Friday, 23 February 2007
HPS Alumni Lecture
When Mechanistic Models Explain: The Hodgkin and Huxley Model of the Action Potential
Carl F. Craver
Washington U. in St. Louis
3:30 pm, 817R Cathedral of Learning
Abstract: In 1952, Hodgkin and Huxley published a mathematical model of the action potential in the squid giant axon. The model is derived in part from laws of physics and chemistry, such as Ohm’s law, Coulomb’s law, and the Nernst equation, and it can be used to derive myriad electrical features of many different kinds of neuron in many different species. Despite this accomplishment, Hodgkin and Huxley insist that their model fails as an explanation. This is curious if one thinks, as many did in the 1950’s, that to explain a phenomenon just is to show that it follows from laws of nature coupled with initial conditions. I argue that Hodgkin and Huxley regarded their mathematical model as a phenomenological model and that they regarded their understanding of the action potential as sketchy at best. I also argue that they were right. There is a widely accepted distinction between merely modeling a mechanism’s behavior and explaining it. Models play many roles in science. They are used to make precise and accurate predictions. They are used to summarize data. They are used as heuristics for designing experiments. They are used to demonstrate surprising and counterintuitive consequences of particular forms of systematic organization. But some models, such as the mechanistic model that continues to develop in the wake of Hodgkin and Huxley’s work, are also explanations. I will discuss some of the ways that models, even very useful models, can fail to provide explanations in order to build a positive account of when mechanistic models explain. I will also show how this mechanistic view of constitutive explanation differs from its nearest neighbors, such as Cummins 1975, Lycan 1989, and Machamer et. al. 2000.
