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New essay: Proliferation

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By Paul Newall (2005)

Arguments for proliferation as a methodological principle are often associated with the philosopher of science Paul Feyerabend (1999) but they date back at least to J.S. Mill (1869 [1991]) and take the same form.

In the latter’s On Liberty of 1869, four reasons were given to advocate proliferation of theories and "forms of life".


First, if any opinion is compelled to silence, that opinion may, for aught we can certainly know, be true. To deny this is to assume our own infallibility.

The history of science is (often unfortunately) littered with examples of theories that were true without doubt and yet crumbled all the same in spite of this certainty. Although case studies such as the so-called Galileo Affair have shown that the relationship between early science and religious strictures was considerably more nuanced than had previously been believed, such that the claim that science was "held back" by religion is problematic, nevertheless the assumption of infallibility has consequences for the speed with which we can discover an error. After all, why question a surety? It has tended to take people with extreme tenacity like Galileo to adduce doubt when there is little reason to do so before the erroneous nature of the certainty can eventually become clear, of which more below.


Secondly, though the silenced opinion be an error, it may, and very commonly does, contain a portion of truth; and since the general or prevailing opinion on any subject is rarely or never the whole truth, it is only by the collision of adverse opinions that the remainder of the truth has any chance of being supplied.

It is now straightforwardly accepted that science is a fallible venture, such that our theories are never certain and are always assumed to contain some errors (although obtaining this admission where it pays in rhetorical terms not to mention it is sometimes a painful process). Given that this is so, we can take two points from Mill's remarks: firstly, that although other theories may be flawed they may still be partly true (or possess some degree of verisimilitude, or truthlikeness); and, secondly, that by bringing theories together that conflict in some or all areas we can use one to identify the flaws in the other, and vice versa.

Indeed, this "collision of adverse opinions" is for Mill an important means by which to come by knowledge. Even where an opinion strikes us as deluded or wholly ignorant, the very process of setting out why can be beneficial because it forces us to rehearse the reasons and hence to understand how a theory comes to be considered false rather than relying on an insistence that only a fool would think otherwise. This leads us to the next reason:


Thirdly, even if the received opinion be not only true, but the whole truth; unless it is suffered to be, and actually is, vigorously and earnestly contested, it will, by most of those who receive it, be held in the manner of a prejudice, with little comprehension or feeling of its rational grounds.

Here Mill insists that this business of contesting ideas – no matter how sure we are of them – is valuable insofar as it prevents us holding them without appreciating why they were thought worthwhile originally. There is more, though:


And not only this, but, fourthly, the meaning of the doctrine itself will be in danger of being lost, or enfeebled, and deprived of its vital effect on the character and conduct: the dogma becoming a mere formal profession, inefficacious for good, but cumbering the ground, and preventing the growth of any real and heartfelt conviction, from reason or personal experience.

Not only can an idea unrehearsed become held as a dogma, then, but this state of affairs can also prove a hindrance to further development, whether of the idea in question or others simultaneously or subsequently. It is here that we arrive at the full meaning of Mill's advocacy of pluralism and proliferation: even the best ideas can be improved by their clashing with others, even poor ones, because they are either enriched by their own flaws being highlighted or revealed, or else because the challenge leaves them untouched but better understood, forcing us to articulate them more clearly and to not insist upon them due to the power, prestige or authority of their supporters. Conversely, there is no value in even a true idea that is not continually subjected to challenge by even apparently false ones. Moreover, it has often been the case that the more sure of a theory people have been, the less inclined they are to question its anomalies or continue to work on its development.

One consequence of the principle of proliferation that may not be immediately apparent from Mill's discussion was elaborated upon by Feyerabend and is that these so-called "poor ideas" cannot be dismissed for the very same reasons that the "good ideas" cannot be accepted uncritically. Not only is the question of demarcating between good and bad ideas a thorny one itself, but the good ones typically started life as bad and those they replaced provide them with much of their content through the process of improvement. Any student of the history of science is also familiar with numerous examples of ostensibly hopeless theories that were regarded with scorn by all right-thinking people only to make a comeback (on several occasions in some instances, like atomism), such that it would eventually be thought preposterous that anyone would have imagined otherwise. At the time of Copernicus, say, the arguments marshalled by the Aristotelians against heliocentrism and geokineticism were so strong that Galileo had to appeal to reason over and above the clear evidence of the senses in order to explain why anyone should doubt geocentrism and geostaticism. This is not to say that a theory will make a triumphant return, of course, but only that it might and that, in the meantime, by keeping it in mind we remain aware why we (tentatively) hold to an improvement on it.

Another reason to be interested in proliferation is that theory choice is no longer accepted to be a simple matter of agreement with the evidence. The importance of rhetoric, as well as social, political, economic and thematic factors, amongst others, means that the current superior status of one theory is insufficient grounds for supposing that this circumstance is due solely to the merits of the victor (and here Lysenkoism in the former Soviet Union is perhaps the most chilling example of a theory that succeeded thanks to ideology and at the cost of many lives). Notice also that this situating of theories within a wider context is unavoidable: all the models and ideas we develop have some beneficial aspects or we would not come up with them at all, but the questions are to whom and to what end? Where only one option exists we have no opportunity for comparison and hence no way of knowing whether we have the best of the matter.

Indeed, it can happen that a theory is incorrect in an important way but there is no experimental way of knowing this. An example discussed elsewhere concerns the phenomenological theory of gases, which was replaced by the kinetic theory thanks to Einstein's Investigations on the Theory of Brownian Movement. In this case the consideration of an alternative theory in spite of there being no experimental falsification allowed Einstein to explain the same situation with a new theory that led to novel predictions, which turned the tide against the phenomenological theory and replace it with the kinetic.

Arguments against proliferation have taken several forms. One important and wider issue is that pluralism runs contrary to one of the prevalent thematic ideals: the search for unity that runs through much of physics. From this perspective, it makes little sense to proliferate theories when the aim of science is (or should be) a small number of laws that can account for all phenomena. At base this approach relies on the same notion that both Galileo and the Church insisted upon; namely, that the truth is singular and hence even if our theories may be fallible they are still getting closer and closer to one reality. Methodologically speaking, then, the suggestion is that we should not be hearking back to old, defeated theories but concentrating on the best we have and striving to improve them. In particular, our best theories (such as evolution or quantum mechanics) may be incomplete but it is unthinkable that they could be discarded at some point in the future, so we should work on the few remaining details and not concern ourselves with alternatives just to satisfy otherwise sound advice on understanding ideas.

Notwithstanding that many theories in the past were in precisely this situation (consider the certainty with which Copernicus' writing was rejected, for instance), it is here that Feyerabend's argument applies. This singular approach presupposes that our theories can be straightforwardly developed by further application but the example of Brownian motion shows that sometimes this is not possible. More generally, if a theory T1 predicts circumstances C1 but what actually occurs is C2, even though C2 is (currently) experimentally indistinguishable from C1, then we have no reason to look at alternatives to T1 in spite of it being incorrect. If we instead proliferate theories and find that some T2 predicts C2 then we have a justification for trying to experimentally differentiate the two or, where this remains impossible, for studying the merits of the two otherwise. Another possibility, of course, is that the investigation of T2 allows us to tweak T1 slightly such that it does predict C2 while maintaining its other advantages. In this way proliferation leads to strengthening or deepening the content of theories.

For Mill's part, he was very clear on why it can never suffice to rest content with one theory:


He who knows only his own side of the case, knows little of that. His reasons may be good, and no one may have been able to refute them. But if he is equally unable to refute the reasons on the opposite side; if he does not so much as know what they are, he has no ground for preferring either opinion.

On this view, it is never enough to know a theory inside out; we also need to understand why alternatives exist, why they are believed to be true and why they fail in order to appreciate the value of the theory as an improvement or more deserving of our attention. The clash between advocates and deniers of the phenomenon of global warming, for example, has pushed both sides to reconsider their arguments and strengthen them, allowing flaws to be amplified (although political and other pressures are such that a good argument is rarely enough to change a policy), while the challenge of creationism and the elaboration of intelligent design have forced biologists to enter the public arena and explain why evolutionary theory is so highly confirmed and the foundation of biology.


Nor is it enough that he should hear the arguments of adversaries from his own teachers, presented as they state them, and accompanied by what they offer as refutations. That is not the way to do justice to the arguments, or bring them into real contact with his own mind. He must be able to hear them from persons who actually believe them; who defend them in earnest, and do their very utmost for them. He must know them in their most plausible and persuasive form; he must feel the whole force of the difficulty which the true view of the subject has to encounter and dispose of; else he will never really possess himself of the portion of truth which meets and removes that difficulty.

Here Mill goes further, insisting that not only do "the arguments of adversaries" deserve to be heard just as part of learning about the superiority of the current theory but rather in their very strongest form. This is no recommendation of a superficial treatment, then, but a conviction that by supporting and developing alternatives we contribute to the improvement of our knowledge, even where these alternatives achieve nothing when considered in isolation. This is to say that a theory may be preposterous on its own but becomes of benefit to us when taken as providing an ever-present challenge to others. It goes without saying that the stronger it is, the greater our confidence can be that our current ideas have survived critique.


So essential is this discipline to a real understanding of moral and human subjects, that if opponents of all important truths do not exist, it is indispensable to imagine them, and supply them with the strongest arguments which the most skilful devil's advocate can conjure up.

It was this last principle that Feyerabend embodied, even though there are always enough people who will paint those who apparently depart from orthodoxy as heretics by implication or opponents of clear thinking. On the contrary, it is the effort to expound, support and defend arguments we do not agree with that allows us to truly understand them enough to dissent and prefer an alternative. Proliferation enjoins upon us both a methodological pluralism and a belief that a theory is of no value unless subject to a continuous process of challenge, of which even the most dismissed of ideas is an indispensable part.

Another objection to proliferation – often the most common – is that it may be a good idea in the abstract but not in practice. Unfortunately for scientists and vacuum cleaner salesmen alike, there is only so much money to go around and hence we cannot afford to allocate resources to any and all ideas that come along. A theory can be challenged by a well-developed and plausible alternative, in keeping with the argument so far, but little or nothing is to be gained (indeed, it would even be detrimental) by taking funding from our best theories to support hopeless substitutes.

The first point to note about this rejoinder is that it effectively begs the question against the alternatives: if we deny support to an idea we can hardly criticise it later for being undeveloped and not worthy of consideration. Theories start their lives riddled with internal contradictions and partial (or even complete) disagreement with the evidence but over time may – or may not – prove their worth and begin to be taken seriously. Perhaps it is because this process of acceptance is usually slow (even where so-called "revolutions" in science are taken to have occurred, a claim that is increasingly untenable in historiographic terms) that we fail to notice it and forget that there were times when our best theories were themselves rejected as absurd or unlikely? Rejecting an idea because it is prima facie false would thus have been catastrophic methodological advice for science in the past and there is no reason to think otherwise today unless we presuppose that the current state of science and knowledge is approximately the final one, a conceit that seems to affect all ages.

What is also ignored in this response is that an idea is not only credible in proportion to how much work has been done on it and how much money is behind it. The early quantum theory was rejected with a considerable measure of displeasure by some physicists (Heisenberg told Pauli in a letter of 1926 that "[t]he more I ponder the physical part of Schroedinger's theory, the more disgusting it appears to me" (p.15 in Holton, 1988)) because it disagreed strongly with their thematic preferences, while the notion that there is or is not a higher intelligence involved in the creation or sustaining of the universe is no less unpalatable to some. Moreover, there are power structures involved in science just as anywhere else and those who have an investment in the prestige and financial rewards associated with a successful theory may not be as open to new ideas and the redistribution of funding when appropriate as their claims to the contrary might suggest. Money is thrown at projects with little or no chance of success (for example, missile defence systems) or no proven achievements (such as string theory) not because of some inherent value but because the rhetoric or personalities behind them do a better job of convincing others, while a skilful synthesis can create unity where there was none (Dobzhansky's in evolutionary biology, say) or a consensus can be constructed (like the usage of the DSM in psychiatry). Insisting that time and funding are limited, then, ignores their already unequal distribution and that the relative status of a theory is determined by far more than its empirical support. That the decision between theories is complex and based on many factors, some of them extra-scientific, is no reason to restrict our efforts to few or ignore the arguments for proliferation.

Tempering this realisation is the frequent (and necessary, for Feyerabend) association of proliferation with tenacity, the tendency of scientists (and people in all walks of life) to persist with their ideas even in the face of the most adverse of difficulties. Once again, the history of science is replete with examples of theories or conjectures that by all methodological standards in use should have been discarded but were maintained in spite of experimental results to the contrary or the most grievous of conceptual problems (see the discussion of falsificationism for more instances). Perhaps the finest illustration of tenacity – and its link with thematic commitment – is provided by Einstein's response to the question of how he would have felt if Eddington's expedition of 1919 had failed to confirm his theory of general relativity: "... then I would have been sorry for the dear Lord - the theory is correct."

The coupling of proliferation and tenacity thus goes some way to assuaging the concerns of those who would prefer not to waste time on what they consider to be bad ideas. As Mill noted, proliferation requires that we work with the strongest possible version of defeated theories in order to better our own, and vice versa, and the way to achieve this is to ensure we persist with both even when all seems lost or when it would be absurd to withhold assent that a theory is correct. This is not to say that no division of labour can be employed and that we each have to consider every idea in the marketplace, but only that it is in our interest to see that those who wish to study them are able to. This means that scientists working on a theory do not have to reassign part of their time to develop alternatives in the name of proliferation but that we should not condemn these alternatives out of hand. They may compete for funding, of course, just as we would expect given the parallel principle of tenacity, but we should view their rhetoric and behaviour in these terms rather than as indicative of a neutral claim to superiority and financial support over and above the requirements of proliferation.

When Mill set out his arguments for allowing many "forms of life" he did not have in mind only the laboratory, although proliferation outside science tends to be resisted robustly – especially when it comes to alternative medicines and the spectre of frauds and charlatans putting the health of their victims at risk. However, if we allow the benefits of supporting different ideas covered above then the same applies to methodologies, with the current dominance or pre-eminence of one approach (science) no guarantee of its continuing success – or the demerits of alternatives – any more than this could be said of theories. Extending democracy to all traditions is some way off, though, even where it is agreed that self-determination should have wider application.

Proliferation is thus a principle that makes our attitude to life and learning inclusive, as well as reflexive and genuinely fallible. It is not a rule any more than parsimony is but functions to keep knowledge an open and unfinished process by never letting us stop and be satisfied with what we have.


Selected References:

  • Feyerabend, P.K., Knowledge, Science and Relativism (Cambridge: Cambridge University Press, 1999).
  • Holton, G., Thematic Origins of Scientific Thought (Cambridge: Harvard University Press, 1988).
  • Mill, J.S., On Liberty (Oxford: Oxford University Press, 1991).

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