In his groundbreaking 1962 book, The Structure of Scientific Revolutions, Thomas Kuhn shattered the prevailing view that scientific progress was a piecemeal process resulting from the steady accumulation of new data and new knowledge. Kuhn showed that the history of science was punctuated by dramatic shifts of vision — conceptual revolutions — in which one paradigm was displaced by another. In Conceptual Revolutions, Paul Thagard sets out to better understand how this process unfolds — how one conceptual system supersedes another, and whether the process is a rational one or governed instead, as Kuhn believed, by non-rational and non-empirical factors.

Thagard looks at seven historical cases of conceptual change in science, including the Copernican revolution which displaced the earth-centered cosmology of Ptolemy, Lavoisier's oxygen theory which overturned the phlogiston theory of Stahl, Darwin's evolutionary theory which supplanted the prevailing idea of divine creation of species, Einstein's theory of relativity which replaced and absorbed Newtonian physics, and the geological theory of plate tectonics that established the existence of continental drift. According to Thagard, each of these cases demonstrates that conceptual revolutions involve radical transformations of whole systems of concepts and laws, not the mere revision of beliefs or theories. "The importance of conceptual revolutions is so great," he says, "in part because they are so rare. Science does not make revolutionary leaps very frequently, but when it does the epistemic consequences are enormous."

He analyzes conceptual revolutions from several angles, showing that they invariably involve major additions or deletions of beliefs, the reorganization of concepts, and/or the redefinition of conceptual hierarchies. They may involve "branch jumping" or "tree branching and switching" (shifting a concept from one branch of a hierarchical tree to another), the "sublation" rather than "transplanting" of theories (where some parts of the earlier theory are retained while others are rejected), and the "abduction" or inference to the best hypothesis. In general, he argues, the search for explanatory coherence is the impulse that drives all scientific inquiry and is the final test for the truth of theories.

One of Thagard's most compelling insights is that scientific paradigms are not in fact "incommensurable," as Kuhn suggested, and that conceptual shifts are therefore essentially irrational events. As he sees it, there is enough continuity in scientific revolutions to justify the claim that the adoption of new views is a rational process. "Theory acceptance is not much like religious conversion, and social factors alone do not explain the adoption of new theories," he writes. "Acquiring a new conceptual system is somewhat similar to learning a second language."

Thagard acknowledges that other disciplines, such as politics, economics, sociology, and anthropology, also undergo conceptual transformations on occasion. But because they lack the sort of organizing theories that shape and dominate the major work in the field, these shifts tend to involve changes of basic approach rather than the displacement of axiomatic concepts and principles.

Copyright 2008 by Scott London. All rights reserved.