It is widely acknowledged that a central aim of the scientific enterprise - whether it concerns physics, biology, or social science - is to achieve understanding of the observed phenomena (in addition to accurate description and prediction). But what precisely is scientific understanding? When is it achieved? Many philosophers of science have tried to answer these questions, but this has not led to consensus. Presently, at least with respect to the natural sciences, two competing positions dominate the debate: the view that ‘causal mechanisms’ are essential to scientific understanding, and the view that unification of the discoveries in a social context is essential. Moreover, not only philosophers of science but also scientists themselves disagree about standards of understanding.

Scientific understanding is both exciting and necessary; human cultures are vulnerable systems whose survival is threatened, in the face of which threat we seek moral values embedded within our scientific knowledge. This proceeds through the interweaving of the scientific quest with the search for moral resources that will help us confront threats to human survival.  However, we have yet to go in integrating knowledge and values

In the 1940s and 50s a confident, optimistic vision of science was widely shared by philosophers and historians of science. The goal of science was to discover the truth about nature, and over the centuries science had advanced steadily towards that goal; science discerned the real kinds of things of which the world was composed and the causal relations between them; the methods of science were rational and its deliverances objective; and so on. Only where science failed in some of these respects was there any need to provide external, that is social, political, or individual, explanations to bolster scientific belief.

In the late 50s, and especially subsequent to Kuhn's classic, The Structure of Scientific Revolutions, all this started to change. Historians increasingly insisted that the development of science must be treated just like any other cultural process, which meant embedding the narrative of the growth of scientific knowledge fully in the social and political context in which it occurred. This suggested that the view of science as deriving from a uniquely rational process could no longer be sustained. And, notoriously, Kuhn argued that science could not be seen as cumulative across the most dramatic changes in scientific theory.

Though philosophers have never given up entirely on the old optimistic picture of scientific understanding ("Legend" as Kitcher refers to it), its influence has steadily waned. For various reasons philosophers have become increasingly concerned over whether one could believe scientific claims to be literally true. Influenced by ideas that scientific theory must always be underdetermined by empirical evidence, they became more sympathetic to the possibility that scientific belief must be explained, at least in substantial part, by much more than the rational objective processes envisioned by Legend.

These philosophical doubts existed in uneasy tension with more extreme tendencies toward thoroughgoing relativism or scepticism, and with the movement in the sociology of science to see the whole concern with truth.  Falsity is an irrelevant diversion.

Kitcher argues that science does indeed discover the truth about nature, that it discerns the real kinds and their causal relations, that scientific understanding develops progressively and cumulatively through methods that are rational and ultimately objective.

First, he identifies many of the problems that arose for Legend's view of science as resulting from a too simple conception of science. Instead of describing scientists simply in terms of what they believed, Kitcher introduces the holistic notion of a scientific practice, which includes a scientist's professional language; the questions she identifies as significant; the statements she accepts; the patterns of explanation, or schemata, she accepts; the examples of, and criteria for, credible informants; the paradigms of experimentation and observation, including the instruments that she considers reliable; and the exemplars of good and faulty scientific reasoning .

Of particular note is the idea of schemata for explanation, which Kitcher uses to describe the growing explanatory resources of science. The various developing practices of individual scientists feed into what is the real locus of scientific progress, an advancing consensus practice. A further major respect in which Kitcher diverges from the classical tradition is in emphasizing that scientific practices develop in response to encounters both with nature and with colleagues.

If there is one point on which substantial consensus has been reached in the post-Legend era, it is that claims about the progressiveness of scientific understanding need to be sensitive to the real history of science. The most impressive aspect of Kitcher's book is the seriousness with which he responds to this demand. The central focus of his argument is the detailed treatment of three key episodes in the history of science, Galileo's defence of heliocentrism, Lavoisier's refutation of phlogiston chemistry, and  Darwin's argument for evolution by natural selection.  Most philosophers, believe that Galileo, Lavoisier and Darwin advanced our understanding of nature, but it is not always easy to articulate this belief. The significance of these episodes is that they constitute advance if anything does: they are the crown jewels of scientific history.

Kitcher argues forcefully that Galileo, Lavoisier, and Darwin triumphed in the end because they had the better arguments. And except for a few scientists who were either too strongly committed to the old views for personal or external reasons, or just plain stupid, the scientific community eventually came to see that they had the better arguments. The reason they had the better arguments, moreover, was that what they claimed was true--or at least approximately so.