|The three men: Emil Du Bois-Reymond, Hermann Helmholtz, and Ernst Brucke, of the so-called Helmholtz school, behind the 1842 Reymond-Brucke oath, namely of vow of allegiance to the view that only physicochemical forces, in opposition to any and all “life force” (or vitalism) theories, operate in organisms.|
In 1828, Friedrich Wohler’s synthesis of urea worked to destroy vitalism in chemistry.  The destruction of vitalism in evolution, physiology, psychology, and the humanities, in general, however, would not be so simple; has history has shown.
In 1833, German physiologist Johannes Muller, a tenacious adherent to vitalism theories, became the chair of anatomy and physiology at Humboldt University, and therein founded a famous experimentation-focused medical school laboratory.
In 1838, German medical students Emil Du Bois-Reymond and Ernst Brucke joined Muller's laboratory; soon thereafter a young Hermann Helmholtz joined the group.
In 1841, Helmholtz, enticed by Muller's motto of "creation of new phenomena through experimentation", decided to write his dissertation under Muller's supervision, specifically on the microscopic examination of the nervous system. 
In 1842, Du Bois-Reymond and Brucke, fed up with Muller's vitalism beliefs, pact that: 
“[We pledge] to put in power this truth: no other forces than the common physical chemical ones are active within the organism. In those cases which cannot at the time be explained by these forces one has either to find a specific way or form of their action by means of physical mathematical method, or to assume new forces equal in dignity to the chemical-physical forces inherent in matter, reducible to the force of attraction and repulsion.”
The manifesto was said to have been written by DuBois-Reymond, in collaboration with Brucke; soon after, Helmholtz and Karl Ludwig joined in, and as legend has it they each signed it with their own blood.  In short, the group “pledged a solemn oath” to eschew all nonmaterial explanations of organisms, physiologically, mentally, or operationally.  Second, they agreed that the only way the scientist could learn about these forces was through experimentation and observation. 
The underlying tension was a religious one; as Michael Wertheimer summarizes: 
“Although no one objected to scientific materialism in the physical sciences, the closer that one came to psychology, the greater the general opposition seemed to be, mainly on religious and ethical grounds.”
This credo, in turn, would greatly influence a young Sigmund Freud, Brucke’s soon-to-become medical school student; who in 1895, in his “A Project for Scientific Psychology”, would go onto posit that all mental functioning could be explained, thermodynamically, via bound energy and free energy, something far ahead of its time, even by 21st standards. 
The pact, which became one of the central doctrines of the newly-forming "Helmholtz school" of physical science based medicine, was made in opposition to the vitalist doctrine, adhered to by their mentor Johannes Muller, and or possibly to the notion of animal heat, such as promulgated in Justus Liebig's 1842 Animal Chemistry.
1. Murray, Henry A. and White, Robert W. (2006). The Study of Lives: Essays on Personality in Honor of Henry A. Murray (pg. 372-75). Aldine Transaction.
2. Wertheimer, Michael. (2012). A Brief History of Psychology (pg. 63). Taylor & Francis.
3. Decker, Hannah. (2013). The Making of DSM-IIIRG: A Diagnostic Manual’s Conquest of American Psychiatry (pg. 37). Oxford University Press.
4. Newton, Peter M. (1995). Freud: From Youthful Dream to Mid-Life Crisis (pg. 80). Guilford Press.
5. Cahan, David (1993). Hermann von Helmholtz and the Foundations of Nineteenth-Century Science (pg. 24). University of California Press.