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," http://www.rockefeller.edu/archive.ctr/ru-jl.html (January 17, 2002).

"Jacques Loeb's Influence on Korzybski," http://www.kcmetro.cc.mo.us/pennvalley/biology/lewis/loeb.html (January 17, 2002).

Wozniak, Robert H., "Jacques Loeb, Comparative Physiology of the Brain, and Comparative Psychology," http://www.brynmawr.edu/Acads/Psych/rwozniak/loeb.html (January 17, 2002).

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