Reductionism

The term reductionism has two rather different meanings in science. One is laudable and the other is derogatory. In its laudable sense, reductionism means a search for genuine fundamentals, which can occur at any systems level. Less laudably, and in the terminology of systems theory, reductionism means working at the lower systems levels. This is sometimes called the merological approach. However, it is not true fundamentalism. The lower systems levels are not necessarily more fundamental than any other systems level. It is in this derogatory sense that the term 'reductionism' is used throughout this book. In this sense of the term, reductionism is hazardous, because emergents that are apparent only at the higher systems levels are invisible to investigators who are working exclusively at the lower systems levels.

The converse of derogatory reductionism involves the higher systems levels, and it is called 'holism' or the 'holistic approach'. Excessive reductionism, and excessive holism, lead to biased science. Good science treats all systems levels as being equally worthy of investigation.

It is often argued that the finer the details, or the lower the systems level, of the analysis, the more fundamental the science becomes. Hence, the importance of particle physics. Hence, too, there is a widespread love of reductionism. It is not my intention to denigrate research conducted at the lower systems levels. However, far too many scientific investigations are conducted exclusively at the lower systems levels. And I believe it is self-evident that science must treat all systems levels with equal respect.

Taxonomists are often divided into so-called 'splitters' and 'lumpers'. Splitters are reductionists, and lumpers are holistic. For example, in the taxonomy of Citrus, Tanaka (1954), a splitter, proposed 145 species, while Swingle (1967), a lumper, proposed only sixteen species.

Let us consider reductionism within the discipline of plant pathology. In the middle of the twentieth century, plant pathology was almost exclusively concerned with the functioning of the gene-for-gene relationship (see 4) at the systems level of the individual. That is, at the systems level of one individual host interacting with one individual pathogen. Typically, a single detached leaf or leaf-disk would be inoculated with a single spore, and the interaction would be qualitative. There would either be disease, or no disease. The pathologists would then assemble a set of host differentials that would identify any 'physiologic race' of the pathogen. And they would assemble a set of 'physiologic races', or pathogen differentials, that would identify any resistance. These differentials became an essential tool of plant breeders who were working with this kind of resistance.

Then, for the next two or three decades, mainstream plant pathology moved to a lower systems level, and concentrated on the individual resistance mechanism. In particular, it was concerned with the chemistry of these mechanisms. This was known as 'physiologic' plant pathology and, for some thirty years, it claimed the lion's share of research funds. More recently, almost the entire discipline has moved to an even lower system level and it has embraced molecular biology. This is the lowest systems level of all in biology. Indeed, it is impossible to go any lower, because anyone who works at a lower systems level stops being a biologist and becomes a chemist.

Mid-century plant pathologists could and should have considered the converse trend, the holistic approach, which involves the higher systems levels. The level above the individual host and the individual pathogen involves the interaction of two populations. That is, the pathogen population and the host population interacting with each other. This is the level of the pathosystem. A still higher level is the ecosystem, which involves populations of many different species interacting with each other and their environment.

It is important not to belittle or to over-emphasise the significance of any systems level. They are all important, and a good scientist considers them all equally. What is dangerous, however, is the tendency to exclusiveness, the tendency to claim that the systems level of one's choice is uniquely important.

The importance of the holistic approach is that it does not suboptimise (see 1.11). It does not attempt to analyse or to control the entire system in terms of only one or a few subsystems. Nor does it neglect the emergents of the higher systems levels, which are undetectable at lower systems levels. There is no escaping the fact that modern crop protection is in a mess. And, it seems, this mess is the result of reductionism and suboptimisation.

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