In 1962, Thomas Kuhn published The Structure of Scientific Revolutions. He built on his earlier work in the history and philosophy of science, The Copernican Revolution (1957) which analyzed the way the sun-centered Copernican view of planetary motion replaced the long-standing Ptolemaic earth-centered view as an example of how scientific understanding of the world can change.
In a nutshell, Kuhn says that scientists at any given time usually work within a model or theory, or paradigm as he referred to it, that explains their findings. This paradigm guides what we do every day as we work away at what Kuhn called "normal science". There are always unexplained or even apparently contradictory facts that don't easily fit into our working theory, but we do our very best in normal science to fit, or shoe-horn, these anomalies into our current paradigm. Occasionally, when the lack of fit becomes too great, a 'revolution,' essentially a new theory, is proposed, usually based on a new finding or a new way of synthesizing the data that does a better job of accounting for the anomalies in question (even if it may do less or less well for some known facts).
The new theory dramatically and at one fell swoop accounts for hosts of facts that hadn't fit into the previous working paradigm, including the apparent anomalies. A key point we'll discuss below is that the new view is not just a quantitative improvement in, say, measurement accuracy or something like that. It's not technology. Instead, a defining characteristic is that the new view is "incommensurate" with the view it replaced: you cannot express the new view in terms of its predecessor. It is quickly adopted by the profession in what Kuhn coined a "paradigm shift", which becomes the tool of a new phase of 'normal science'. This was what he called a scientific 'revolution'.
|Motion of Sun, Earth, and Mars according to heliocentrism (left) and to geocentrism (right), before the Copernican-Galilean-Newtonian revolution. Note the retrograde motion of Mars on the right. Yellow dot, Sun; blue, Earth; red, Mars.|
(In order to get a smooth animation, it is assumed that the period of revolution of Mars is exactly 2 years, instead of the actual value, 1.88 years). The orbits are assumed to be circular, in the heliocentric case. Source: Wikipedia, Copernican Revolution
The view that the earth was part of the solar system fundamentally changed the way planetary motion was accounted for. In the older Ptolemaic system, movements that were supposed to be perfect circles in the perfect spheres of the heavens, did not fit astronomical observations. So occasional little circles of movement (called epicycles) were invoked to explain observations and make predictions more accurate and consistent. But if the sun were viewed as the system's center, then one could account for the motions with ellipses and no epicycles. Refinements were to come along with Newton and Kepler, and Tycho Brahe then showed that geocentric mathematics could also work, with a "geo-heliocentric" system in which the Sun and Moon orbit the Earth (see Wikipedia: Tycho Brahe); but in which the other planets go around the Sun.
However, there have been other examples that reflect the basic Kuhnian idea: Darwin's evolutionary theory replaced one of special creation of the earth's species; quantum theory and relativity added truly revolutionary ideas about space, time and even causal determinism; plate tectonics (continental drift) replaced a diversity of ad hoc accounts for geological forms and changes, and so on. The basic notions of normal science, working paradigms, and essentially incommensurable replacement of one theory by another may be criticized in detail, but Kuhn's way of explaining the dynamics of science has much to recommend it.
The phrase "paradigm shift" has become canonized in modern science parlance. It glamorizes the genius (Copernicus, Einstein, Darwin) who was responsible for the change of view, often neglecting others who had roughly the same idea or whose work triggered the iconic figure's work. And for that reason, and because scientists are mainly middle class drudges who need to feel important, we throw the phrase around rather loosely (often referring to our own work!). We speak of scientific revolutions now rather casually as if they are occurring, whenever some new finding or technology comes along. But is that justified?
Generalizations about classical 'paradigm shifts' and revolutions in science
We were lead to write about this because of comments on our recent post on the faith component of science having to do with how we in science view what we think we know. This and the following post tomorrow are reflections about this, and not intended as an argument with the commenter.
A key relevant question is how we decide that what we assert today is better than what we said yesterday. If it is different, but not better, then where can we find a sense that we know more, or are closer to the truth? What if there is no single truth that we hope science is asymptotically approaching--with each new discovery getting closer to a perfect understanding?
At least one aspect of the answer lies in the idea of incommensurability between 'paradigms' as opposed to accuracy within a given paradigm. Here, I'll focus on genetics and evolution, fields I know at least something substantial about.
Prior to Darwin, in Western culture the prevailing view of life was that species had been individually created by God for His own reasons. Species might change under husbandry and so on, but they were basically static (though they might become extinct, again for some reason in God's plan), but they didn't morph one into another. After Darwin, species were viewed as the result of a historical physical process, evolution taking place over time due to physical constraints (natural selection). In a Darwinian view one cannot measure the nature or arrival of species in terms of events of special creation. Humans cannot be viewed as specially created at the Beginning with the rest of life created for our use. Evolution is not just a quantitative description of special creation. The two views are incommensurable.
In the new 'paradigm', everything changed. Species and their traits are viewed in terms of historical usage history, context-specific factors that affect what forms could succeed better than other forms relative to each other at the time, not in any external Creator's eye. Evolution was truly a revolutionary change in the understanding of global diversity in life. It has had at least as much impact as any other revolutionary conceptual change in any science. But is it more 'true' or has it given us the truth about life?
Of course, even if the process of speciation is an historical one that takes place gradually by Darwinian means, each species must arise at some specific time. Is this so different? Yes! It's different first because the definition of 'species' is a human-imposed cultural one and because the many processes that could lead populations to be mating-incompatible (the usual definition of 'species') may arise by single events (mutations in chromosome regions required for mating, for example) but they were historical, random changes in DNA, etc. They were not guided from without with any purpose. And generally, diversity accumulates along with mating incompatibility, gradually.
And what about natural selection? It is the theoretically accepted origin of complex traits in living species. It is a gradual process even if each life or death or conception may be discrete events in time and place. And, after Darwin, we have had to add chance (genetic drift) into the picture of how genomic structures and what they cause have changed over time. But such additions modify, but do not at all overthrow the idea of evolution. They introduce no paradigm shift.
Nor does the discovery that chromosomes contain more than just protein-coding DNA sequence--they have regulatory sequences, sequences involved in DNA's own packaging, and so on. The idea of gene regulation, or of genes being made of discrete, interrupted sequence regions (coding exons, introns, etc) added new theoretical elements to biology, but they are entirely commensurable with prior views that were non-specific as to just what genes 'are'. The discovery of the base-pairing nature of DNA and its use of a code for protein sequence and other functions added to our understanding of life, and produced a new theory of genetic causation. But that theory didn't replace some earlier specific theory about what genes were. None of this in any serious way was a paradigm shift, even though these discoveries were of momentous importance to our understanding of life.
And then there's the origin of 'life'. Mustn't that, too, have had a moment of creation? Biochemists will have to assert that the possibility has always existed since the beginning of the cosmos, but that only when the right ingredients (molecules, pH, temperature, etc.) existed at the same time and place did life start. It may have had countless molecular origins, but here on earth at least only one such led to life as we know it today. That is, in a sense, a theory of a moment of occurrence--though not of 'creation'. So in our modern view it's part of the historical process that is life.
So, biology has had its scientific revolution, and one that shook the earth in very Kuhnian terms. But whether we are closer to the 'truth' about what life is, is itself a rather vague or even unanswerable question. As technology advances, we could be getting a better and better understanding, and a more complete explanation of the essential nature of life. Or, forces at work within organisms might be discovered which will lead to fundamentally different kinds of understanding of life. How can we ever know unless or until that happens?
One way to rephrase this question is to ask whether we can know how 'close' we are to understanding the truth. We can compare origin theories from many different cultures, including our own Biblical one, but we can't really concoct a quantitative measure of how true they are even relative to each other. In a sense, all have zero truth except evolution, but that's not very useful, because we have no way to know what new idea may come along to challenge the one that we now believe to be true. Of course, some people, even some otherwise scientists, accept religious explanations and will simply not acknowledge what I've been saying because they have an incommensurable truth that cannot be compared in this way to evolution other than by forced contortions such as that the Bible should be taken metaphorically and the like. Or others have a mystical view of universal unity and reincarnation etc. which, like Biblical explanations, cannot really be compared because it doesn't attempt to explain the same things.
But there is another very different way to view scientific progress, typically referred to by the term 'Bayesian', which is often implicitly equated with 'revolutions' or 'paradigm shifts', as a systematic rather than episodic way for scientific truth to become known. We'll discuss that tomorrow.