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Newton’s ‘Phenomena’ continued…

Kirsten Walsh writes…

In my last post, I considered the phenomena in book 3 of Newton’s Principia.  Newton’s decision to label these propositions ‘phenomena’ is puzzling, as they do not seem to fit any standard definition of the term.  In this post, I’ll consider Bogen & Woodward’s (1988) distinction between data, phenomena and theories, and suggest that it sheds light both on Newton’s use of ‘phenomena’ and on the connection between his methodology in Opticks and Principia.

Bogen & Woodward (B&W) have argued for a 3-level picture of scientific theories in which:

  1. ‘Data’ are records produced by measurement and experiment that serve as evidence or features of phenomena.  E.g. bubble chamber photographs, and patterns of discharge in electronic particle detectors.
  2. ‘Phenomena’ are features of the world that in principle could recur under different contexts or conditions.  E.g. weak neutral currents, and the decay of a proton.
  3. ‘Theories’ are explanations of the phenomena.

B&W argue that theories explain phenomena, but not data.  Data usually reflect many causal influences besides the explanatory target, while phenomena typically reflect single, or small, manageable numbers of causal influences.  For example, General Relativity explains the phenomenon of bending light, but doesn’t explain the workings of the cameras, optical telescopes, etc. that causally influence the data.

Can we characterise Newton’s phenomena in terms of these three levels of theory?  Let’s consider phenomenon 1:

    “The circumjovial planets, by radii drawn to the centre of Jupiter, describe areas proportional to the times, and their periodic times – the fixed stars being at rest – are as the 3/2 powers of their distances from that centre.”

In his discussion of this phenomenon Newton explained, “This is established from astronomical observations.”  He provided the following table:

These observations are not data in the ‘pure’ sense that B&W discuss.  Rather, they are generalisations: average distances and calculated periods of orbit.  Moreover, the bottom row contains the average distances calculated from the period and the Harmonic rule (that the periods are as the 3/2 power of the semidiameters of their orbits).  These calculations illustrate the ‘fit’ between the expected distance and the observed distance.  Nevertheless, they provide a good example of how we might get from a set of data to a phenomenon.  So perhaps we can think of them as ‘data’ in a methodological sense: they are records from which phenomenal patterns can be drawn.

I have another reason for considering these calculations ‘data’ in B&W’s sense of the term.  In his discussion of phenomenon 1, Newton indicated that these calculations reflect a number of causal influences besides gravity.  For instance, he explained that the length of the telescope affected the measurement of Jupiter’s diameter, because

    “the light of Jupiter is somewhat dilated by its nonuniform refrangibility, and this dilation has a smaller ratio to the diameter of Jupiter in longer and more perfect telescopes than in shorter and less perfect ones.”

This is a nice illustration of B&W’s notion of the shift from data to phenomena.  By attending to his theory about telescopes, Newton was able to manipulate the data to control for distortion.

Now let’s consider the role of phenomenon 1 in Principia.  Phenomenon 1 is employed (in conjunction with proposition 2 or 3, book 1, and corollary 6 to proposition 4, book 1) to support proposition 1, theorem 1, book 3:

    “The forces by which the circumjovial planets are continually drawn away from rectilinear motions and are maintained in their respective orbits are directed to the centre of Jupiter and are inversely as the squares of the distances of their places from that centre.”

This theorem doesn’t contain any information about the sizes or positions of the satellites of Jupiter, or about the workings of telescopes.  So, while it explains the phenomenon, it gives no direct explanation of the data.  This suggests that, in the Principia, data and phenomena are methodologically distinct.

B&W’s distinction between ‘data’ and ‘phenomena’ reveals two methodological features of Newton’s phenomena:

Firstly, Newton’s phenomena are explananda, but not appearances.  Traditionally, ‘phenomenon’ seems to have been synonymous with both ‘appearance’ and ‘explanandum’.  For example, the ancient Greeks were concerned to construct a system that explained and preserved the motions of the celestial bodies as they appeared to terrestrial observers.  2000 years later, Galileo and Cardinal Bellarmine argued over which system, heliocentric or geocentric, provided a better fit and explanation of these appearances.  This suggests that, traditionally, there was no real difference between phenomena and data.  For Newton, however, these come apart.  The six phenomena of Principia describe the motions of celestial bodies, but not as they appear to terrestrial observers.  In this sense, they are not appearances, but they do require an explanation.

Secondly, this reveals a continuity in Newton’s methodology.  The point of Newton’s articulation of ‘phenomena’ in Principia is the same as his experiments in Opticks.  Both identify and isolate a pattern or regularity.  In the Opticks, Newton isolated his explanatory targets by making observations under controlled, experimental conditions.  In Principia, Newton isolated his explanatory targets mathematically: from astronomical data, he calculated the motions of bodies with respect to a central focus.  Viewed in this way, Newton’s phenomena and experiments are different ways of achieving the same thing: isolating explananda.

These considerations are admittedly speculative, so I’m keen to hear what our readers think.  Does this look like a good way of characterising Newton’s phenomena?

Newton’s ‘Phenomena’

Kirsten Walsh writes…

On this blog, I have often argued that Newton’s Principia should be characterised as a work of experimental philosophy (for example, here, here and here).  To support this argument, I have tended to emphasise similarities between Newton’s work in optics and mechanics.  Recently, however, I have noted that some aspects of Newton’s methodology varied according to context.  For example, in the Opticks, Newton employed ‘experiments’, but in the Principia, he employed ‘phenomena’.  Given that experimental philosophy emphasises observation- and experiment-based knowledge, it is important for my project that I understand Newton’s use of phenomena, and its relationship to observation.  In this post, I’ll discuss the phenomena in Principia, and in my next, I’ll discuss the relationship between phenomena and experiments in more detail.

Firstly, let’s consider the origin of the phenomena of Principia.  In the first edition of Principia (1687), book 3 contained nine hypotheses.  But in the second edition (1713), Newton re-structured book 3 so that it contained only two hypotheses.  Five of the old hypotheses were re-labelled ‘phenomena’, and he added one more (phenomenon 2), to bring the total to six:

Phenomenon 1: The circumjovial planets, by radii drawn to the centre of Jupiter, describe areas proportional to the times, and their periodic times – the fixed stars being at rest – are as the 3/2 powers of their distances from that centre.

Phenomenon 2: The circumsaturnian planets, by radii drawn to the centre of Saturn, describe areas proportional to the times, and their periodic times – the fixed stars being at rest – are as the 3/2 powers of their distances from that centre.

Phenomenon 3: The orbits of the five primary planets – Mercury, Venus, Mars, Jupiter, and Saturn – encircle the sun.

Phenomenon 4: The periodic times of the five primary planets and of either the sun about the earth or the earth about the sun – the fixed stars being at rest – are as the 3/2 powers of their mean distances from the sun.

Phenomenon 5: The primary planets, by radii drawn to the earth, describe areas in no way proportional to the times but, by radii drawn to the sun, traverse areas proportional to the times.

Phenomenon 6: The moon, by a radius drawn to the centre of the earth, describes areas proportional to the times.

There are several things to notice about these phenomena.  Firstly, they are distinct from data, in that they describe general patterns of motion, rather than measurements of the positions of planetary bodies at particular times.  So, while the phenomena are detected and supported by astronomical observations, they are not observed or perceived directly.

Secondly, they are distinct from noumena (or the nature or essence of things), in that they are facts inferred from the observable, measurable properties of the world.  They describe the motions, sizes and locations of bodies, but not the substance or causes of these properties of bodies.

Thirdly, they describe relative motions of bodies.  That is, in each case, the orbit is described around a fixed point.  For example, phenomenon 1 describes the motions of the satellites of Jupiter around Jupiter, which is taken as a stationary body for the purposes of this proposition.  In phenomena 4 and 5, the motion of Jupiter is described around the sun, which is taken as stationary.

Fourthly, these phenomena do not prioritise the observer.  Rather, each motion is described from the ideal standpoint of the centre of the relevant system: the satellites of Jupiter and Saturn are described from the standpoints of Jupiter and Saturn respectively, the primary planets are described from the standpoint of the sun, and the moon is described from the standpoint of the Earth.  And because Newton doesn’t prioritise the observer, effects such the phases and retrograde motions of the planets are not phenomena but only evidence of phenomena.

The re-labelling of these propositions as ‘phenomena’ is somewhat puzzling.  The term ‘phenomenon’ has a variety of uses, such as:*

  1. A particular (kind of) fact, occurrence, or change, which is perceived or observed, the cause or explanation of which is in question;
  2. An immediate object of sensation or perception (often as distinguished from a real thing or substance); or
  3. An exceptional or unaccountable thing, fact or occurrence.

But, as we’ve seen, Newton’s ‘phenomena’ don’t properly fit any of these definitions.  Can any reader shed light on what Newton really meant by the term?


* Definitions (a) and (c) feature in both C18th and C21st dictionaries, but in the C21st, definition (b) has become more prominent, particularly in philosophy.


UPDATE: I have written a follow-up post.