Tag Archives: Hypothesis

Listen and learn: the language of science and scepticism

The Conversation

Peter Ellerton, The University of Queensland

As scientists, one of our responsibilities should be to promote clarity. A lot of problems are caused by an incorrect or incomplete understanding of terms we regularly, and even lovingly, use.

When I use the word “evidence”, what I think I mean is a function of many things, not least my education in science and philosophy.

It’s also the product of many discussions with people about science, superstition, psychology, pseudoscience and subjectivity.

These discussions have added nuance to my understanding of the nature of evidence. They’ve also alerted me to the fact this nature changes in certain circumstances and through certain worldviews. In other words, what I intend to say is sometimes heard as something else entirely.

This type of miscommunication can be bad enough when dealing with someone who isn’t using the terms in a scientific way, but it’s particularly frustrating when it happens when talking to teachers and communicators of science.

I’d like to take a shot, then, at defining some key terms in the name of clarity.

P Shanks

Scientific law

People might think scientific law is about the highest sort of truth you can get; they might think something “proven” scientifically has the status of certainty, which is to say it’s always true: nature will always behave so as to be in accord with this law.

While in some way accurate, that interpretation is fundamentally flawed. It conflates (or worse, ignores) important concepts and creates a brittleness in the public conception of science that erodes confidence and trust.

First and foremost, laws in science are seldom proven: they are demonstrated, and they are demonstrated because they are demonstrable, which is to say they are descriptive.

Newton’s inverse square law of gravity outlines how the force of gravity between two massive objects varies with distance. Basically, if you double the distance, the force is reduced by a factor of four. Triple it and the force reduces by a factor of nine, and so on.

The same relationship with distance holds for the intensity of omnidirectional radiation, as shown below. What’s significant about a law like this is that while it describes the effect it does not really explain it.


Newton himself was famously silent on the question of what gravity was and why it would behave this way. To get an explanation of what gravity is, we needed Einstein. And we needed a theory.

Modelling reality

General relativity explains the phenomena associated with gravity by postulating that the presence of mass warps, and hence affects movement through, space-time. This theory – or model – of how the universe works, when “run” through the process of mathematical calculation, produces outcomes that correspond to possible states of the world.

These states are checked against reality to test their veracity. The more times the model produces results that agree with observation, the more confidence we have in the model as an accurate representation of how the world works.

The example above shows nicely the difference between a model and a law: the former is a representation of reality, the latter a descriptive account.

It’s worth noting, of course, that “model” can be both a noun and a verb (and sometimes both at once). We can build a model of the solar system, or we can model weather on a computer. Either way, the terminology holds.

To put this another way, a law describes what happens and to what degree, but if we want to find out why it happens we need a theory – a model that represents reality.

A model can give us a more satisfying insight into the possible mechanisms of the universe – it’s an analogy (for rarely is it completely accurate) that betters our comprehension, as analogies are designed to do.

Both theories and laws have predictive power and are subject to being refining, falsified or confirmed; although in the case of laws refining is best done in the light of theoretical change (i.e. explaining the law by the theory/model).

Observing the law

We generalise to laws through observation, and support our generalisations with theoretical understanding. But it can be very tricky to determine that something is true in all cases (we can’t test the potential law in all possible places and at all possible times) or just happens to be true every time we check.

When stating something is universally true (even if parameters need to be defined), we must be very careful to determine whether we mean it’s true because it must be that way, or simply because it happens to be that way.

It may be a necessary condition of the universe that all like charges repel each other. But what about a generalisation such as “all posters are held up by drawing pins”?

The posters in my room and all those in my building are held up by drawing pins, but this hardly seems a necessary condition of posters: surely something else would do the job just as well. These are extreme examples, but many “laws” of nature may not be necessary laws – which seems to suggest they really shouldn’t be called laws in the first place.

Calling something a law certainly does not mean it is unchallengeable.

Laws do not develop from theories. To put it another way, theories do not become laws. I have thrown out science textbooks from several schools because they outline an unrealistic progression: from hypothesis to theory to law.

These three concepts are different creatures, and one does not morph into the other. One of the most significant misunderstandings in science exists because of this type of thinking.


In as much as science can make us sure of anything, we are sure evolution occurred in the manner generally accepted by evolutionary biologists; it is a fact about the world.

Darwin, as is generally known, developed a theory – a model – to explain evolution. This model is natural selection. It’s unfortunate that the lovely phrase “the theory of evolution by natural selection” has been truncated into the misleading, inaccurate, confusing and very wrong phrase “the theory of evolution” – including on this very website.

The “theory of evolution” is wrong for two reasons (when scientists use it they know of what they speak, but this is not my point). First, evolution is not the model – natural selection is. So we immediately conflate two very different ideas – that of evolution and the model of natural selection.

When added to the mistaken belief that theories become laws, adherents of young earth creationism (for there are really no other serious evolution opposers) can claim evolution as a tentative conclusion, akin to vague, hand-waving notions, that culminated in Ronald Reagan’s famous dismissal of evolution as “only a theory”.

The consequences for both the teaching of evolution and the credibility of science are enormous. And yet I have never seen a defender of science articulate this misunderstanding.



Just as a theory is a model, and law is a generalisation, a hypothesis is a statement about the world that could be true or false.

Moreover, the statement must be testable, which means it must be falsifiable, or inherently disprovable.

Phrased like this, hypotheses seem to have more in common with laws than they do with theories, considering that Newton could easily have hypothesised the inverse square law of gravity without going through any theoretical modelling of gravity.

But, of course, the creative act of devising a model of the universe, or a part of it, is to hypothesise that the world is really like that, and the hypothesis becomes that the model is an accurate representation.

Hypotheses, then, are ways of talking about building theories and laws, but not in the common way of theories being intermediate between hypotheses and laws.

While hypotheses can stand alone or inform both theories and laws, the interplay in practice between various hypotheses, theories and laws is web-like and complex and exists at nearly every level of operation from the experiment of the day to the paradigm of the century.

The idea of a hypothesis-to-theory-to-law progression is seriously flawed, and this needs to be articulated as the root cause of much misunderstanding.


“Prove” comes from the Latin probare, meaning “to test”. It’s also the origin of the word “probe”.

An older term – “proving ground” – for a testing area or trial shows we have not entirely lost that interpretation. But in the everyday use of the term, “proof” has come to indicate certitude.


What remains poorly understood is that “proof”, as such, is a deductive creature that really does not sit comfortably in science (at least not in an affirming sense). In mathematics a proof conveys that, within the bounds of the axioms in use, there is a truth to be discovered or a certainty to be expressed.

For its theoretical claims, and indeed for its laws, inductive science can only boast confirming instances.

Headlines that (routinely) claim “Einstein proved right“ would, we know from his own words, make the great man turn in his grave.

He often spoke of the exquisite sensitivity of his theories to falsification, saying that it would not matter how many times experiment agreed with him, it had only to disagree once to prove him wrong (granted, of course, the validity of the experiment, as recent neutrino-based dramas have shown).

The simple fact that we can never test his theories under all conditions in all places at all times creates conclusions that are tentative, even though the level of confidence may be very high.

We may “prove” facts about the world, such as Earth being more or less spherical, but this does not extend to our laws and theories to the extent we might like to think.

So proof works best in science to falsify, not to affirm, though this is the opposite of common belief.

If we are clear on the above, we have a better appreciation of what makes an idea scientific, as opposed to pseudo-scientific.

We know that the best scientific hypotheses and theories are those with great explanatory power and high sensitivity to falsification, and that these are often the results of highly creative thinking, as are the experimental attempts to confirm or falsify them.

This is a very beautiful idea, but one that can’t be appreciated unless you know science does not spend its time stamping into place dry facts about the world, but grows as a vigorous and exhilarating human enterprise showcasing the best of collective human achievement.

Clarifying these ideas will, I hold, go a very long way indeed into increasing people’s understanding of science and their confidence in scientific findings.

The ConversationPeter Ellerton, Lecturer in Critical Thinking, The University of Queensland

This article was originally published on The Conversation. (Reblogged by permission). Read the original article.



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Theories of Generation

by Tim Harding

Does Darwin successfully combine the two explanatory paradigms of inheritance, and growth and development?  If he does, is this a problem for Kuhn’s view of scientific change?

In this essay, I intend to show how Darwin’s hypothesis of pangenesis created a synthesis between the then competing paradigms relating to inheritance and development.  I think that this synthesis does present a problem for Kuhn’s view of scientific change, and that Popper’s account may actually be more relevant than Kuhn’s to certain aspects of this particular case study.

Before discussing Darwin’s theories, it is necessary to briefly describe the two paradigms that attempted to explain inheritance (how characteristics are transferred to offspring from parents), and growth and development (how the growth of offspring is organised).  These two paradigms were characterised by competing Hippocratic theories and preformationist theories (Verdnik, 1998:126, 157).

Before the advent of genetics, Hippocratic theories attempted to explain inheritance in terms of a blending of fluids extracted from all parts of both male and female bodies during intercourse.  It was thought that the characteristics of the offspring are determined by the relative amounts and strength of fluids from each part of the body of each parent (Verdnik, 1998:126).

On the other hand, preformationist theories held that the new mammalian offspring is already preformed in miniature, either within the egg of its mother (ovist preformationism) or in the semen of its father (animalculist preformationism).  Both of these types of theories incorporated emboîtment (encasement), which was the thesis that God created all future organisms in miniature, and that reproduction was just the growth and development of these miniatures (Verdnik, 1998:128).

Hippocratic theories were very good at explaining inheritance but very bad at explaining growth and development; whilst preformationist theories were the opposite – very good at explaining growth and development but very bad at explaining inheritance (Verdnik, 1998:126).  To give some examples, Hippocratic theories were unable to adequately explain phenomena such as the regeneration of freshwater polyps as observed by Trembley (Trembley 1744: 148); while preformationist theories were unable to adequately explain how the mating of a mare with a donkey produces a mule (Maupertuis 1745: 172).

Darwin came to his hypothesis of pangenesis, from a different direction – to fill a gap left in his theory of evolution, as published in his 1859 book The Origin of Species (Darwin, 1859).  Natural selection provided a mechanism for variation and eventual speciation, but it did not explain the inheritance of variation.  Without some way to explain the inheritance of characteristics acted on by natural selection, his theory would be incomplete.  Darwin’s breeding experiments on domestic animals (mainly pigeons) in the 1850s and 60s[1] were part of his attempts to complete his evolution theory (Bartley, 1992: 308).  He was attempting in these experiments to show just how quickly varying characteristics can be amplified by domestic breeding, and therefore how natural selection can operate (Verdnik, 1998:156).


The Laws Of Inheritance & Pangenesis

Darwin called his explanation of inheritance ‘the hypothesis of Pangenesis’, which he published in Part II of Chapter XXVII of Variation Under Domestication (Darwin, 1875: 374-404).  However, he provides a more succinct description of this hypothesis in an earlier unpublished manuscript on pangenesis sent to Huxley in 1865:

‘Furthermore, I am led to believe from analogies immediately to be given that protoplasm or formative matter which is throughout the whole organisation, is generated by each different tissue and cell or aggregate of similar cells; – that as each tissue or cell becomes developed, a superabundant atom or gemmule as may be called of the formative matter is thrown off; – that these almost infinitely numerous and infinitely minute gemmules unite together in due proportion to form the true germ; – that they have the power of self-increase or propagation; and that they here run through the same course of development, as that which the true germ, of which they are to constitute elements, has to run through, before they can be developed into their parent tissues or cells. This may be called the hypothesis of Pangenesis’ (Olby 1963: 259).

Darwin further proposed that his hypothesis would not only account for inheritance, but also for development:

‘The development of each being, including all the forms of metamorphosis and metagenesis, as well as the so-called growth of the higher animals, in which structure changes, though not in a striking manner, depends on the presence of gemmules thrown off at each period of life, and on their development, at a corresponding period, in union with the preceding cells ‘ (Darwin, 1875: 403-404).

Through these mechanisms, Darwin proposed that inheritance and development were tied together – not only in the generation of offspring and early stages of embryonic life, but throughout the life of the organism (Bartley, 1992: 310).  By giving ‘gemmules’ the power to be modified throughout the life of an organism and then be transferred to the next generation, he argued that inheritance should be viewed as a form of growth (Bartley, 1992: 331).

By means of this single hypothesis, Darwin not only filled a gap in his theory of evolution, but whether he meant to or not, he created a synthesis between the then competing paradigms relating to inheritance and development.

The preceding discussion raises some significant philosophical issues.  How does it compare with Kuhn’s view of scientific change; and in particular, is Darwin’s paradigm synthesis a problem for the Kuhnian account?

According to Kuhn, ‘normal science’ operates within scientific paradigms[2] that not only determine which scientific theories are acceptable, but define scientific communities and even the areas of research undertaken (Kuhn, 1962: 10-11).

Kuhn says that in so far as he is engaged in normal science, the researcher is a solver of puzzles, not a tester of paradigms.

Paradigm-testing occurs only after persistent failure to solve a noteworthy puzzle has given rise to crisis.  And even then it occurs only after the sense of crisis has evoked an alternative candidate for a paradigm.  In the sciences, the testing situation never consists, as puzzle-solving does, simply in the comparison of a single paradigm with nature.  Instead, testing occurs as part of the competition between two rival paradigms for the allegiance of the scientific community (Kuhn, 1962: 144-145).

Kuhn notes that the schools guided by different paradigms are ‘always slightly at cross-purposes’ and that they ‘fail to make complete contact with each other’s viewpoints’, a phenomenon which describes as ‘incommensurability’ (Kuhn, 1962: 112, 148-150).  The failure of the adherents of Hippocratic and preformationist theories to focus on the strengths and weaknesses of both paradigms, and to engage in the testing of both paradigms, tends to support Kuhn’s notion of incommensurability.  No crisis occurred within each paradigm, because their adherents failed to focus on how well each paradigm explained both inheritance and development.

The formulation of Darwin’s hypothesis does not accord with the Kuhnian account.  He did not conduct his breeding experiments on domestic animals to test either the Hippocratic or the preformationist paradigms, but to fill a gap in his quite separate theory of evolution.  Similarly, his synthesis between the then competing paradigms relating to inheritance and development, was not done to resolve any crisis in these paradigms, but for a separate purpose.  A further problem for the Kuhnian account is that the scientific method Darwin used to formulate his hypothesis was essentially inductive, although this may also be a problem for the Popperian account.

Another aspect of the Kuhnian account is that the failure of experimental results to conform to the prevailing paradigm is seen as an ‘anomaly’, rather than as an instance of Popperian falsification.[3]  If scientists are confronted by anomalies, they will often devise ad hoc modifications in order to eliminate any apparent conflict.  Anomalies do not cause the abandonment of paradigm or theory unless and until the level of anomalies builds to a crisis where the prevailing paradigm or theory is replaced by a new one (Kuhn, 1962: 77-78).

After reading Variation Under Domestication, Francis Galton (a cousin of Darwin’s) arranged for a series of experiments to be conducted on rabbits initially housed in the Zoological Gardens of London and later at his Kensington home.  His intention was to demonstrate the transmission of ‘gemmules’ to succeeding generations via blood injected from one rabbit to another, using coat colour as a marker.  Galton ultimately found that not a single instance of induced variation of coat colour occurred in a total of 88 offspring from blood transfused parents, and in 1871 published his results in Nature (Brown, 2002: 290-291).  Although intended to verify Darwin’s hypothesis, it seems that Galton’s experiments provided a Popperian falsification instance.  Whether the purpose was verification or falsification, Galton’s experiments do not accord with Kuhn’s account of paradigm-testing discussed above.

In later editions of Variation Under Domestication, Darwin admitted in a footnote that he would have expected to find ‘gemmules’ in the blood, although their presence was not absolutely necessary to his hypothesis (Brown, 2002: 292).  I find Darwin’s response unconvincing, as he provides no alternative explanation as to how the ‘gemmules’ are transmitted from the parents’ somatic cells to the germ cells.  He made no real attempt to modify his hypothesis in response to Galton’s falsification of it.

I therefore conclude that whilst the Hippocratic and preformationist paradigms accord with Kuhn’s notion of incommensurability, Darwin’s synthesis of these two paradigms does not accord with the Kuhnian account of science.  Furthermore, Darwin’s failure to make an ad hoc modification to his hypothesis after the discovery by Galton of an anomaly, as would be expected under the Kuhnian account, supports a Popperian rather than a Kuhnian account of the scientific process in this case.


Bartley, M.M. (1992) Darwin and Domestication: Studies on Inheritance. Journal of the History of Biology, Vol 25, No. 2 pp.307-333.

Browne, J. (2002) Charles Darwin – The Power of Place, Volume II of a Biography. London: Pimlico.

Darwin, C. (1859) The Origin Of Species By Means Of Natural Selection, Or The Preservation Of Favoured Races In The Struggle For Life. 6th ed. 1873. London: John Murray.

Darwin, C. (1875) The Variation of Animals and Plants Under Domestication, Vol II London: John Murray.

Kuhn, T.S. (1962) The Structure of Scientific Revolutions 3rd ed. Chicago: University of Chicago Press.

Maupertuis, P. (1745) The Earthly Venus in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Olby, R.C. (1963) Charles Darwin’s Manuscript of Pangenesis, British Journal for Philosophy of Science 1: 251-263.

Popper, K. (1959) The Logic of Scientific Discovery. rev. ed. 1968, in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Trembley, A. (1744) Memoires pour server a l’histoire d’un genre de polypes d’eau douce in Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.

Verdnik, D. (ed.) (1998) Thinking about Science. Study Guide and Readings. Churchill: Monash Distance Education Centre.


[1] Darwin reported the results of these experiments in both The Origin Of Species and The Variation of Animals and Plants Under Domestication.

[2] Kuhn does not provide a concise definition of a scientific paradigm, but the Merriam-Webster Online dictionary defines it as ‘a philosophical and theoretical framework of a scientific school or discipline within which theories, laws, and generalizations and the experiments performed in support of them are formulated; broadly: a philosophical or theoretical framework of any kind.’

[3] Popper proposes falsifiability as the criterion of demarcation between scientific and non-scientific statements.  In other words, a theory is scientific if it is falsifiable (Verdnik, 1998:76).

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