The Nature of Scientific Theory
Science is a way of thinking and scientists use that way of thinking to conceive conceptual schemes or theories that are rational representations of collective human experience. The function of theory is to help us grasp the whole picture. Following our previous comments, we can define a scientific theory in the following way:
Scientific Theory: A theory is a conceptual scheme which we invent or postulate in order to explain to ourselves, and to others, observed phenomena and the relationships between them. Thus the theory brings together in one structure the observations, concepts, hypotheses, principles, and laws from often very widely different fields.
Theories are of value to science because they unify, and in so doing they simplify. A theory is assumed to have universal applicability when it agrees with what we already know and can be repeatedly validated by future experiences to which it is applied.
An important question is where do theories come from? Is a theory entirely suggested by a scientist's examination of data, or is it purely a product of imagination in which the scientist's mind has been set into action by contact with data? Alternatively, could the theory be the result of a combination of observation and imagination? To such questions there are no right or wrong answers. In fact, there is not even a consensus among those scholars who care about such issues.
Of great concern now as in the past to scientists and nonscientists alike is whether or not science captures reality? This long-standing question can be said to be roughly equivalent to asking which of the philosophies, realism or idealism, characterizes what scientific theories reveal. Realism is the belief that experiences that come by way of our senses must reveal a "real" world that exists independent of any human perceiver or acts of perception. In contrast to realism is the philosophical position of idealism, which is the belief that there is no objective or absolute reality apart from the products of our imagination or mental constructs. According to idealism humanity can fashion only imperfect copies of that ultimate reality of our imaginations.
Idealism and realism are philosophical orientations whose origins can be traced back to Plato (427-347 B.C.) and his pupil and successor Aristotle, respectively. Thus the philosophical orientation of scientists never has and probably never will be as rigidly prescribed as one might suppose it to be. Einstein approached this difficult problem in an illuminating way. He observed that the practicing scientist "appears as realist insofar as he seeks to describe the world independent of the act of perception, [but] as idealist insofar as he looks upon the concepts and theories as free invention of the human spirit." Thus in Einstein's view both realism and idealism may have a place in answering the question of reality in scientific theories.
The Role of Scientific Models
Underlying theories are notions called scientific models and they have been utilized in science for a very long time. Ancient science relied frequently on analogies with the behavior and drives of organisms, and thus their theories can be said to be based on organismic models. But from Newton up to the beginning of the present century, scientific models have been mechanistic models, and much of currently accepted physical theory is a product of mechanistic thinking. However, experience in the twentieth century has shown that conceptual schemes cannot always be cast in terms of some mechanical model, and there are historical examples of progress being delayed by too strong a belief in a mechanistic model. As this century has progressed scientific models have become more mathematicised conceptual schemes of mental images than purely mechanistic models. Let us define scientific models as follows:
Scientific Model: A scientific model is a mental picture, or idealization, based on physical concepts and aesthetic notions that accounts for what scientists see regarding a particular phenomenon. Such a model allows scientists to predict a future course for the phenomenon in question.
The requirements of explanation and prediction as constraints on scientific models date only from the sixteenth and seventeenth centuries. Before then, theories of nature often had to satisfy only aesthetic or theological constraints.
Whether models are devised for something as all-encompassing as the Universe or for such limited phenomena as lunar eclipses, models are widespread in astronomy. However even though models are widely used, they must be carefully distinguished from the real world. For like metaphors in poetry, their early versions may be more figurative than actual; they are only vague approximations to the world of our experience. Our thoughts proceed, as it were, on crutches, and so depend on these mental schemes no matter how incomplete they are.
The Role of Successful Theories in Science
The role of theories in science consists in correlating many separate and possibly diverse facts into a logical, easily grasped structure of thought. Such a structure may then suggest new relations that launch human imagination along previously unsuspected lines of inquiry that ultimately connect old and new facts. Theories also stimulate the making of predictions or speculations about new situations in nature, particularly in the case of quantitative theories where the prediction can be a numerical one.
History shows that successful theories are most often based on a few simple assumptions or hypotheses. And these assumptions are reasonable plausible ones to scientists, even if they are not subject to rigorious testing. Thus the whole tenor of a new theory is not in conflict with contemporary ideas. The fitness of a theory is most advantageously shaped and most convincingly demonstrated in a vigorous contest of ideas in which predictions play a vital role. Predictions are in essence tests of theories, and such tests may show that a theory continues to be valid, that it is in need of modification, or that it should be discarded and a new theory adopted to replace it. Thus successful theories are those that are flexible enough to grow, and to undergo modification where necessary. But if, after a full life, a theory dies, it dies gracefully, leaving a minimum of wreckage and preferably one or more descendants.
Although we are able to discern a few criteria by which successful theories are judged, one should not presume that any theory is necessarily rejected solely because it fails to meet these criteria. The decision making process is more complicated than that. With these notions of theories and their role in mind, one can see how Newton's theory of motion shaped the evolution of science.