Bold hypothesis (or bold conjecture, the terms are used interchangeably) is a concept originally created by the liberal philosopher Karl Popper when he tried to define the meaning of scientific thinking and scientific progress. It was first explained in his debut The Logic of Scientific Discovery (1935) and subsequently elaborated in writings such as Conjectures and Refutations: The Growth of Scientific Knowledge (1963), Objective knowledge: an evolutionary approach (1972),Popper, Karl R. Objective knowledge: an evolutionary approach. Oxford: Clarendon Press, 1972. and Replies to my critics (1974).Karl Popper, “Replies to My Critics.” In: Paul Arthur Schilpp (ed.), The Philosophy of Karl Popper, Vol. II, La Salle, IL: Open Court, 1974, pp. 961–1197. For Popper's collected writings, see: Troels Eggers Hansen (ed. & rev.), "Bibliography of the writings of Karl Popper". In: Paul Arthur Schilpp (ed.), The philosophy of Karl Popper, Part II. La Salle, IL: The Open Court Publishing Co, 1974, pp. 961-1197, at pp. 1201-1287. In successive texts written in the course of four decades, Popper added some nuances to the definition of the concept.
Broadly speaking, the idea of a bold hypothesis refers to a new, testable theoretical proposition (or theoretical claim) made within a subject area which, if it is true, would be able to explain and predict much more about the subject than was previously possible. It would significantly alter existing scientific understandings about the subject, in a positive way. The new idea would be very useful for scientific research about the subject. This contrasts with other types of scientific claims which, if they turn out to be true, make little difference to what scientists already know about the subject. The contrary of a bold hypothesis is an "ad hoc hypothesis" which protects a well-established scientific theory from refutation, by means of some convenient modification of this theory which has no new testable consequences.Alan F. Chalmers, What is this thing called science? (4th edition) Maidenhead, UK: Open University Press, 2013, p. 70. Alan F. Chalmers summarizes that, in Popper's philosophy of science,
The idea of a "bold hypothesis" is nowadays widely used in the philosophy of science and in the philosophy of knowledge. It is also used in the social, historical and physical sciences, to refer to "significant new ways to understand something". Bold hypotheses are often understood to be an important ingredient of major scientific breakthroughs, scientific progress and scientific innovation. Bold hypotheses can help to move scientific understanding forward, by creating new concepts and formulations to understand something and testing them out. That is also why bold hypotheses have an important place in Karl Popper's theory of scientific progress.
However, when bold hypotheses are formulated, they don't necessarily meet all the scientific criteria required to test them yet. And not all of them survive the scientific tests that are eventually done. Some hypotheses stand up to the evidence, while others do not. Testing a bold hypothesis conclusively can be difficult. It can sometimes take a long time, before a bold hypothesis is definitely accepted or rejected. There are also bold hypotheses which, although they were ultimately rejected after relevant tests, nevertheless triggered a lot of new and valuable ideas about what might (or might not) be the case. They might help to rule out a lot of possibilities, and focus the research with new leads. Sometimes it is helpful to know what cannot be true, in order to find out what is true.For example, a bold hypothesis might be: "what if something which everyone always believed to be true, is in reality not true?". Brief explanation Karl Popper's argument was that scientific progress occurs through formulating bold hypotheses, and trying to refute (disprove or falsify) them. Popper believed that:
He makes this point more specific in a 1953 lecture, where he argues that, if we aim to explain the world, then:
In his 1974 reply to critics, Popper described the concept of bold hypotheses some more:
He added that the characteristic of bold conjectures, in combination with tests and refutations, is what distinguishes science from non-science, from prescientific myths and from metaphysics:
This passage makes it very clear, that the idea of bold hypotheses also has a central role in Popper's solution for the demarcation problem, i.e. the problem of how scientific knowledge and scientific practice can be distinguished from non-scientific beliefs and practices.
Defining the boldness of hypotheses
A "bold" hypothesis is a new scientific idea which, if it was true, would be able to predict and/or explain a lot, or a lot more, about the subject being theorized about. The "boldness" of a scientific hypothesis can be evaluated with the following criteria: Testability: the degree to which the hypothesis can be comprehensively tested (or, the extent to which the hypothesis can be definitely proved right or wrong, with available scientific methods).Karl Popper, The Logic of Scientific Discovery. London: Routledge, 1992, chapter 6. Risk: the likelihood that the hypothesis which is to be tested will turn out to be wrong when tests are carried out.Karl Popper, Objective Knowledge: an evolutionary approach. Oxford: Oxford University Press, 1972, p. 53. Novelty: the extent to which the hypothesis represents a genuinely new departure from what scientists already know from established scientific ideas (or compared to background knowledge).Alan F. Chalmers, What is this thing called science? (4th edition) Maidenhead, UK: Open University Press, 2013, p. 76. Heuristic power: whether the hypothesis stimulates new, innovative research that goes beyond the conventional or established approaches. Predictive power: whether the hypothesis enables genuinely new and better predictions involving new phenomena (or forecasts which are not trivial), if the hypothesis is true. Scientific impact: whether the hypothesis would have a major impact on existing scientific thinking, if it is true (or, how much it could change scientific thinking or scientific theory)."If we call the complex of scientific theories generally accepted and well established at some stage in the history of science the background knowledge of the time, then we can say that a conjecture will be bold if its claims are unlikely in the light of the background knowledge of the time." – Alan F. Chalmers, "What is this thing called science?" (4th edition) Maidenhead, UK: Open University Press, 2013, p. 76. Explanatory power/depth: the scope or reach of the hypothesis – the size, number and variety of phenomena which it would explain, if it is true.
Testing hypotheses Once a bold hypothesis has been mooted, Popper argues, scientists try to investigate and test how well the bold hypothesis can stand up to the known evidence. They try to find counter-arguments that would refute or falsify the bold hypothesis. In this process of testing and criticism, new scientific knowledge is generated. Even if the bold hypothesis turns out to have been wrong, testing it may well generate useful knowledge about what can and cannot be the case. Often it stimulates new research.
Inversely, if a hypothesis lacks the quality of boldness, then it would make very little difference to what scientists already know. It is not "a big deal", i.e. it is not very significant for the knowledge which exists already. It contributes little to advancing scientific progress, because it does not expand or add anything much to scientific understanding.
Tentative knowledge According to Popper,
In one of his later writings, Objective Knowledge (1972), Popper argued that:
This interpretation was criticized by Adolf Grünbaum.Adolf Grünbaum, "Is the Method of Bold Conjectures and Attempted Refutations Justifiably the Method of Science?". The British Journal for the Philosophy of Science, Vol. 27, No. 2 (Jun., 1976), pp. 105–136.
Main criticisms Popper's idea of the role of bold hypotheses in scientific progress has attracted four main kinds of criticisms. These concern (1) the meaning of "boldness", (2) the issue of testability, (3) the issue of falsifiability, and (4) the realities of normal science. Boldness issue The idea of a bold hypothesis is itself somewhat fuzzy, because exactly "how bold is bold" in scientific boldness? Is boldness always a good thing in scientific hypothesizing?Timothy Cleveland and Paul T. Sagal, "Bold hypotheses: the bolder the better?", in: Ratio, Vol. 2, No. 2, December 1989, pp. 109–121, at p. 110. How we assess the degree of boldness may just be a subjective matter (it may depends on how you care to look at it). Some new ideas, although they are rather modest in themselves, can make a very large difference to the advancement of scientific research. At a more concrete level, the degree of "boldness" could refer (i) to the content of the hypothesis (considered relative to other possible hypotheses), or (ii) to the manner or context in which the hypothesis is presented, (iii) to its expected importance for research, or (iv) to the attitude involved. The degree of boldness could be downplayed for the sake of credibility or compatibility with established ideas, or it could be exaggerated to create the image of a "big scientific breakthrough" (to obtain research funding). The timing of the evaluation of a hypothesis is also important: a hypothesis which does not seem "bold" right now, could be judged "very bold" later, when historians know much more about the full impact it turned out to have (and vice versa). Alan F. Chalmers comments that “What rates as a bold conjecture at one stage in the history of science may no longer be bold at some later stage.”Alan F. Chalmers, What is this thing called science? (4th edition) Maidenhead, UK: Open University Press, 2013, p. 75. Scientists may not know in advance exactly how bold a hypothesis actually is, even if they know it is a very new idea. The degree of "boldness" of a new hypothesis may be admitted only in the light of a rational reconstruction of the history of a science. A truth discovered at the margins of society may later go mainstream, and eventually become accepted by most people, although that was not foreseen when the discovery was made. Inversely, a hypothesis which seemed really bold at the time it was first mooted, can later appear to have been "not such a big deal". A fashionable scientist with a lot of sponsors might be presented as doing new and bold things, while in reality it was more hype than substance. So it remains somewhat fuzzy what kinds of criteria or relativizations we might use, to credit new hypotheses as "bold" or not. There could be an element of propaganda in the "boldness" attributed to a scientific idea.
Testability issue In Popper's philosophy of science, scientific statements are always provisional, they have limits of application, and they could always be wrong. If a statement cannot even in principle be proved wrong, it cannot be a scientific statement. Thus, in Popper's eyes, the falsifiability criterion clearly demarcates "science" from "non-science".
This Popperian idea has been very controversial, however. The reason is that it can be quite difficult to test scientifically how true a particular idea is. Even if scientists do want to test an idea, they may not know yet how exactly to test it conclusively. Yet, scientists also don't want to abandon a hunch that seems useful, simply because they don't know how to verify it yet. This point is especially important for "bold" new hypotheses, because the very "boldness" of the new hypothesis could mean that it would take a lot of work before adequate tests could be designed and carried out.