May 22, 2025

Reflecting on a person's life is rarely a black-and-white process. While we can highlight experiences that are inspiring, instructive, or encouraging, they may not always apply to different contexts and situations. Moreover, success or failure in one area does not necessarily define an individual’s journey in other aspects of life. With this in mind, I would like to briefly explore two facets of a hero’s life in this article: Louis Pasteur.

Pasteur as a Scientist

Before the end of his adolescence, little indicated that Louis Pasteur would become Pasteur. Born in Dole, France, in 1822, he was not an especially distinguished student, and the surviving documents from his youth in Arbois suggest a stronger inclination towards drawing and painting than science. However, he soon abandoned charcoal and pastels for the microscope and laboratory. Indeed, years later, upon moving to Paris, Pasteur began to show a deep interest in chemistry, a field in which he would eventually earn his doctorate.

Tartaric acid was the subject of his earliest investigations, during which, thanks to optical phenomena, he discovered two distinct forms of the compound. Such substances, architecturally asymmetric and non-superimposable yet having identical molecular composition, are termed "chiral." Pasteur is credited with discovering these molecular “isomers”.

Not long after, Pasteur was appointed professor at the University of Strasbourg, where, in 1849, he married the daughter of the rector of the Faculty of Sciences. The prolific nature of Pasteur’s research was fueled by tragic circumstances: three of his five children would die prematurely. His relentless pursuit of a cause – and a solution – for infectious diseases was intensified by despair and grief. Along with these personal losses came the ailment of hemiplegia. Yet, Pasteur did not falter nor abandon his goals. He recognized that his endeavors were not merely a temporary distraction from his personal suffering but potentially the key to addressing pressing public health challenges. Thus, he chose to channel his grief and tears into work for the benefit of those who remained and those who would come after.

Pasteur proposed that microorganisms were responsible for certain pathologies, and that fermentation and decay were also caused by the same type of entities. These represented biological phenomena, rather than chemical ones. The proposed solution was, undeniably, elegant: the theory unified three fields of experience that would otherwise remain disconnected. Philosophers of science refer to this epistemic virtue as the “unifying power” of an empirical theory.

Regardless of epistemological approval, the significance of his theory lies in its profound impact—dramatically increasing life expectancy for millions, including us. Moreover, his research led to multiple applications in various fields.[1]

Of particular interest for the subject of origins is Pasteur’s theory of biogenesis, a consequence of his research into fermentation, during which he confronted the old theory of spontaneous generation, still in circulation in his time, bolstered by Félix Pouchet’s Hétérogénie, ou traité de la génération spontanée.

Pouchet was fully aware of the objections that had been raised against spontaneous generation for centuries, including the famous experiments of Francesco Redi on this matter. However, Pouchet claimed to have new empirical evidence (the book’s frontispiece bears the subtitle: 'Based on New Experiences,' and cites the microscope as his primary tool) suggesting that at least some microbes emerge from dead and decomposing organic matter. Pouchet argues that this process does not always necessarily involve the presence and/or intervention of living organisms. Furthermore, he contends, still unknown plastic forces tend to aggregate molecules, imparting them with vitality and resulting in the formation of a new being, which then interacts with the environmental elements from which it originated (Pouchet 1859, 7-8). If matter can aggregate in the ovary to give rise to a new being, why could a similar aggregation not occur in a different medium? This proposal represents a particular form of spontaneous generation of life, which he termed 'heterogeny.' If this is correct, some feared, it could have multiple consequences, including the endorsement of both reductionist approaches to life and a materialistic view of the world.[2]

Pasteur believed that the correctness or rejection of heterogeny should be based on experimental evidence. Thus, he devised a rigorous four-stage experiment to test the model that was conducted in Arbois in 1860. Following the norms of his day, the results were presented at the French Academy of Sciences prior to publication (Pasteur 1861). The most famous public lecture on this subject took place in April 1864 at the Sorbonne. Confident, Pasteur asserted that life comes from life, not from the self-organization of environmental elements. Air, he argued, is simply a vehicle – not the origin – of microorganisms, which are vulnerable to heat. Although Pasteur's arguments were very sound, the debate on spontaneous generation would not be resolved until a few decades later. However, Pasteur's experiments were decisive in securing the victory. At the same time, they became a model of proper experimental methodology.

Pasteur as a Man of Faith and “The Great Problems”

Pasteur was raised in a Christian household, and he did the same with his own children. His reluctance to write publicly about organized religion explains the absence of unanimity among his biographers on this subject. Nonetheless, there is consensus that his devotion never wavered. Pasteur saw no contradiction between seeking natural solutions to epidemics and his belief in a benevolent Deity. While he made references to the Creator in some of his public addresses, he did not blend spiritual concepts with his scientific work. Diseases, he believed, were not caused by spiritual entities, but by natural ones. Simultaneously, germs emerge from other germs, but the primordial origin of life was something beyond human understanding. For Pasteur, there was no conflict between science and religion, and none of his discoveries seemed to have distanced him from his spiritual convictions.

Joseph Needham, a Catholic, had embraced spontaneous generation; Lazzaro Spallanzani, also Catholic, had opposed it. Was the theory heretical? Pasteur did not address this directly, confining himself to assessing its empirical validity, not its alignment with his faith. However, some scholars argue that, in part, his campaign against this theory — which, for him, supported a materialistic view of nature — was conducted in defense of his religious beliefs. (Caballero 1945, 88, 103; Zweig 1942, 51-57; Geison 1995, 121-125; cf. Pasteur 1861, 3; Roll-Hansen 1979-275-276). If true, it was these religious convictions (which some might label "biases") that, a priori, positioned him in opposition to the aforementioned theory.[3]

Hétérogénie was published in 1859. That same year, another profoundly influential work was released: On the Origin of Species by Charles Darwin. This book did not leave him indifferent. At the outset of his renowned address at the Sorbonne, he acknowledged the turbulent scientific climate of his time. Aware of the magnitude of the debates surrounding him, he remarked:

“Today, with keen minds, we witness the debate of great problems: the unity or plurality of human races; the creation of man thousands of years ago or millennia ago; the permanence of species or the gradual and progressive transformation of one into another; eternal matter and, beyond it, nothingness; the idea of God rendered superfluous; these are some of the issues that men today dispute. […] But alongside them, near these mysteries, there lies […] the question of whether living beings can come into the world without parents, without grandparents. […] What a triumph, gentlemen, it would be for materialism if it could affirm that it rests on the established fact of matter organizing itself, taking on life of itself, matter which already has in it all known forces! Ah! If we could add to it this other force which is called life what would be more natural than to deify such matter? What good then would it be to resort to the idea of a primordial creation, before which mystery it is necessary to bow? Of what use then would be the idea of a Creator-God?” (Fide Geison, 110-111)

Although Pasteur was always focused on his research, this statement reveals a new facet of his character. Pasteur was a sagacious individual, not only thoroughly informed about the issues of his time but also aware of the significance of certain topics. Pasteur understood the potential impact that a naturalistic worldview could have on our vision of reality. A contemporary of these debates, his experiments helped him to remain committed to Christianity, and the belief in a Creator God.

Finally, let us note that this nineteenth-century debate can be interpreted as part of a broader discussion that continues to this day. I am referring to the fact that the well-known naturalistic methodology of science constrains research to seek ways in which the appearance of life in the universe could have taken place without the intervention of a divine intelligence. The example of Pasteur teaches us that, even within these tight constraints, empirical results may be sufficient to build significant objections to a theory, irrespective of the potential undesirable consequences in non-empirical realms that would arise from accepting that theory.

Sources:

Caballero, R. (1945) Pasteur. Estudio integral de sus obras. El hombre, el filósofo, el creyente. Rosario: FCM.

Duclaux, E. (1920) Pasteur. The History of a Mind. Philadelphia: W. B. Saunders Co.

Geison, G. (1995) The Private Science of Louis Pasteur. Princeton: Princeton University Press.

Houssay, B. (1945) “Luis Pasteur”, Conferencia en la Academia Nacional de Medicina, s.d.e.

Pasteur, L. (1861) Sur les corpuscules organisés qui existent dans l'atmosphère: examen de la doctrine des générations spontanées. Paris: Charles Lahure et Gia.

Roll-Hansen, N. (1979) “Experimental Method and Spontaneous Generation: The Controversy between Pasteur and Pouchet, 1859-1864”, Journal of the History of Medicine and Allied Sciences 34(3):273-292.

Vallery-Radot, R. (1885) Louis Pasteur; His Life and Labors. New York: Appleton & Co.

Zweig, M. (1942) Vida y obra de Luis Pasteur. Buenos Aires: Claridad.

Endnotes

[1] The list includes (1) freeing sericulture from pebrine, a disease that decimated silkworm populations; (2) improving winemaking quality and the reputation of distilleries by perfecting fermentation techniques; (3) enhancing beer production through the incorporation of heat and high-pressure storage to prevent the intrusion of foreign ferments; (4) tackling chicken cholera in the regional farms by inoculating chickens with a weakened bacillus; (5) applying the same method to combat swine erysipelas; (6) bolstering the livestock industry by synthesizing a vaccine against anthrax; and (7) contributing to the development of an effective treatment for rabies, transitioning from a preventative to a therapeutic approach. Pasteur laid the groundwork for mass vaccination efforts, a practice to which we are now so accustomed. Hence, (8) he transformed public health policies, (9) influencing the history of medicine through his innovative concept of immunity (that microorganisms can be used both to prevent and cure the very diseases they cause), the pursuit of asepsis in operating rooms, and the adoption of an etiological – rather than symptomatic – approach in medical practice.

[2] Pouchet himself was a man of faith and saw no conflict between his theory and his religion. He wrote: “les théories des hétérogénistes, loin d'énerver les attributs du Créateur, ne font qu'en augmenter la divine majesté” (Pouchet 1859, 98; cf. 42, 95-137).

[3] The history of science clearly demonstrates that not all religious preconceptions led scientists down the path we now regard as correct, but Pasteur’s career shows that, in some instances, it did.

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Daniel Blanco, PhD
Universidad Nacional del Litoral, Santa Fe, Argentina

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Suggested citation: Blanco, D. (2025, May 22). Pasteur and Spontaneous Generation. Geoscience Research Institute. https://www.grisda.org/pasteur