Proponents of the semantic conception of theories explored alternative notions of reduction. For some philosophers, however, conceiving of theories as families of models provided a useful way of capturing what they saw as the piecemeal character of contemporary scientific work. Instead of viewing the sciences as directed at large generalizations, they suggested that researchers offer a patchwork of models, successful in different respects and to different degrees at characterizing the behaviour of bits and pieces of the natural world. This theme was thoroughly pursued by the American philosopher Nancy Cartwright, who emerged in the late 20th century as the most vigorous critic of unified science.
Cartwright opposed the kind of reduction considered above (“vertical reduction”), but she believed that the standard critiques did not go far enough. She argued that philosophers should also be skeptical of “horizontal reduction,” the idea that models and generalizations have broad scope. Traditional philosophy of science took for granted the possibility of extrapolating regularities beyond the limited contexts in which they can be successfully applied. As a powerful illustration, Cartwright invited readers to consider their confidence in Newton’s second law, which states that force is equal to the product of mass and acceleration (see Newton’s laws of motion). The law can be used to account for the motions of particular kinds of bodies; more exactly, the solar system, the pendulum, and so forth can be modeled as Newtonian systems. There are many natural settings, however, in which it is hard to create Newtonian order. Imagine, for example, someone dropping a piece of paper money from a high window overlooking a public square. Does Newton’s second law determine the trajectory? A standard response would be that it does in principle, though in practice the forces operating would be exceedingly hard to specify. Cartwright questioned whether this reponse is correct. She suggested instead that modern science should be thought of in terms of a history of successful building of Newtonian models for a limited range of situations and that it is only a “fundamentalist faith” that such models can be applied everywhere and always. It is consistent with current scientific knowledge, she argued, that the world is thoroughly “dappled,” containing some pockets of order in which modeling works well and pockets of disorder that cannot be captured by the kinds of models that human beings can formulate. Scientific change
Although some of the proposals discussed in the previous sections were influenced by the critical reaction to logical empiricism, the topics are those that figured on the logical-empiricist agenda. In many philosophical circles, that agenda continues to be central to the philosophy of science, sometimes accompanied by the dismissal of critiques of logical empiricism and sometimes by an attempt to integrate critical insights into the discussion of traditional questions. For some philosophers, however, the philosophy of science was profoundly transformed by a succession of criticisms that began in the 1950s as some historically minded scholars pondered issues about scientific change.
The historicist critique was initiated by the philosophers N.R. Hanson (1924–67), Stephen Toulmin, Paul Feyerabend (1924–94), and Thomas Kuhn. Although these authors differed on many points, they shared the view that standard logical-empiricist accounts of confirmation, theory, and other topics were quite inadequate to explain the major transitions that have occurred in the history of the sciences. Feyerabend, the most radical and flamboyant of the group, put the fundamental challenge with characteristic brio: if one seeks a methodological rule that will account for all of the historical episodes that philosophers of science are inclined to celebrate—the triumph of the Copernican system, the birth of modern chemistry, the Darwinian revolution, the transition to the theories of relativity, and so forth—then the best candidate is “anything goes.” Even in less-provocative forms, however, philosophical reconstructions of parts of the history of science had the effect of calling into question the very concepts of scientific progress and rationality.
A natural conception of scientific progress is that it consists in the accumulation of truth. In the heyday of logical empiricism, a more qualified version might have seemed preferable: scientific progress consists in accumulating truths in the “observation language.” Philosophers of science in this period also thought that they had a clear view of scientific rationality: to be rational is to accept and reject hypotheses according to the rules of method, or perhaps to distribute degrees of confirmation in accordance with Bayesian standards. The historicist challenge consisted in arguing, with respect to detailed historical examples, that the very transitions in which great scientific advances seem to be made cannot be seen as the result of the simple accumulation of truth. Further, the participants in the major scientific controversies of the past did not divide neatly into irrational losers and rational winners; all too frequently, it was suggested, the heroes flouted the canons of rationality, while the reasoning of the supposed reactionaries was exemplary. The work of Thomas Kuhn
In the 1960s it was unclear which version of the historicist critique would have the most impact, but during subsequent decades Kuhn’s monograph emerged as the seminal text. The Structure of Scientific Revolutions offered a general pattern of scientific change. Inquiries in a given field start with a clash of different perspectives. Eventually one approach manages to resolve some concrete issue, and investigators concur in pursuing it—they follow the “paradigm.” Commitment to the approach begins a tradition of normal science in which there are well-defined problems, or “puzzles,” for researchers to solve. In the practice of normal science, the failure to solve a puzzle does not reflect badly on the paradigm but rather does so on the skill of the researcher. Only when puzzles repeatedly prove recalcitrant does the community begin to develop a sense that something may be amiss; the unsolved puzzles acquire a new status, being seen as anomalies. Even so, the normal scientific tradition will continue so long as there are no available alternatives. If a rival does emerge, and if it succeeds in attracting a new consensus, then a revolution occurs: the old paradigm is replaced by a new one, and investigators pursue a new normal scientific tradition. Puzzle solving is now directed by the victorious paradigm, and the old pattern may be repeated, with some puzzles deepening into anomalies and generating a sense of crisis, which ultimately gives way to a new revolution, a new normal scientific tradition, and so on indefinitely.
Kuhn’s proposals can be read in a number of ways. Many scientists have found that his account of normal science offers insights into their own experiences and that the idea of puzzle solving is particularly apt. In addition, from a strictly historical perspective, Kuhn offered a novel historiography of the sciences. However, although a few scholars attempted to apply his approach, most historians of science were skeptical of Kuhnian categories. Philosophers of science, on the other hand, focused neither on his suggestions about normal science nor on his general historiography, concentrating instead on Kuhn’s treatment of the episodes he termed “revolutions.” For it is in discussing scientific revolutions that he challenged traditional ideas about progress and rationality.
At the basis of the challenge is Kuhn’s claim that paradigms are incommensurable with each other. His complicated notion of incommensurability begins from a mathematical metaphor, alluding to the Pythagorean discovery of numbers (such as Square root of√2) that could not be expressed as rationals; irrational and rational lengths share no common measure. He considered three aspects of the incommensurability of paradigms (which he did not always clearly separate). First, paradigms are conceptually incommensurable in that the languages in which they describe nature cannot readily be translated into one another; communication in revolutionary debates, he suggested, is inevitably partial. Second, paradigms are observationally incommensurable in that workers in different paradigms will respond in different ways to the same stimuli—or, as he sometimes put it, they will see different things when looking in the same places. Third, paradigms are methodologically incommensurable in that they have different criteria for success, attributing different values to questions and to proposed ways of answering them. In combination, Kuhn argued, these forms of incommensurability are so deep that, after a scientific revolution, there will be a sense in which scientists work in a different world.