Scientific Mechanisms, Past and Present

Today's scientists, it could be said, frequently explain observed phenomena by finding chemical, biological, or physical mechanisms of some form, shape, or size. In a panel session at the recent History of Science Society meeting in Pittsburgh, presenters traced the roots of this perceived tendency and provided high-profile examples of scientists' apparent captivation with mechanisms--defined broadly as entities and activities that produce regular changes from start or setup to finish or termination. 1 The speakers suggested that studying legitimate scientific mechanisms is a fruitful way of tracking the evolution of scientific discovery. Presenter Peter Machamer, a historian and philosopher of science at the University of Pittsburgh, claimed that the drive for mechanisms, though somewhat evident in ancient Greece and medieval times, can essentially be traced to the 17th century. Subsequent presenters offered examples of 20th century research that has been colored by the need to find mechanisms. "The reason we want to tell this story is primarily because [a large amount] of science, if not all of science, [looks] for the mechanisms by which the world works, specifically, ... the way various phenomena or ... events occur," commented Machamer. He contends that 17th century astronomer and physicist Galileo Galilei was the first to popularize the mechanical world view. Much of Galileo's mathematics related to the nature of matter and motion can be elegantly illustrated and realized via simple machines--or mechanisms. Soon after, Rene Descartes ' collision laws, Dutch mathematician Christiaan Huygens ' laws of motion, and, most notably, Sir Isaac Newton 's introduction of forces all influenced how mathematical laws lend themselves to actual mechanisms in theory and practice. "It's not just having the mathematics," explained Machamer. "It's saying here's a problem in the physical world, now think about it this way. ... It sets the way in which you think, and how you have to structure problems, and gives you the canon for solution." Graphic: Cathleen Heard This mechanistic way of thinking, according to Machamer, eventually seeped into biology and chemistry, a claim evidenced by the presentations of the other speakers. Historian of science Jeffrey L. Ramsey of Oregon State University in Corvallis traced the history of the photochemical reaction between chlorine and hydrogen, which yields hydrochloric acid. He noted that the reaction was well characterized in the 1920s and '30s, but the mechanisms by which it took place were not. Chemists constructed mechanisms, said Ramsey, after a series of failed attempts to use only physical and chemical laws to explain chemical reactions. "The appeal to mechanism did at least two things," remarked Ramsey, who plans to eventually publish a version of his comments. "It allowed this group of chemists to negotiate chemistry's relative theoretical autonomy with respect to physics, and it introduced a level of explanation independent of the experiments and the laws." University of Maryland philosophy professor Lindley Darden suggested that the mechanism of protein synthesis was viewed differently by biochemists and molecular biologists when first described in detail in the 1950s. Molecular biologists looked to weak hydrogen bonding between nucleic acid bases; biochemists looked to covalent bonding between molecules. Molecular biologists relied on depictions of the 3-D structures of macromolecules and the role of the RNA; biochemists relied on equations and emphasized enzymes and energy activation. Darden argued that scientists from each field eventually unified their theories and composed an "integrating mechanism schema." Carl Craver, an assistant professor of philosophy at Florida International University in Miami, used the example of long term potentiation (LTP) to show the significance of mechanisms in recent and current investigations. LTP, which many scientists believe to be intimately involved in long-term learning and memory, is the protracted enhancement of an excitatory postsynaptic potential, the long-lasting increase in efficacy of a cortical synapse. LTP's controversial role 2 has had a major impact on contemporary neuroscience. Scientists currently publish an average of four papers per day on the phenomenon; at a recent five-day neuroscience conference in Miami, researchers presented no fewer than 200 papers on LTP. According to Craver, since it's never actually been definitively linked to the mechanism of learning and memory, LTP, sometimes referred to as the "gene of neuropsychology," was in a sense created as a mechanism rather than discovered when the first research on LTP was done more than 30 years ago. Craver outlined several reasons for LTP's growth from its humble beginnings as a laboratory tool: It has been demonstrated experimentally; it occurs in the central nervous system (not in the periphery) and, even better, in the hippocampus; it can be evoked in unanaesthetized animals; and it lasts for a long time. But most important, said Craver, is the "multilevel structure" into which LTP is situated. LTP can, he remarked, "accommodate work of different scientists in different labs with different educational backgrounds using different experimental techniques on different experimental models." Craver, Darden, and Machamer plan to publish their general account of mechanisms in the March issue of Philosophy of Science. Analyzing the "history of science" in terms of a "history of mechanisms" and asking why scientists have gravitated toward mechanistic explanations of nature may not only say something about the scientific enterprise, but about human nature itself. Said Ramsey, mechanisms are "one of the ways we have made things intelligible to ourselves." The possibility of having some device that can, for example, explain why and how we remember what we do, is quite enticing. "Part of the appeal for mechanisms," commented Machamer, is "you have this way of literally showing how the damn thing works." Eugene Russo can be contacted at erusso@the-scientist.com. 1. C. Craver, L. Darden, P. Machamer, Philosophy of Science, March 2000. 2. E. Russo, "Controversy surrounds memory mechanism," The Scientist, 13[5]:1, March 1, 1999.

Scientific Mechanisms, Past and Present

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Eugene Russo

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December 1999

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