However, in The Structure of Scientific Revolutions, Thomas Kuhn relinquished the notion of science as truth-seeking. In place of scientific realism he substituted a non-continuous model of scientific progress that had as its goal efficient puzzle solving. In abandoning the notion that scientists search for truth, Kuhn also abandoned scientific realism, thus challenging a defining characteristic of modern science since the scientific revolution of the sixteenth and seventeenth centuries.
Modern Science: Realism, Truth, and Evolution
Copernicus, with the 1543 publication of On the Revolutions of the Celestial Spheres, laid the foundation for modern science when he propounded that his sun-centered model of the universe explained physical reality. The Aristotelian-Ptolemiac theory dominant at the time, on the other hand, was such a complex system that nobody believed that it corresponded to the physical reality of the universe. Although the Ptolemaic system accounted for observations-"saved the appearances"-its epicycles and deferents were never intended be anything more than a mathematical model to use in predicting the position of heavenly bodies.  As historian of science A. Rupert Hall explains, the medieval scientists, like the ancient Greeks from whom they inherited the notion of saving appearances, (22)believed that "mathematical science could not explain things by revealing the structure of reality and its inner logic, it could only give the possibility of predicting future results from stated antecedents."  Copernicus found this practice of saving appearances to be "a confession of ignorance and confusion,"  and instead advocated scientific realism for his system.  He believed that the earth really moved. Probably from fear of animosity towards his heliocentric conception of the universe, he did not publish his theory until he neared death; however, his idea of an earth in motion left a legacy of problems-" about the nature of matter, the nature of the planets, the sun, the moon, the stars, and the nature and actions of force in relation to motion"-that would be taken up by later astronomers such as Galileo and Newton. 
In 1610, the Italian mathematician Galileo published The Starry Messenger, in which he wrote of his support for Copernican astronomy and the telescopic observations that had convinced him to accept the heliocentric model. The reaction to Galileo reveals the strength of the opposition to realism. Many scientists opposed him because they believed that the telescope deceived their eyes, or they did not find the empirical evidence to be relevant.  In 1616, the Church added Copernicus's De Revolutionibus to the Index of Forbidden Books, condemning the section where he stated that the motion of the Earth was a physical reality.  At that time, Cardinal Bellarmine, representing the Inquisition, informed Galileo that he was free to continue his work with Copernican theory if he agreed that the theory did not describe physical reality but was merely one of the many potential mathematical models.  Galileo continued to work, and while he "formally (23)claimed to prove nothing,"  he passed his mathematical advances and his observational data to Newton, who would not only invent a new mathematics but would solve the remaining problems posed by Copernicus. 
Newton, with his "Natural Philosophy," proposed a new scientific method. Newton's method consisted of "general induction from phenomena" and resulted in knowledge that was "accurately or very nearly true."  Like Copernicus and Galileo, he was a realist and "argued straightforwardly that universal gravity 'really exists."'  In his "Rules of Reasoning in Philosophy," he showed his belief that science uncovered physical realities when he insisted that scientists not hypothesize outside the bounds of empirical evidence: "induction may not be evaded by hypotheses."  Although he would "frame no hypotheses"  about the cause of gravity, because he could make no observations of the cause, he insisted that gravity was a real phenomena and that his laws accurately described the effects of gravity. Thus without pretending that his method could find the underlying causes of things such as gravity, Newton believed that his method produced theory, based upon empirical evidence, that was a close approximation of physical reality.
The notion of scientific realism was perhaps crucial to the development of modern science. Medieval science was guided by "logical consistency."  In order to move beyond Aristotelian science, the new goal of discovering physical reality was needed. Edward Grant, historian of medieval science, argues that the idea of a "quest for physical reality" was a necessity for a new science to replace Aristotelianism.  As historian of eighteenth-century science A. Rupert Hall concludes, the idea of "scientific truth" furnished the "metaphysical substrate"-the intellectual foundation-that produced the scientific revolution. 
Newtonian science, during the eighteenth-century Enlightenment it helped create, led to the evolutionary notion of progress that permeated the thought of the time and became standard throughout the next century. The Enlightenment view of the universe as a precise machine governed by knowable, absolute laws supported an idea of progress that was continuous and cumulative, (24)with each new piece of knowledge adding to the last; the structures and processes of the physical world could be uncovered by means of observation and reason. Although scientific knowledge was not held to be certain, it was assumed that with each new discovery science moved a step closer to representing accurately physical reality. Scientists of the nineteenth and twentieth centuries inherited both scientific realism and the evolutionary understanding of scientific progress.
The logical empiricists of the twentieth century represent the final school of support for scientific realism and the evolutionary development of science. As the name, "logical empiricist" implies, this movement combined induction, based on empiricism, and deduction in the form of logic. Carl Hempel, one of the later advocates of logical empiricism, in Philosophy of Natural Science (1966) argued against those who "deny the existence of 'theoretical entities' or regard theoretical assumptions about them as ingeniously contrived fictions."  Although Hempel recognized that many theoretical entities and processes cannot be directly observed (e. g. gravity cannot be observed; only the effects of gravity can be observed), as a scientific realist he believed that a theory well-confirmed by experiment translated to a high probability that the entities and processes of the theory really did exist.
Because of his belief in scientific realism, Hempel also believed that science evolved in a continuous manner. New theory did not contradict past theory: "theory does not simply refute the earlier empirical generalizations in its field; rather, it shows that within a certain limited range defined by qualifying conditions, the generalizations hold true in fairly close approximation."  New theory is more comprehensive; the old theory can be derived from the newer one and is one special manifestation"  of the more comprehensive new theory. The logical empiricists would agree, for instance, that Newtonian physics is a special case of, and can be derived from, Einsteinian physics. The logical empiricist's conception of scientific progress was thus a continuous one; more comprehensive theory replaced compatible, older theory. Each successive theory's explanation was closer to the truth than the theory before. It was the truth, and the prediction and control that came with it, that was the goal of logical-empirical science.
The notion of scientific realism held by Newton led to the evolutionary view of the progress of science. The entities and processes of theory were believed to exist in nature, and science should discover those entities and processes. The course of nineteenth- and twentieth-century science eventually threatened the idea of scientific realism. Particularly disturbing discoveries were made in the area of atomic physics. For instance, Heisenberg's indeterminacy (25)principle, according to historian of science Cecil Schneer, yielded the conclusion that "the world of nature is indeterminate. The behavior of the particle is uncertain and therefore the behavior of the atom is an uncertainty."  Thus at the atomic level, "even the fundamental principle of causality fail[ed] ."  Despite these problems, it was not until the second half of the twentieth century that the preservers of the evolutionary idea of scientific progress, the logical empiricists, were seriously challenged. Although Thomas Kuhn was not the first critic of traditional views of science, his work held the most important implications about the rationality of science. 
Thomas Kuhn: Revolution Against Scientific Realism
In 1962 a new historiography-of-science emerged with Thomas Kuhn's The Structure of Scientific Revolutions, first published as part of the "Foundations of the Unity of Science" series. In his book, Kuhn outlined a revolutionary model of scientific change and examined the role of the scientific community in preventing and then accepting change. Kuhn's conception of scientific change occurring through revolutions undermined the traditional scientific goal, finding "truth" in nature.
Kuhn's notion of scientific progress rested upon his concept of a paradigm: the common terminology and basic theories of a scientific community and that community's fundamental assumptions about methodology and what questions a scientist can legitimately ask. Textbooks inform scientists-to-be about this common body of knowledge and understanding. Scientific research necessarily takes place within a paradigm, for the world is too huge and complex to be explored randomly. Within a paradigm, a scientist knows what facts are relevant and can build on past research. Those who deviate from the dominant paradigm are not scientists at all; the scientific community considers them to be chasing superstitions.
During "normal science," research that occurs within a paradigm, scientists are busy "puzzle solving," an activity conducted to "add to the scope and precision with which the paradigm can be applied."  The scientist's research is like solving a puzzle because the scientist, guided by the paradigm, asks questions that can be answered and that have an easily recognizable solution. The paradigm thus shapes both the questions and the answers.
(26)Normal science, as defined by Kuhn, is cumulative. New knowledge fills a gap of ignorance. But normal science does not permit for advancement by means of revolutionary theories. As Kuhn pointed out, "one standard product of the scientific enterprise is missing. Normal science does not aim at novelties of fact or theory and, when successful, finds none."  However, normal science does contain a mechanism that uncovers anomaly, inconsistencies within the paradigm. Because normal science has precision as its goal, it focuses on details; eventually, details arise that are inconsistent with the current paradigm. In most cases, these inconsistencies are eventually resolved or are ignored. However, if the inconsistent details significantly threaten a paradigm, perhaps because they concern a topic of central importance, a crisis occurs and normal science comes to a halt. Such a crisis requires that the scientists re-examine the foundations of their science that they had been taking for granted.
During a crisis, alternate paradigms are proposed, usually by scientists who are young or new to the field and thus more open-minded. Slowly, one of the alternate paradigms triumphs over the competing paradigms for several possible reasons: it resolves the crisis better than the others, it offers promise for future research, and it is more aesthetic than its competitors. The reasons for converting to a new paradigm are never completely rational. Because different paradigms justify themselves with their own terms, one must actually step into a paradigm to understand it. Kuhn even used the word 'faith' to describe a conversion. As the scientific community is converted to the new paradigm, normal science begins anew under a new set of basic assumptions. The converted scientists, argued Kuhn, did not merely reinterpret old data in new ways, but rather "work[ed] in a different world"  after their conversion.
Kuhn departed from traditional evolutionary views with his argument that a new paradigm with its new foundation is "incommensurable" with the old paradigm. Unlike evolutionary science, in which new knowledge fills a gap of ignorance, in Kuhn's model new knowledge replaces incompatible knowledge. Thus science is not a continuous or cumulative endeavor: when a paradigm shift occurs there is a revolution similar to a political revolution, with fundamental and pervasive changes in method and understanding. Each successive vision about the nature of the universe makes the past vision obsolete; predictions, though more precise, remain similar to the predictions of the past paradigm in their general orientation, but the new explanations do not accommodate the old.
Kuhn argued against scientific realism. Each new paradigm increases predictive accuracy, but scientists have no reason to believe that the accuracy of explanation is closer to corresponding to what is "really there." He saw that the reason that one paradigm survives and another dies is because one solves puzzles better, not because it is a more accurate representation of reality:
A scientific theory is usually felt to be better than its predecessors not only in the sense that it is a better instrument for discovering and solving puzzles but also because it is somehow a better representation of what nature is really like. One often hears that successive theories grow ever closer to, or approximate more and more closely to, the truth. Apparently generalizations like that refer not to the puzzle-solutions and the concrete predictions derived from a theory but rather to its ontology, to the match, that is, between the entities with which the theory populates nature and what is "really there. " 
When he looked at history, Kuhn believed that he could "design a list of criteria that would enable an uncommitted observer to distinguish the earlier from the more recent theory time after time,"  but this list would include nothing about approaching truth.
Judging from the history of science, Kuhn believed that it was "implausible" to say that theory is approaching truth. There is no linear advancement of theory toward truth:
Newton's mechanics improves on Aristotle's and ... Einstein's improves on Newton's as instruments for puzzle-solving. But I can see in their succession no coherent direction of ontological development. On the contrary, in some important respects, though by no means in all, Einstein's general theory of relativity is closer to Aristotle's than... to Newton's. 
According to Kuhn, Einstein's theory is not merely a more complex version of Newton's. Einsteinian theory heads in its own direction; there is "no coherent direction of ontological development." This statement embodies, and indeed follows from, the idea of "Revolution" for which Kuhn argued.
In the closing chapter of his book, Kuhn proposed the need for a goal to guide science to replace the idea of progressing toward the truth:
The development process described in this essay has been a process of evolution from primitive beginnings-a process whose successive stages are characterized by an increasingly detailed and refined understanding of nature. But nothing that has been or will be said makes it a process of evolution toward anything.... We are all deeply accustomed to seeing science as the one enterprise that draws constantly nearer to some goal set by nature in advance. 
Kuhn thus argued against the notion of science as an activity approximating more and more closely the truth in nature. With his suggestion that human beings are forever separate from truth, Kuhn implied that truth does not guide science and thus removed from science the teleological goal of finding truth. (28)Truth cannot be observed and therefore cannot be leading scientists to better puzzle solving. Kuhn explained away truth using the analogy of Darwin's theory of evolution: "the entire process may have occurred, as we now suppose biological evolution did, without the benefit of a set goal, a permanent fixed scientific truth, of which each stage in the development of scientific knowledge is a better exemplar."  Science is not pulled forward by truth; science is propelled forward by the puzzles solved during normal science. As McMullin explained Kuhn's theory, as more puzzles are solved, scientists are not led to "a new level of understanding," but to "an illusion of understanding."  The "illusion of understanding" that Kuhn implied threatens traditional scientific rationality, for "illusion" is not at all what Newton and the logical empiricists believed to be the product of science. 
Kuhn issued a challenge to scientific realism and to scientific rationality itself. His theory raised many questions about the rationality of science that have been feeding a lasting controversy. The challenges facing scientific realism-the idea that guided modern science from its beginnings in the scientific revolution until the twentieth century-are such that it will probably never be restored. In a sense, we have circled back to the ancient and medieval practice of separating scientific theory from physical reality; both medieval scientists and Kuhn would agree that no theory corresponds to reality and therefore any number of theories might equally well explain a natural phenomenon.  Neither twentieth-century atomic theorists nor medieval astronomers are able to claim that their theories accurately describe physical phenomena. The inability to return to scientific realism suggests a tripartite division of the history of science, with a period of scientific realism fitting between two periods in which there is no insistence that theory correspond to reality. Although both scientific realism and the evolutionary idea of scientific progress appeal to common sense, both existed for only a few hundred years.