In the early sixties, Thomas Kuhn wrote an exceedingly important and still controversial book  called The Structure of Scientific Revolutions.  In his book, Kuhn challenges the model of science as a cumulative process whereby each new discovery adds one more item to the scientist's world.  In the cumulative process view, the scientist’s activities are determined by the facts and structure already existing in the world of nature which constrain and guide the scientist’s work – he is discovering or uncovering the Truth of the natural world.  But Kuhn argues that there is a set of tacit assumptions and beliefs which he calls a paradigm within which research is conducted – the functions of a paradigm are to supply puzzles for scientists to solve and to provide the tools for their solution.  These  assumptions, however,  rather than being derived from the observations of facts may  actually be determinative OF the facts observable.  I see what fits in to my interpretation of the facts.  This means that when scientific paradigms conflict, there is no recourse in looking at the facts because this is precisely what is at issue – what are the facts?   A fact for one paradigm may simply be a mistake for another.

What this means then is that science doesn’t proceed by offering its findings to the independent judgment of Nature but rather when those from one scientific paradigm present their case in a way that the other paradigms find compelling and convincing.  Paradigm shifts tend to be most dramatic in sciences that appear to be stable and mature, as in physics at the end of the 19th century. At that time, physics seemed to be a discipline filling in the last few details of a largely worked-out system. In 1900, Lord Kelvin famously stated, "There is nothing new to be discovered in physics now. All that remains is more and more precise measurement." Five years later, Albert Einstein published his paper on special relativity, which challenged the very simple set of rules laid down by Newtonian mechanics, which had been used to describe force and motion for over three hundred years. In this case, the new paradigm reduces the old to a special case (Newtonian mechanics is an excellent approximation for speeds that are slow compared to the speed of light).
Kuhn observes that in scientific disagreements “Each party must try by persuasion to convert the other.”  Science then has no more access to a neutral observation language than any other discipline.

There cannot be a paradigm shift however without a crisis or rather a paradigm-in-crisis which displays the following characteristics: (1) a confusing multiplicity of variables in introduced into the paradigm.  The puzzles presented by the paradigm repeatedly fail to be solved resulting in new concepts being added/invented in the hope that they will solve the puzzle. (2) Failures in puzzle-solving  can be seen in continually recurring counterinstances.  One puzzle-solver will embarrass the efforts of other scientists by showing that the puzzle was not properly solved.   (3) The defenders of the crumbling paradigm offer a variety of conflicting articulations of data.  Each counterinstance is the signal for the creation of more and more vague ad hoc modifications of the paradigm, so that the theory becomes gross and inaccurate  (Sarbin & Mancuso).

Sound familiar?  The bio-medical model currently being applied in psychiatry is clearly very much a paradigm-in-crisis.  More on this later.