A defining feature of the mature sciences is their cumulative nature. Knowledge progresses with research programs building on what has gone before. Should psychiatric nosology strive toward such a goal? For critics of psychiatric diagnoses who view them as social constructions, this is an incoherent project. If there is no truth out there, we cannot expect to get closer to it. For those who adopt either realist or pragmatist perspectives on psychiatric nosology – that there are things or inter-related sets of things out there in the real world that correspond to individual psychiatric illnesses – it is a more rational and, I would argue, vital task. I here explicate this approach through the concept of iteration.
The idea of iteration originates in mathematics and is defined as a computational method that generates a series of increasingly accurate estimations of a desired parameter. In a properly working iterative system, each estimate improves on its predecessor. With a sufficient number of iterations, this process asymptotes to a stable and accurate parameter estimate. Iterative processes are robust in that they can begin with widely divergent starting values and reliably converge to the same correct solution.
In a recent thoughtful book with important, albeit indirect, lessons for psychiatry, Chang (2004)
expands on this concept to develop what he terms ‘epistemic iteration’ and applies it to the history of the science of temperature. According to Chang, epistemic iteration (where ‘epistemic’ refers to the acquisition of knowledge) is an historical and scientific process in which successive stages of knowledge in a given area build in a sequential manner upon each other. Directly analogous to its original mathematical meaning, when correctly applied, the process of epistemic iteration should lead through successive stages of scientific research toward a better and better approximation of reality in ‘a spiral of improvement’.
How does this apply to the history of thermometers? In the measurement of temperature, someone had to start somewhere with a crude measuring device; in this case an air thermometer developed by Robert Fludd in 1638. This initial approach required all sorts of assumptions that at first were untestable (such as the increase in air volume with temperature would be linear across the temperature range under investigation). Over the next two centuries, the accuracy of thermometers was evaluated and improved by cross-calibration with a variety of different physical designs, calibration scales and substances that, because of their expanding–contracting nature, reflected temperature (including, in addition to air, alcohol, mercury and clay) (Chang, 2004
). Chang documents the deep controversy between scientists and their followers over the ‘best’ thermometer and the challenges, all eventually overcome, of expanding the range of temperature measurement to the very cold (below the freezing point of most substances) and the very hot (beyond the boiling point of most substances). This in part required the intercalation of measurements from different media and also extensive research on the nature of the boiling and freezing of water, as these temperatures were quickly established as important benchmarks. It was a messy, conflict-filled process including colorful characters and trips to Siberia. Yet, over time, relentlessly, the range and accuracy of the measurement of temperature improved. Chang claims that an inevitably untidy but fundamentally iterative process was at work. The development of the thermometer reflected a series of increasingly successful approximations. Chang, concludes from his history, that
in an iterative process, point-by-point justification of each and every step is neither possible nor necessary; what matters is that each stage leads on to the next one with some improvement. (Chang, 2004
, p. 215)
The concept of epistemic iteration poses a substantial challenge to psychiatric nosology. If our current methods for validating psychiatric disorders, including description, genetics, imaging, treatment response and follow-up studies, reflect aspects of an objective truth out there in the world and we want our nosology to describe those truths with increasing accuracy, the only way to achieve this is to assure ourselves that each periodic revision of our manuals contains improvements on its predecessor. That is, changes are only made on the basis of convincing evidence that, using an agreed upon set of validators, the new diagnostic criteria improve upon the performance of their predecessor.
We noted above the inherent historically contingent nature of many of our clinical categories. Let us take for a moment the influence of Emil Kraepelin. We argued that, despite his deep clinical experience, Kraepelin simply could not know that long-term course should be the defining criteria for psychotic disorders. (Indeed, his concept that dementia praecox was fundamentally a deteriorative disorder may be mistaken; Menezes et al. 2006
.) So, we cannot be sure that we are starting off at the right spot in our iterative process. However, a wonderful property of iteration is its capacity to get to the real solution regardless of the starting point. Although the process might be slow, as long as every iteration improves on its predecessor, the logic is relentless.
Epistemic iteration might work well for a clearly defined, unidimensional physical phenomenon such as temperature, where progress is easy to measure (e.g. how well does your thermometer agree with my thermometer?). Psychiatric disorders are ‘messier’ and can be viewed from several perspectives that do not always agree with each other (Kendler, 1990
). In addition, historical and cultural factors can influence conceptions of psychopathology. Can the iterative process work well with such complex constructs?
It will not work if our nosologic revisions reflect largely a power struggle between different branches of psychiatry, each with its own essentialist views of the true nature of mental illness (e.g. social, psychodynamic, genetic, neurobiological). In this pessimistic view, different constituencies within psychiatry would, over time, vie with each other for influence and control of the nosologic process and the professional status that it brings. When it gains control, each group in turn reshapes the nosologic system in their own a priori image. Instead of a gradual iterative process bringing us toward increasing validity, we would instead have wide fluctuations between different systems with divergent theoretical perspectives and no net progress.
However, between this gloomy view of a non-progressive nosologic process and the consistently cumulative progression in the development of thermometers, lies a middle ground of wobbly iterations that our nosologic process should continue to occupy. We do not yet and probably never will possess a diagnostic construct as simply and clearly measured as temperature. The most important source of wobble will probably be shifts in the importance attached to one validating perspective on psychiatric illness versus others. Because psychiatry is both a science and a practical medical discipline, we must allow for the impact of both empirical and pragmatic factors in our nosology. If these changes are kept modest – empirically driven small differences in emphasis on one perspective versus another – this will retard but not derail the iterative process. To operationalize this approach will require maturity and a consistency of vision in a rapidly shifting historical landscape. This is the best way in which psychiatry can follow biology in maturing historically from top-down essentialist views of our categories to bottom-up empirically defined entities that reflect with increasingly accuracy the world as we can best understand it.