56, No. 4, 1998, pp. 255-262 and Sharon Ann McMullen for doing the word processing.