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Jacques Loeb

Jacques Loeb

From an early age German physiologist Jacques Loeb (1859-1924) was interested in the question of whether or not free will existed. Rather than pursue his interests via a philosophical approach, Loeb used science to address his question. Using biological experiments on a wide range of specimens, including dogs, caterpillars, and marine animals, Loeb concluded that there was not free will. He believed that all animals, including people, operated mechanistically, as a result of physical and chemical reactions to stimuli. He took this position a step further by arguing that, once scientists understood the mechanics of biology, they could ultimately control development.

Born in Mayen, Germany, in 1859, Jacques Loeb, born Isaak Loeb, was the first of two sons born to Benedict and Barbara Isay Loeb. His father worked as an importer, but was very interested in science, literature, and collecting books. He was especially interested in 18th century French humanists and he exposed his children to these ideas. In 1873 Loeb's mother passed away from pneumonia and his father died three years later of tuberculosis. At the age of 16 Loeb went to Berlin to work in his uncle's bank. However, he soon became bored with this line of work so he decided to continue his education at the Askanische Gymnasium, a school for Berlin's Jewish elite that taught Latin, Greek, German literature, math, physics, and philosophy. There Loeb was exposed to the writings of philosophers such as Spinoza, Kant, Nietzche, and Schopenhauer. Even at this young age, he showed a great interest in the issues of instincts and free will.

In 1880 Loeb enrolled in the University of Berlin and also changed his name from Isaak to Jacques. This change to a French name symbolized both his secularization and his disapproval of the German Nationalism that was gaining in strength as a result of the Franco-Prussian War. At this time Loeb also decided to pursue a career in medicine rather than continue his classical training in philosophy. After one semester in Berlin Loeb transferred to the University of Munich and then to the University of Strasbourg. He became involved in a research project with Friedrick Goltz on the localization of brain functions, which he found fascinating. This experience convinced him to pursue physiology as a career. In 1884 Loeb received his medical degree from Strasbourg and he passed the government medical exams in 1885. He then returned to Berlin to study the effects of brain lesions at the Agricultural College.

Began Research on Animal Tropisms

In 1886 Loeb began working as an assistant for Adolf Fick, a physiology professor at Wurzburg, and befriended Julius von Sachs, a famous botanist. Von Sachs was researching plant tropism: the ability of plants to respond to external stimuli, such as light or gravity, like simple machines. Loeb saw this approach as a way to answer his philosophical questions about free will and viewed animal tropism as a way to see if free will could be controlled. If he could show that "lower" animals were affected by external stimuli just like plants, then he would prove that animals had no free will.

Loeb's first experiment in animal tropisms involved caterpillars' reaction to light. When caterpillars hatch from cocoons they climb to the tips of branches for food. It was believed that caterpillars had an instinct for where to find food. Loeb, however, had a more mechanistic hypothesis. He believe that the caterpillars had no such instinct and were simply responding to the external stimulus of light. His experiments proved his theory correct. Caterpillars were given the choice of light or food and they chose light even though they starved to death. Loeb worked feverishly on this idea for two years until he published his first paper on animal tropisms. This was the beginning of a lifelong pursuit to show how life could be manipulated by science if the ruling mechanisms were known. As Loeb wrote in The Mechanistic Conception of Life, "Our wishes and hopes, disappointments and sufferings have their source in instincts which are comparable to the light instinct of the heliotropic animals."

In 1888 Loeb returned to the University of Strasbourg to work as an assistant in Goltz's physiological institute. His main responsibility was teaching and he worked with several students on psychophysiological problems. However, political and financial problems led Loeb to leave Strasbourg after only a year, and from 1889 to 1990 he spent his summers in Zurich where his brother was attending medical school. In the winters he worked at the Naples biological station, where he studied the depth migrations of pelagic invertebrates and regeneration in marine animals in an effort to learn how to control development. Philip J. Pauly, in Controlling Life, explained Loeb's broad range of experimental interests: "In contrast to most physiologists, he was interested not only in functions of adult vertebrates, but also invertebrates and embryos; he was concerned not only with routine functions but with behavior, development, and ultimately … evolution. Loeb's program was not applied science. It was a refocusing of biological inquiry itself around what Loeb conceived as the activity of the engineer."

In the spring of 1890 Loeb met his future wife while visiting physiology Professor Justus Gaule in Zurich. Anne Leonard was a young American who had just earned her Ph.D. in philology from the University of Zurich. The couple married in October of 1890 and had three children together: the eldest child, Leonard, became a physicist, Robert became a teaching physician, and daughter Anne attended Barnard College prior to her marriage.

Developed Mechanistic Conception of Biology

Between 1889 and 1991 Loeb developed his views on biological engineering. He was greatly influenced by physicist Ernst Mach, who believed that sensation, perception, and behavior were physical, rather than mental or emotional, reactions. In particular, Loeb moved from the narrow perception of biological change exhibited by his work on animal tropisms to a more radical view of the nature of biology and the scientist's ability to manipulate it. According to Charles Rasmussen and Rich Tillman in Jacques Loeb: His Science and Social Activism and Their Philosophical Foundations, "This prominent scientist was not only a major influence on Loeb's mechanistic and engineering notions for a biology of behavior, his influence reinforced and shaped Loeb's belief in the importance of social issues in his life as a practicing scientist; in this regard, the influence of Mach can be traced through the remainder of Loeb's life."

Continued Academic Career in America

Once married, Loeb searched for an academic job which would offer his family more financial stability. He was increasingly unhappy with the political situation in Germany, so the young couple moved to America. Loeb was offered a teaching job at Bryn Mawr College in Pennsylvania. Although he enjoyed his new position, the research facilities were not suitable to his needs. In January 1892 he was asked to join the new University of Chicago. He stayed at this university for the next ten years, eventually becoming head of the physiology department, and he spent his summers teaching physiology at Woods Hole, Massachusetts.

Loeb extended his work on free will to see if it was possible to control the entire developmental process. To this end, he conducted experiments in parthenogenesis, reproduction without fertilization. He subjected eggs to both chemical and physical stimuli and discovered several methods by which an egg could develop without sperm fertilization. Loeb's work on artificial parthenogenesis gained him fame in both the scientific community and the popular press and established him as a major figure in biology. Though Loeb never won the Nobel Prize for science or medicine, he was nominated for the prestigious award in 1901. As Philip J. Pauly explained in Controlling Life: Jacques Loeb and the Engineering Ideal in Biology, "In the period from 1890 to 1915 Loeb was the major public advocate of what can be termed 'the engineering standpoint.' … By the turn of the century he had come to symbolize both the appeal and the temptation of open-ended experimentation among biologists in America, and he became the center of scientific and popular controversies over the place of manipulation in the life sciences."

Loeb spent the winter of 1898-1899 in Pacific Grove, California, where he planned to work on marine research. During this visit he wrote Comparative Physiology of the Brain and Comparative Psychology in German and his wife translated it to English. This book, a tribute to Mach, summarized Loeb's career in neurophysiological and behavioral research. It also introduced to new areas that Loeb was interested in, particularly colloidal substances (molecular structures such as proteins) and reflexes. Loeb was impressed with the California climate and liked the possibility of doing marine research all year round. In 1902 he accepted a position at the University of California and a laboratory was built for him in Pacific Grove. The only drawback to working in California was that Loeb was isolated from his professional colleagues. He did not like to travel so he attended few professional meetings. As a result of this isolation, Loeb decided to accept a position as head of experimental biology at the Rockefeller Institute for Medical Research in 1910, which allowed him to dedicate all of his time to research. There he pursued his research on animal tropisms as well as bioelectrical phenomena, regeneration, and the properties of proteins. In the same year he was also elected to the National Academy of Sciences. In 1912 Loeb published a collection of his essays in a volume titled The Mechanistic Conception of Life. In 1918 he started the Journal of General Physiology with Winthrop J.V. Osterhout, as well as a series of Monographs on Experimental Biology.


While on vacation in Bermuda, Loeb suffered from angina and died on February 11, 1924. His ashes were brought to Woods Hole and a memorial was placed there at the Marine Biological Institute, as well as at the Rockefeller Institute for Medical Research. Throughout his prolific career, Loeb published over 400 scientific works. His engineering approach to science impacted other scientists in many fields, including behaviorism, genetics, biochemistry, and physiology. Loeb was a hard worker who was passionate about science and its role in society. As Osterhout wrote in The Journal of General Physiology: Jacques Loeb Memorial Volume, "He had a passionate love of truth and what appeared to him to be true had to be so expressed that all could feel the inspiration and see the beauty of what he saw." In Controlling Life, Philip J. Pauly stated that "Loeb was important primarily as a model, both of what it meant to be a scientist, and of a particular approach to biological research."


The Journal of General Physiology: Jacques Loeb Memorial Volume, edited by W. J. Crozier, John H. Northrop, and W.J.V. Osterhout, The Rockefeller Institute for Medical Research, 1928.

Loeb, Jacques, The Mechanistic Conception of Life, University of Chicago Press, 1912.

Pauly, Philip J., Controlling Life: Jacques Loeb and the Engineering Ideal in Biology, Oxford University Press, 1987.

Rasmussen, Charles, and Rick Tillman, Jacques Loeb: His Science and Social Activism and Their Philosophical Foundations, American Philosophical Society, 1998.


Science, September 8, 2000.

Scientific America, September 2000.


"Jacques Loeb Papers, 1906-1924," (January 17, 2002).

"Jacques Loeb's Influence on Korzybski," (January 17, 2002).

Wozniak, Robert H., "Jacques Loeb, Comparative Physiology of the Brain, and Comparative Psychology," (January 17, 2002). □

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Loeb, Jacques

Loeb, Jacques

(b. Mayen, Rhine Province, Prussia, 7 April 1859;d. Hamilton, Bermuda, II February 1924)

physiology, biology.

The apostle of mechanistic conceptions in biology, Loeb was the elder son of a prosperous Jewish importer. His brother Leo later became professor of pathology at Washington University, St. Louis. His father, an ardent Francophile, encouraged his son to read the classics of eighteenth-century thought. The mature Loeb always sought to harness his empirical researches to the wide-ranging political, social, and philosophical concerns of the philosophes. There is no direct evidence that he was influenced by the cognate tradition of the German-speaking medical materialists, led by Jacob Moleschott, Ludwig Büchner, and Carl Vogt, who published their major pronunciamentos in the 1850’s. But it would be extraordinary if Loeb had not been acquainted with a celebrated essay by a kindred spirit of the medical materialists—“Die mechanistische Auffassung des Lebens” by Rudolf Virchow.

When he prepared to enter the University of Berlin in 1880, Loeb was not interested in science as a career. Having become intoxicated by reading Schopenhauer and Hartmann, the philosophers of the will, he resolved to become a philosopher in order to discover if freedom of the will existed—preferably to establish that it did not. To his bitter disillusionment, he felt the professors of philosophy at Berlin to be mere “wordmongers,” uninterested in resolving the issues they posed and incapable of it.

Rebounding from this disappointment, he turned to science. His quest remained the same—to find a “starting point for an experimental analysis of the will.” He thus enrolled at Strasbourg in 1880 to study the localization of function in the brain under Goltz. Loeb presumably conceived of brain surgery upon experimental animals as forcing their wills. He took his M.D. in 1884 with a thesis written under Goltz on blindness induced by injury to the cerebral cortex. Loeb regarded his five years at Strasbourg as totally wasted for his chosen purpose.

Loeb finally found the concatenation of men and impulses that he was looking for at Würzburg in 1886, where he had become assistant to the professor of physiology Adolf Fick, one of the chief pioneers in the application of physics to biology and medicine. The decisive contact of Loeb’s career was his intimate friendship with Julius von Sachs, from whom he learned of plant tropisms, obligatory movements elicited by physical stimuli such as light and gravity. The concept was ultimately traceable to T. A. Knight and Candolle but had been greatly amended and enhanced in interest by Sachs’s own research. Loeb’s task in science suddenly became clear: to establish a concept of tropisms in animals by which they too could be shown to be irresistibly driven by external stimuli and impotent to interpose their wills. He could simultaneously allay his metaphysical anxieties and prosecute a clear-cut program of empirical researches.

Loeb’s final period of scientific initiation came in the winter of 1889-1890 at the marine biological station at Naples, where he found himself in an intellectual environment dominated by Wilhelm Roux’s Entwicklungsmechanik. In this context, he first encountered a group of brilliant young American embryologists and cytologists, including Thomas Hunt Morgan. In 1890 Loeb married an American, Anne Leonard, who had taken a Ph.D. in philology at Zurich; they settled in the United States in 1891. He later wrote that he “could not live in a regime of oppression such as Bismarck had created” and was also disturbed by reading Heinrich von Treitschke, “the court historian of the King of Prussia,” on the superiority of the German race. In the United States, Loeb taught successively at Bryn Mawr College, the University of Chicago, and the University of California at Berkeley, and from 1910 until his death at the Rockefeller Institute in New York. He spent his summers at the marine biological laboratories at Pacific Grove, California, or Woods Hole, Massachusetts, Two of his three children, Leonard and Robert, became scientists.

Loeb’s publications on animal tropisms began in 1888. Certain caterpillars on emerging in the spring had long been observed to climb to the tips of branches and to feed upon the buds; and this behavior had been attributed to an infallible instinct for self-preservation. Loeb showed, however, that if the only source of light was in the opposite direction from food, the caterpillars would move toward the light and starve to death. The alleged instinct, Loeb maintained, was merely a product of heliotropism: the caterpillars were photochemical machines enslaved to the light. He demonstrated by ingenious experiments that they strove to achieve a frontal fixation upon a light source, with the plane of symmetry bisecting the source of stimulation. Loeb thought that he had found the solution to the problem of will by applying the appropriate tropic stimulus, “forcing, by external agencies, any number of individuals of a given kind of animals to move in a definite direction by means of their locomotor apparatus.” He showed that certain organisms, thought to be exempt from tropistic behavior, could be oriented in a single direction by such simple expedients as adding carbonic acid to the medium.

Although some organisms indubitably manifest tropistic behavior, Loeb’s conception of animal tropisms as generally applicable soon came under severe attack. The classic refutation was given by the American biologist Herbert Spencer Jennings in his Behavior of the Lower Organisms (1906). He showed that some apparently tropistic responses were merely “avoiding reactions,” unspecifically elicited by many stimuli and totally unaffected by the direction from which the stimulus was applied. More generally, Jennings invoked the concept, originated by C. Lloyd Morgan and Edward Lee Thorndike, of trial and error reactions to unfamiliar stimuli, leading by chance to rewarded behavior which tended to recur in the future.

Undaunted by such criticism, Loeb saw himself as having suppressed one form of biological mysticism by substituting tropisms for instincts. In the field of development mechanics to which he had been introduced at Naples, he thought that the equivalent achievement would be “the substitution of well-known physico-chemical agencies for the mysterious action of the spermatozoon.” To test the hypothesis that salts acted upon the living organism by the combination of their ions with protoplasm, Loeb immersed fertilized sea urchin eggs in salt water the osmotic pressure of which had been raised by the addition of sodium chloride. When replaced in ordinary seawater, they underwent multicellular segmentation. T. H. Morgan then subjected unfertilized eggs to the same process and found that they too could be induced to start segmentation, although without producing any larvae. It was Loeb who first succeeded, in 1899, in raising larvae by this technique —the first notable triumph in achieving artificial parthenogenesis. Although Loeb greatly relished the philosophical implications of artificial parthenogenesis, he also saw it as an addition to the repertory of experimental embryologists.

The third of Loeb’s major lines of research, from 1918 until his death in 1924, was summarized in Proteins and the Theory of Colloidal Behavior (1922). He demonstrated that proteins are amphoteric electrolytes, capable of reacting chemically either as an acid or as a base, and that many observed properties of proteins and protein solutions are explicable in terms of the creation of a Donnan equilibrium between two solutions separated by a semipermeable membrane. This view seemed to many to be an epoch-making advance in the unification of physiology and physical chemistry. As the Donnan theory postulates the existence of salt solutions, Loeb’s concern with these issues was directly linked to his earlier research on artificial parthenogenesis.

Few scientists of Loeb’s generation were as well known to the American public. As a materialist in philosophy, a mechanist in science, and a socialist in politics, he offended against the prevalent American orthodoxies. But he was correspondingly idolized by the dissenters and debunkers—including Veblen, Mencken, and Sinclair Lewis—who increasingly set the intellectual tone. Lewis, in his satirical delineation of the Rockefeller Institute in Arrowsmith (1925), drew significantly upon Loeb for the only senior scientist consistently exempted from the author’s scorn and derision.

Loeb’s principal statement of his basic philosophy for a lay audience was his famous address “The Mechanistic Conception of Life,” delivered before the First International Congress of Monists in Hamburg in September 1911 and published as the title piece of his most widely read book (1912). In this work Loeb argued that the mechanistic conception had made colossal strides in the first decade of the twentieth century, largely through his own researches: the activation of the egg “completely reduced to a physicochemical explanation”; the instincts of men equated with tropisms; and the field of heredity, traditionally “the stamping ground of the rhetorician and metaphysician,” now transformed by Mendelism into “the most exact and rationalistic part of biology.” The one great remaining task was to explain the origin of life, but he did not expect this problem to be insurmountable.

The long-term impact of Loeb’s work and orientation was substantial. Jennings, his chief critic on tropisms, always affirmed his indebtedness to Loeb as the great pioneer in the objective analysis of behavior, the man who first made the study of behavior a rigorously experimental science. While a graduate student at the University of Chicago, J. B. Watson, the founder of behaviorism in psychology, chose Loeb as his thesis director but was dissuaded by other professors who told him that Loeb was not “safe” for a student to work with. It is a fair inference, however, that Loeb had already instilled in him a latent hostility toward subjective modes of analysis in psychology. Loeb may justly be described as Watson’s greatest precursor in exemplifying and proselytizing for this attitude.


An exhaustive bibliography of Loeb’s publications compiled by Nina Kobelt is appended to the standard biographical account, W. J. V. Osterhout, “Jacques Loeb” in Journal of General Physiology, 8 (1928), ix-xcii. The other extended account of Loeb is Donald Fleming, “Introduction,” in Jacques Loeb, The Mechanistic Conception of Life, repr. ed. (Cambridge, Mass., 1964), vii-xli. On Loeb’s relationship to Jennings and Watson, see Donald D. Jensen, “Foreword,” in H. S. Jennings, Behavior of the Lower Organisms, repr. ed. (Bloomington, Ind., 1962), ix-xvii.

Donald Fleming

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Loeb, Jacques

Jacques Loeb (lōb), 1859–1924, American physiologist, b. Germany, M.D. Univ. of Strasbourg, 1884. He came to the United States in 1891 and taught at Bryn Mawr, the Univ. of Chicago, and the Univ. of California. From 1910 he was a member of the Rockefeller Institute (now Rockefeller Univ.). Best known for his tropism theory and for his experiments in inducing parthenogenesis and regeneration by chemical stimulus, he also propounded the mechanistic philosophy that all ethics were the outgrowth of humanity's inherited tropisms. He was a founder and editor of the Journal of General Physiology. His works include The Mechanistic Conception of Life (1912), Artificial Parthenogenesis and Fertilization (1913), and The Organism as a Whole (1916).

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