Published online 2 March 2009 | Nature | doi:10.1038/news.2009.128

Column: Muse

What does it all mean?

Science depends on clear terms and definitions — but the world doesn't always oblige, says Philip Ball.

What's wrong with this statement: 'The acceleration of an object is proportional to the force acting on it.' You might think no one could object to this expression of Newton's second law. But Nobel laureate physicist Frank Wilczek does. This law, he admits, "is the soul of classical mechanics". But he adds that, "like other souls, it is insubstantial"1.

Bertrand Russell went further. In 1925 he called for the abolition of the concept of force in physics, and claimed that if people learned to do without it, this "would alter not only their physical imagination, but probably also their morals and politics"2.

That seems an awfully heavy burden for a word that most scientists will use unquestioningly. It's a disconcerting reminder that even supposedly precise scientific terminology is often much more mutable and ambiguous than we think — which makes it prone to misuse, abuse and confusion3,4. But why should that be so?


Some scientific words are simply misapplied, often because their definition is ignored in favour of something less precise. Can't we just stamp out such transgressions? Not necessarily, for science can't expect to evade the transformations that any language undergoes through changing conventions of usage. When misuse becomes endemic, we must sometimes accept that a word's definition has changed.

Similarly, it is now routine to speak of protein molecules undergoing phase transitions, which they cannot in the strict sense because phase transitions are only defined in systems that can be extrapolated to infinite size. Here, however, the implication is clear, and inventing a new term is arguably unhelpful.

Perhaps word misuse matters less when it simply alters or broadens meaning — the widespread use of 'momentarily' to indicate 'in a moment' is wrong and ugly, but it is scarcely disastrous to tolerate it. Misuse is, however, more problematic when it threatens to traduce logic. 'Fertility', for example, now often connotes birth rate — among demographers as well as in general use — allowing the existence of fertile people who have zero fertility.

Everyday words used in science

In 1911 the geologist John W. Gregory, chairman of the British Association for the Advancement of Science, warned of the dangers of appropriating everyday words into science5. Worms, elements, rocks — all, he suggested, run risks of securing "specious simplicity at the price of subsequent confusion".

Even some scientific terms can be a little fuzzy.

Interestingly, Gregory also worried about the differing uses of 'metal' in chemistry and geology; what would he have said, one wonders, about the later redefinition of the term by astronomers to mean any element heavier than helium. Such Humpty-Dumpty-style assertions that a familiar word can mean whatever one chooses are characteristic of the excesses of postmodern philosophy that scientists often lament.

There are hazards in trying to assign new and precise meanings to old and imprecise terms. For instance, experts in nonlinear dynamics can scarcely complain about misuses of 'chaos' when it already had several perfectly good meanings before they came along.

When scientific words become fashionable, haziness is an exploitable commodity. One begins to suspect that there are few areas of science that cannot be portrayed as involving 'complexity' or 'nanotechnology'. It recently became popular to assert a fractal nature in almost any convoluted shape, until some researchers eventually began to baulk at the term being applied to structures (such as ferns) whose self-similarity barely extends beyond a couple of levels of magnification6.

Teaching the 'wrong' ideas

The reasons for Wilczek's scepticism about force are too subtle to describe here, but they don't leave him calling for its abolition. He points out that it holds meaning because it fits our intuitions — we feel forces and see their effects, even if we don't strictly need them theoretically. In short, the concept of force is easy to work with: it has heuristic value.

Science is full of concepts that lack sharp definition, or even logic, but help us to understand the world. It is possible that one day the notion of a gene may create more confusion than enlightenment7, but at present it doesn't seem feasible to understand heredity or evolution without it.


One could argue that 'wrong' ideas that nonetheless systematize observations are harmful only when they refuse to give way to better ones (pace Aristotelian physics and phlogiston), while teaching science is a matter of finding useful (as opposed to 'true') hierarchies of knowledge that organize natural phenomena.

The world also does not fit into boxes, as the furore over the meaning of 'planet' illustrated8 – a classic example of the tension between word use sanctioned by definition versus convention.

The same applies to 'meteorite'. According to one, perfectly logical, definition of a meteorite, it is not possible for a meteorite ever to strike Earth (because it becomes one only after having done so). Certainly, the common rule of thumb that meteors are extraterrestrial bodies that enter the atmosphere but don't hit the surface, while meteorites do, is not one that planetary scientists will endorse. There is no apparent consensus about what they will endorse, which seems to be a result of trying to define processes on the basis of the objects they involve.

All of this suggests some possible rules of thumb for anyone contemplating a scientific neologism. Don't invent a new word without really good reason (for example, don't use it to patch over ignorance). Don't neglect to check whether one exists already (we don't need both amphiphilic and amphipathic). Don't assume you can put an old word to new use. Make the definition transparent, and think carefully about its boundaries. Oh, and try to make it easy to pronounce — not just in Cambridge but in Tokyo too. 

  • References

    1. Wilczek, F. Physics Today 57, 11–12 (2004).
    2. Russell, B. The ABC of Relativity, 5th edn, p. 135 (Routledge, London, 1997).
    3. Nature 455, 1023–1028 (2008).
    4. Parsons, J. & Wand, Y. Nature 455, 1040–1041 (2008).
    5. Gregory, J. W. Nature 87, 538–541 (1911).
    6. Avnir, D., Biham, O., Lidar, D. & Malcar, O. Science 279, 39–40 (1998).
    7. Pearson, H. Nature 441, 398–401 (2006).
    8. Giles, J. Nature 437, 456–457 (2005).
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