Skip to Navigation Skip to Content Skip to Search Skip to Site Map

Tag Archives: optics

Observation, experiment and intervention in Newton’s Opticks

Kirsten Walsh writes…

In my last post, my analysis of the phenomena in Principia revealed a continuity in Newton’s methodology.  I said:

    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.

In this post, I’ll have a closer look at Newton’s method of isolating explananda in the Opticks.  It looks like Newton made a distinction between experiment and observation: book 1, contained ‘experiments’, but books 2 and 3, contained ‘observations’.  I’ll argue that the distinction in operation here was not the standard one, which turns on level of intervention.

In current philosophy of science, the distinction between experiment and observation concerns the level of intervention involved.  In scientific investigation, intervention has two related functions: isolating a target system, and creating novel scenarios.  On this view, experiment involves intervention on a target system, and manipulation of independent variables.  In contrast, the term ‘observation’ is usually applied to any empirical investigation that does not involve intervention or manipulation.  This distinction is fuzzy at best: usually level of intervention is seen as a continuum, with observation nearer to one end and experiment nearer to the other.

If Newton was working with this sort of distinction, then we should find that the experiments in book 1 involve a higher level of intervention than the observations in books 2 and 3.  That is, in contrast to the experiments in book 1, the observations should involve fewer prisms, lenses, isolated light rays, and artificially created scenarios.  However, this is not what we find.  Instead we find that, in every book of the Opticks, Newton employed instruments to create novel scenarios that allowed him to isolate and identify certain properties of light.  It is difficult to quantify the level of intervention involved, but it seems safe to conclude that Newton’s use of the terms ‘observation’ and ‘experiment’ doesn’t reflect this distinction.

To understand what kind of distinction Newton was making, we need to look at the experiments and observations more closely.  In Opticks book 1, Newton employed a method of ‘proof by experiments’ to support his propositions.  Each experiment was designed to reveal a specific property of light.  Consider for example, proposition 1, part I: Lights which differ in Colour, differ also in Degrees of Refrangibility.  Newton provided two experiments to support this proposition.  These experiments involved the use of prisms, lenses, candles, and red and blue coloured paper.  From these experiments, Newton concluded that blue light refracts to a greater degree than red light, and hence that blue light is more refrangible than red light.

Opticks, part I, figure 12.

In the scholium that followed, Newton pointed out that the red and blue light in these experiments was not strictly homogeneous.  Rather, both colours were, to some extent, heterogeneous mixtures of different colours.  So it’s not the case, when conducting these experiments, that all the blue light was more refrangible than all the red light.  And yet, these experiments demonstrate a general effect.  This highlights the fact that, in book 1, Newton was describing ideal experiments in which the target system had been perfectly isolated.

Book 2 concerned the phenomenon now known as ‘Newton’s Rings’: the coloured rings produced by a thin film of air or water compressed between two glasses.  It had a different structure to book 1: Newton listed twenty-four observations in part I, then compiled the results in part II, explained them in propositions in part III, and described a new set of observations in part IV.  The observations in parts I and IV explored the phenomena of coloured rings in a sequence of increasingly sophisticated experiments.

Consider, for example, the observations in part I.  Observation 1 was relatively simple: Newton pressed together two prisms, and noticed that, at the point where the two prisms touched, there was a transparent spot.  The next couple of observations were variations on that first one: Newton rotated the prisms and noticed that coloured rings became visible when the incident rays hit the prisms at a particular angle.  But Newton steadily progressed, step-by-step, from prisms to convex lenses, and then to bubbles and thin plates of glass.  He varied the amount, colour and angle of the incident light, and the angle of observation.  The result was a detailed, but open ended, survey of the phenomena.

I have argued that Newton’s experiments and observations cannot be differentiated on the basis of intervention, but there are two other differences worth noting.  Firstly, whereas the experiments described in book 1 were ideal experiments, involving perfectly isolated explanatory targets, the observations in books 2 and 3 were not ideal.  Rather, through a complex sequence of observations, as the level of sophistication increased, the explanatory target was increasingly well isolated.  When viewed in this way, the phenomena of Principia seem to have more in common with the experiments of book 1 than the observations of books 2 and 3.

Secondly, the experiments of book 1 were employed to support particular propositions, and so, individually, they were held to be particularly relevant and informative.  In contrast, the observations of books 2 and 3 were only collectively relevant and informative.  Moreover, the sequence of observations was open ended: there were always more variations one could try.

What are we to make of these differences between observation and experiment in the Opticks?  I have previously argued that, while Newton never constructed Baconian natural histories, his work contained other features of the Baconian experimental philosophy, such as experiments, queries and an anti-hypothetical stance.  However, in viewing them as complex, open ended series’ of experiments, I now suggest that the observations of books 2 and 3 look a lot like what Bacon called experientia literata, the method by which natural histories are generated.  I’ll discuss this in my next post, but in the mean time, I’d like to hear what our readers think.

Hypotheses versus Queries in Newton’s Opticks

Kirsten Walsh writes…

A while ago I argued that the queries in Newton’s early optical papers are not hypotheses.  Rather, they are empirical questions that may be resolved by experiment.  In Newton’s Opticks, however, his queries become increasingly speculative – especially the famous ‘Query 31’.  What should we make of this?  Did Newton abandon his early distinction between hypotheses and queries?

In his early optical papers, Newton explains that “the proper Method for inquiring after the properties of things is to deduce them from Experiments”.  Having obtained a theory in this way, one should proceed as follows:

  1. specify queries that suggest experiments that will test the theory; and
  2. carry out those experiments.

He tells us that hypotheses have a role in this procedure.  They may be useful for: (a) suggesting further experiments, as the first step toward specifying queries; and (b) ‘illustrating’ the theory to assist understanding.

The queries in Newton’s Opticks have been much talked about, and often Newton has been accused of slipping hypotheses into his work under the guise of the more-respectable query.  To examine this claim, I looked at the draft manuscripts* of Newton’s Opticks; in particular, “The fourth book concerning the nature of light & ye power of bodies to refract & reflect it” (Add. 3970, 337-8).

The draft begins, as many of the other books of Opticks begin, with a list of observations, followed by numbered propositions.  However, it contains little in the way of argument and virtually no discussion of experimental evidence.  Shapiro points out that this is because this is a draft of an outline or plan of a book; not a draft of the book itself.  The propositions are things that Newton hoped to prove.  For example:

    Prop. 1.  The refracting power of bodies in vacuo is proportional to their specific gravities.
    Prop. 2.  The refracting power of two contiguous bodies is the difference of their refracting powers in vacuo.

The draft contains a section entitled ‘The conclusion’, which contains five ‘hypotheses’.  I am interested in ‘Hypothesis 2’:

    As all the great motions in the world depend upon a certain kind of force (wch in this earth we call gravity) whereby great bodies attract one another at great distances: so all the little motions in ye world depend upon certain kinds of forces whereby minute bodies attract or dispell one another at little distances.
    How the great bodies of ye earth Sun moon & Planets gravitate towards one another what are ye laws of & quantities of their gravitating forces at all distance from them & how all ye motions of those bodies are regulated by those their gravities I shewed in my Mathematical Principles of Philosophy to the satisfaction of my readers: And if Nature be most simple & fully consonant to her self she observes the same method in regulating the motions of smaller bodies wch she doth in regulating those of the greater… The truth of this Hypothesis I assert not because I cannot prove it.  But I think it very probable because a great part of the phaenomena of nature do easily flow from it wch seem otherways inexplicable…

I. Bernard Cohen describes this as “a ‘whale’ of an hypothesis” – and he’s right!  When Newton started writing out this statement, he intended for it to be ‘Proposition 18’.  But at some point, he has scratched out ‘Prop 18’, and re-branded it as ‘Hypoth 2’.  There is no real semantic difference between a proposition and a hypothesis, but, for Newton, there is an epistemic difference.  Propositions are things that he is able to assert as true.  Hypotheses are things that he is unable to assert, because he does not have the evidence.  Newton clearly hoped to assert Proposition 18.  But as he started to explicate it, he must have realised that he couldn’t prove it.  Thus, he re-labelled it as a hypothesis.

When Newton abandoned the fourth book, and restructured the rest of his Opticks, this ‘Hypothesis 2’ appears to have been re-worked to become ‘Query 31’ in Opticks, 2nd edition (1717):

    Have not the small Particles of Bodies certain Powers, Virtues, or Forces, by which they act at a distance, not only upon the Rays of Light for reflecting, refracting, and inflecting them, but also upon one another for producing a great Part of the Phaenomena of Nature?  For it’s well known, that Bodies act one upon another by the Attractions of Gravity, Magnetism, and Electricity; and these Instances shew the Tenor and Course of Nature, and make it not improbable but that there may be more attractive Powers than these.  For Nature is very consonant and conformable to her self…

Here, there is an obvious semantic shift between hypothesis and query: the query is stated as a question.  Some scholars have argued that this is the only difference between hypotheses and queries: in the Opticks, queries are simply Newton’s way of getting around his self-imposed ban on hypotheses.  I claim that there is more to the shift than this.  Newton is using the semantic structure of the query to explore a possible future research program.  The epistemic difference between the query and the hypothesis is similar to the epistemic difference between Popper’s falsifiable and unfalsifiable theories.  The former is testable-in-principle, whereas the latter is not; and testability is a necessary condition of something becoming well-tested.

There is a difference between Newton’s early queries and his later queries: the former are part of the process of justification; but the latter are part of the process of discovery.  In a previous post I noted that:

    While Newton’s [early] method of queries is experimental, it does not appear to be strictly Baconian.  For the Baconian-experimental philosopher, queries serve “to provoke and stimulate further inquiry”.  Thus, for the Baconian-experimental philosopher, queries are part of the process of discovery.  However, for Newton, queries serve to test the theory and to answer criticisms.  Thus, they are part of the process of justification.

The queries in Newton’s later work seem closer to the Baconian tradition that inspired him.

That the themes of Hypothesis 2 and Query 31 appear in Rule 3 of Principia, raises questions about the status of Newton’s ‘Rules of Philosophising’ and how we should interpret the re-branding of ‘hypotheses’ as ‘rules’ in later editions of Principia.  I’d love to hear what you think!


* Recently, Cambridge University put Newton’s papers online, making it possible for those of us who live ‘down under’ to examine copies of many of Newton’s manuscripts!

Explicating Newton’s Natural Philosophical Methodology: Part II

This is the second part of Steffen Ducheyne’s presentation of his new book, The main Business of Natural Philosophy:” Isaac Newton’s Natural-Philosophical Methodology. You can find the first part here.

Steffen Ducheyne writes …

In the Principia (1687), Newton developed a detailed picture of how one may deduce causes from phenomena (for the technical details I refer to Chapters 2 and 3). Newton’s expression ‘deductions from phenomena’ has oftentimes been considered as a rhetorical tool by which he sought to distance himself from his opponents. However, close scrutiny shows, I believe, that Newton’s ‘deductions from phenomena’ have profound methodological significance as well. I do not, however, endorse the view that Newton’s Principia-style methodology was therefore non-hypothetical. Rather, what makes it methodologically interesting is that it encompassed procedures to minimize speculation and inductive risk in the process of theory formation. What is distinctive of Newton’s Principia-style methodology is that he established bi-conditional dependencies between causes and their effects from the laws of motion. In other words, the causes which Newton would later infer in Book III were backed-up and constrained by the laws of motion. Given these dependencies, Newton was able to present his derivations of the centripetal forces acting in our solar system as deductions and, hence, as ‘deductions from phenomena’. I want to emphasize, however, that Newton’s proceeding from phenomena to theory, i.e. his presenting of certain inferences as deductions from phenomena, taken as such is not what makes his method essentially different from hypothetico-deductivism. Rather, proceeding from phenomena to theory is the by-product of what genuinely makes Newton’s method distinctive from hypothetico-deductivism: the establishment of systematic dependencies backed-up by the laws of motion. These systematic dependencies, in other words, mediate between experimental or astronomical results and the very causes which account for these phenomena.

Portrait of Isaac Newton (1702)

Once he had finished the Principia, Newton returned to his optical studies, which would eventually lead to the publication of the Opticks in 1704. Could he now methodize optics according to the highly sophisticated standards which he had developed in the Principia? In my view, the answer is negative. For instance, I have argued that Newton’s argument for the heterogeneity of light rests on an argument of uniformity that cannot be licensed by Newton’s second rule of philosophizing. I have also paid considerable attention to the problem of transduction which Newton encountered in his optical studies. In mechanics, the affected entities, i.e. the explananda – bodies moving along specific trajectories, and their constituent elements, namely, the particles constituting these very bodies – all have a theoretically salient property in common, namely, mass. Because gravity is proportional to mass and because the latter is additive, gravity is likewise additive. This allowed Newton to show that a body’s overall force can be decomposed into the individual forces of each of the bodies constituting that body and vice versa. In optics, by contrast, we do not know – at least not without speculating on the matter – the constituting elements of the explananda. In the Opticks Newton could not establish ‘deductions from phenomena’ because, in contrast to the physico-mechanical theory of the Principia, a mixed science describes a given phenomenon mathematically without an accompanying explanatory story. In other words, in the Opticks the inference of causes could not be constrained by a set of laws which carry information about the proximate causes involved.

By way of outro and also as a teaser, I would like to conclude by devoting some words to the provisionalism that characterized Newton’s later methodological thought. Newton’s provisionalism pervades the third and especially the fourth regula philosophandi, which were added in the second (1713) and third (1726) edition of the Principia, respectively. The provisionalism which Newton envisioned did not apply to the ‘deductions from phenomena’, but rather to propositions ‘rendered general by induction’ – at least evidence from Newton’s manuscripts leads me to believe so. Based on a careful study of Newton’s manuscripts, I have also succeeded in clarifying what Newton understood by qualities which cannot be “intended and remitted” and, on the basis of this, I have concluded that the Cohen-Whitman translation of “intendi et remitti” as “increased and diminished” is incorrect. I could say much more about my book, but I hope that this will suffice to get you interested in reading it.

Hypotheses and Newton’s Rings

Kirsten Walsh writes…

In Ian Lawson’s recent post, he mentioned Hooke’s work on colours in thin films.  In this post, I’ll look at how Newton used his hypotheses on light to build on Hooke’s work in some interesting and important ways.

In his optical work of the early 1670s, while Newton prefers theories to hypotheses, he thinks that hypotheses are acceptable, even useful, for two purposes:

  1. To ‘illustrate’ (i.e. provide an intuitively plausible explanation of) the theory; and
  2. To ‘suggest’ experiments.

However, he insists that hypotheses should always be removed from the final version of the theory.  Recall Newton’s claim from his 1672 paper: “I shall not mingle conjectures with certainties”.

In December 1675, Newton wrote his paper, “An hypothesis explaining the Properties of Light”.  Here, he published his hypotheses on the nature of light for the first time.  To summarise them briefly:

  1. There is an ‘aethereal medium’;
  2. Aether vibrates, carrying sounds, smells and light;
  3. Aether penetrates and passes through the pores of solid substances;
  4. Light is neither the aether itself, nor the vibrations, but a substance that is propagated from ‘lucid’ bodies and travels through the aether;
  5. Light warms the aether and the aether refracts the light; and
  6. The rays (or bodies) of which light consists differ from one another physically.

In this paper, Newton claims that he is only discussing these hypotheses for the purposes of ‘illuminating’ his theory.  Moreover, he does not assert that these hypotheses are true, and emphatically does not use them to support his theory.  For example, when he discusses hypothesis (4), Newton is careful not to push too forcefully for any particular account of light.  He says one might suppose light to be “an aggregate of various peripatetic qualities”, or “unimaginably small and swift” corpuscles of various sizes, or “any other corporeal emanation or impulse or motion of any other medium diffused through the body of the aether”:

    Onely whatever Light be, I would suppose, it consists of Successive rayes differing from one another in contingent circumstances, as bignes, forme or vigour…  And further I would suppose it divers from the vibrations of the aether.

In this paper, there is a notable emphasis on experiment.  For example, when Newton discusses hypothesis (1), he gives an account of a new electrical experiment which seems to support his claim.  And when he discusses hypothesis (3), he discusses the implications for Boyle’s tadpole experiments.  But the most important experiments in this paper are his investigations on the colours that appear between two glass surfaces.

Alan Shapiro notes that Newton began these investigations while he was reading Hooke’s Micrographia.  But his experiments and mathematical descriptions quickly developed into something well beyond the scope of Hooke’s investigations.  Hooke described the colours that appear when two thin sheets of glass are placed one on top of the other.  When he made the thin film of air between the two sheets thicker or thinner by pressing the two sheets together with greater or lesser force, the colours changed.  He observed that different colours appeared at different thicknesses, but he was unable to quantify this observation as he was unable to measure accurately the thickness of the film at any given point.  Newton had the idea of placing a convex lens on top of a flat sheet of glass.  This enabled him to easily calculate the thickness of the film of air, and the colours appeared as a set of concentric coloured circles centred at the point of contact between the two surfaces.  These concentric circles are now known as ‘Newton’s Rings’.

Opticks, Book 2, Figure 3






Next Newton considered his hypotheses.  According to hypothesis (2) the vibrations of the aether vary in size, according to hypothesis (3) aether passes through the pores of solid substances, and according to hypothesis (6) rays of different colours will cause aethereal vibrations of different sizes.  If these hypotheses were correct, he argued, then light of a particular colour would be reflected either when the length of the vibration, or some multiple of the length of the vibration, matched the thickness of the film, and transmitted otherwise.  So he predicted that:

    if the Glasses in this posture be looked upon, there ought to appear at A [the centre], the contact of the Glasses, a black spott, & about that many concentric circles of light & darknesse, the squares of whose semidiameters are to sense in arithmetical progression.

Newton’s “Hypothesis” paper provides a good example of his method of hypotheses.  He remains carefully detached from his own hypothesis, using it only to ‘illustrate’ his theory and to suggest further experiments.  Newton was also careful to keep his hypotheses well separate from his theory; the paper ends with a series of ‘Observations’ that contain no reference to his hypotheses at all!

Hooke’s Knowledge of Optics

This is a guest post by Ian Lawson.

Robert Hooke knew how light worked. He worked with the stuff day in day out during the early 1660s and in Observation IX of his Micrographia (1665) he presents quite a systematic theory of optics.

He presents his theory as the result of a startling observation about the colours of the rainbow observable in thin sheets of muscovy glass (mica). This observation he takes to be an ‘experimentum crucis’ against Descartes’ optical theory, ‘serving as a Guide or Land-mark, by which to direct our course in the search after the true cause of Colours’ (Micrographia, p. 54). His positive thesis starts by outlining a hypothesis about light based on some widely accepted principles (though I won’t go into the details here). This hypothesis he checks against more evidence, this time a glass globe filled with water. He finds his idea consistent with the phenomenon, while Descartes is again lacking. An ‘instantia crucis‘ this time – a sure sign he’s on the right track (ibid., p. 59).

A schema from the Micrographia

To refine his theory, Hooke continues experimenting. Now he uses water in a long glass tube and sheets of muscovy glass split to varying thicknesses. He adds detail until he feels he can account for all kinds of colour phenomena. ‘By this Hypothesis there is no one experiment of colour that I have yet met with, but may be, I conceive, very rationally solv’d, and perhaps, had I time to examine several particulars requisite to the demonstration of it, I might prove it more than probable…’ (ibid., p. 69).

Hooke presents his theory in an ordered and structured way. First he disproves the leading existing theory, then puts forward his own hypothesis. He returns to experiment to check factual adequacy, and uses further trials to refine the general idea. Focusing on his theory as it is presented, though, makes several features of his account mysterious. Why is it tacked on to the end of an observation about colours in a mineral? Why should colour even be the main part of an optical theory? And given that it is, why does he never mention prisms?

Hooke's experimental apparatus

Hooke's apparatus

What is worth noting is the experiments and observations Hooke makes. There are four primary apparatus he uses:

1. Muscovy glass

2. Glass lenses with water between them

3. Water globes

4. Glass vials filled with water.

Prisms, that paradigmatic optical experimentation device used by Descartes, Boyle, Power, and Newton in their experimenting about colour, are conspicuous by their absence. Rather, all of the experiments mentioned by Hooke are, in fact, part of his everyday set up for making microscopical observations. Numbers 2) and 4) are simply water microscopes, which he mentions using in the Preface to Micrographia. Number 3) is a scotoscope, used for concentrating light rays onto a small area to provide illumination, and also described in the Preface. And number 1) was Hooke’s preferred choice of microscope slide, as he explains when recounting his replication of Antoni van Leeuwenhoek’s observations in the late 1670s. It is unlikely that someone in possession of mica, ornate microscopes, and with connections among the fellows of the Royal Society as well as the instrument makers of London, was unable to obtain prisms with which to make experiments.

What is more likely is that, having spent four years making microscopical observations, Hooke stumbled again and again upon the incidental production of colours by his instruments. Chromatic aberration was a well known problem in microscopes and telescopes, which would not be solved until Dollond’s innovations in the eighteenth century. But using muscovy glass for specimen slides, and a water globe for illuminating his subjects, exposed Hooke two forms of colour production others may not have noticed. What’s more, in his Preface Hooke provides not only a detailed drawing of his primary instrument, but instructions on how it is made, and other versions suitable for other situations. Hooke doesn’t seem to have thought of his microscope as a static, finished product. Rather, he used one instrumental set up to make observations of distant objects such as the moon, and another to view things nearby and in his control. Even this could vary depending on whether the subject was translucent or opaque, and on the amount of light required to illuminate it. He notes trialling lenses made not just of glass, but resin, gum, oil, salt, and arsenic. All of this points to a man very aware of the behaviour of light and the process of refraction by which objects are magnified, and who was able to alter his instruments for the best results.

Hooke's microscope

Hooke's microscope

Some features of his theory are better explained by noting this likely route to Hooke’s knowledge of light, but a perhaps more difficult historical question is raised. Why did he present his observations as a constructed, systematic theory of colours rather than simply part of a history, as Boyle had done the previous year? It is likely the answer has something to do with ambition and rhetoric, and the role both Hooke and the other Fellows thought Micrographia would play in the early days of the Society.

Keith Hutchison on ‘De Gravitatione’ and Newton’s Mathematical Method

Keith Hutchison writes…

The core of Kirsten Walsh’s paper is a defence of her proposal that Newton’s De Gravitatione was composed after the publication of the new theory of colours (in 1672-3).  Kirsten compares the methodology of the optical writings with that of De Grav. and notes that despite the similarity there are significant differences. Yet the methodology of De Grav. is effectively identical to that of the Principia, so is plausibly interpreted as the one preferred by Newton.  So Newton would have displayed this methodology in the optical writings, Kirsten concludes, had De Grav. already been composed.

Isaac Newton, 1689

Isaac Newton, 1642-1727

Though I am (tentatively) happy with Kirsten’s observation that it is uncontroversial to see Newton’s Principia as deploying the methodology of De Grav., part of the reason for this is surely the fact that the discussion of methodology in De Grav. is so brief, and hardly exemplified in the actual science that Newton so fleetingly displays in his text.  The little that we find in De Grav. does indeed seem concordant with much that happens in the Principia, but it is easy – too easy – to find agreement between a pair of texts if one of them is vague enough.  Given that the identity between the two methodologies is so important to Kirsten’s case, she needs to find some way of sharpening this step of the argument.

She could, for instance, identify far more thoroughly the small differences between the methodology of the optical writings and that of De Grav.  If each of these differences could be consistently found in the Principia as well, Kirsten would have a much better case, as long as there were not something about the optical investigations that required the alternative approach.  Kirsten notes indeed, that Cohen has suggested that the Principia is primarily a mathematical investigation, but the optical work is overwhelmingly experimental.  Cohen seems to be significantly wrong here, for investigations of the context of Newton’s treatment of chromatic aberration show that Newton originally dreamt of creating a mathematical science of colours – until he found that refraction was puzzlingly idiosyncratic, and so unlike the extremely orderly gravitational interaction that provided much of the mathematics of the Principia.  But it remains true that the optical work is saturated with experiment, and it could be this that allows an earlier (?) De Grav. to seem more like the Principia.

From Experimental Philosophy to Empiricism: 20 Theses for Discussion

Before our recent symposium, we decided to imitate our early modern heroes by preparing a set of queries or articles of inquiry. They are a list of 20 claims that we are sharing with you below. They summarize what we take to be our main claims and findings so far in our study of early modern experimental philosophy and the genesis of empiricism.

After many posts on rather specific points, hopefully our 20 theses will give you an idea of the big picture within which all the topics we blog about fit together, from Baconian natural histories and optical experiments to moral inquiries or long-forgotten historians of philosophy.

Most importantly, we’d love to hear your thoughts! Do you find any of our claims unconvincing, inaccurate, or plainly wrong? Do let us know in the comments!

Is there some important piece of evidence that you’d like to point our attention to? Please get in touch!

Are you working on any of these areas and you’d like to share your thoughts? We’d like to hear from you (our contacts are listed here).

Would you like to know more on some of our 20 claims? Please tell us, we might write a post on that (or see if there’s anything hidden in the archives that may satisfy your curiosity).

Here are our articles, divided into six handy categories:


1. The distinction between experimental and speculative philosophy (ESD) provided the most widespread terms of reference for philosophy from the 1660s until Kant.

2. The ESD emerged in England in the late 1650s, and while a practical/speculative distinction in philosophy can be traced back to Aristotle, the ESD cannot be found in the late Renaissance or the early seventeenth century.

3. The main way in which the experimental philosophy was practised from the 1660s until the 1690s was according to the Baconian method of natural history.

4. The Baconian method of natural history fell into serious decline in the 1690s and is all but absent in the eighteenth century. The Baconian method of natural history was superseded by an approach to natural philosophy that emulated Newton’s mathematical experimental philosophy.


5. The ESD is operative in Newton’s early optical papers.

6. In his early optical papers, Newton’s use of queries represents both a Baconian influence and (conversely) a break with Baconian experimental philosophy.

7. While Newton’s anti-hypothetical stance was typical of Fellows of the early Royal Society and consistent with their methodology, his mathematisation of optics and claims to absolute certainty were not.

8. The development of Newton’s method from 1672 to 1687 appears to display a shift in emphasis from experiment to mathematics.


9. Unlike natural philosophy, where a Baconian methodology was supplanted by a Newtonian one, moral philosophers borrowed their methods from both traditions. This is revealed in the range of different approaches to moral philosophy in the Scottish Enlightenment, approaches that were all unified under the banner of experimental philosophy.

10. Two distinctive features of the texts on moral philosophy in the Scottish Enlightenment are: first, the appeal to the experimental method; and second, the explicit rejection of conjectures and unfounded hypotheses.

11. Experimental philosophy provided learned societies (like the Aberdeen Philosophical Society and the Philosophical Society of Edinburgh) with an approach to knowledge that placed an emphasis on the practical outcomes of science.


12. The ESD is prominent in the methodological writings of the French philosophes associated with Diderot’s Encyclopédie project, including the writings of Condillac, d’Alembert, Helvétius and Diderot himself.


13. German philosophers in the first decades of the eighteenth century knew the main works of British experimental philosophers, including Boyle, Hooke, other members of the Royal Society, Locke, Newton, and the Newtonians.

14. Christian Wolff emphasized the importance of experiments and placed limitations on the use of hypotheses. Yet unlike British experimental philosophers, Wolff held that data collection and theory building are simultaneous and interdependent and he stressed the importance of a priori principles for natural philosophy.

15. Most German philosophers between 1770 and 1790 regarded themselves as experimental philosophers (in their terms, “observational philosophers”). They regarded experimental philosophy as a tradition initiated by Bacon, extended to the study of the mind by Locke, and developed by Hume and Reid.

16. Friends and foes of Kantian and post-Kantian philosophies in the 1780s and 1790s saw them as examples of speculative philosophy, in competition with the experimental tradition.

From Experimental Philosophy to Empiricism

17. Kant coined the now-standard epistemological definitions of empiricism and rationalism, but he did not regard them as purely epistemological positions. He saw them as comprehensive philosophical options, with a core rooted in epistemology and philosophy of mind and consequences for natural philosophy, metaphysics, and ethics.

18. Karl Leonhard Reinhold was the first philosopher to outline a schema for the interpretation of early modern philosophy based (a) on the opposition between Lockean empiricism (leading to Humean scepticism) and Leibnizian rationalism, and (b) Kant’s Critical synthesis of empiricism and rationalism.

19. Wilhelm Gottlieb Tennemann was the first historian to craft a detailed, historically accurate, and methodologically sophisticated history of early modern philosophy based on Reinhold’s schema. [Possibly with the exception of Johann Gottlieb Buhle.]

20. Tennemann’s direct and indirect influence is partially responsible for the popularity of the standard narratives of early modern philosophy based on the conflict between empiricism and rationalism.

That’s it for now. Come back next Monday for Gideon Manning‘s comments on the origins of the experimental-speculative distinction.

(in their terms, “observational philosophers”)

Newton’s Early Queries are not Hypotheses

Kirsten Walsh writes…

In an earlier post I demonstrated that, in his early optical papers, Newton is working with a clear distinction between theory and hypothesis.  Newton takes a strong anti-hypothetical stance, giving theories higher epistemic status than hypotheses.  Newton’s corpuscular hypothesis appears to challenge his commitment to this anti-hypothetical position.  Today I will discuss a second challenge to this anti-hypotheticalism: Newton’s use of queries.

Newton’s queries have often been interpreted as hypotheses-in-disguise.  But in his early optical papers, Newton’s queries are not hypotheses.  In fact, he is building on the method of queries prescribed by Francis Bacon, for whom assembling queries is a specific step in the acquisition and development of natural philosophical knowledge.

To begin, what is Newton’s method of queries?  In a letter to Oldenburg, Newton explains that

    “the proper Method for inquiring after the properties of things is to deduce them from Experiments.”

Having obtained a theory in this way, one should proceed as follows: (1) specify queries that suggest experiments that will test the theory; and (2) carry out those experiments.

He then lists eight queries relating to his theory of light and colours, e.g.:

    “4. Whether the colour of any sort of rays apart may be changed by refraction?
    “5. Whether colours by coalescing do really change one another to produce a new colour, or produce it by mixing onely?”

He ends the letter, saying:

    “To determin by experiments these & such like Queries which involve the propounded Theory seemes the most proper & direct way to a conclusion.  And therefore I could wish all objections were suspended, taken from Hypotheses or any other Heads than these two; Of showing the insufficiency of experiments to determin these Queries or prove any other parts of my Theory, by assigning the flaws & defects in my Conclusions drawn from them; Or of producing other Experiments which directly contradict me, if any such may seem to occur.  For if the Experiments, which I urge be defective it cannot be difficult to show the defects, but if valid, then by proving the Theory they must render all other Objections invalid.”

While Newton’s method of queries is experimental, it does not appear to be strictly Baconian.  For the Baconian-experimental philosopher, queries serve “to provoke and stimulate further inquiry”.  Thus, for the Baconian-experimental philosopher, queries are part of the process of discovery.  However, for Newton, queries serve to test the theory and to answer criticisms.  Thus, they are part of the process of justification.

Newton uses queries to identify points of difference between his theory and its opponents.  For example, in a letter to Hooke he writes:

    “I shall now in the last place proceed to abstract the difficulties involved in Mr Hooks discourse, & without having regard to any Hypothesis consider them in general termes.  And they may be reduced to these three Queries.  [1] Whether the unequal refractions made without respect to any inequality of incidence, be caused by the different refrangibility of several rays, or by the splitting breaking or dissipating the same ray into diverging parts; [2] Whether there be more then two sorts of colours; & [3] whether whitenesse be a mixture of all colours.”

And in a letter to Huygens, Newton says:

    “Meane time since M. Hu[y]gens seems to allow that white is a composition of two colours at least if not of more; give me leave to rejoyn these Quæres.
    “1. Whether the whiteness of the suns light be compounded of the like colours?
    “2. Whether the colours that emerg by refracting that light be those component colours separated by the different refrangibility of the rays in which they inhere?”

In both cases, Newton is using queries to steer the debate towards claims that can be tested and resolved by experiment.  On both occasions, Newton devotes a considerable amount of space to discussing the experiments that might determine these queries.

These early queries are not hypotheses.  Rather, they are empirical questions that may be resolved by experiment.  This is not merely a matter of semantics. In the same letter to Hooke, Newton demonstrates this by distinguishing between philosophical queries and hypothetical queries.  A philosophical query is one that can be determined by experiment, a hypothetical query cannot.  Newton argues that philosophical queries are the only acceptable queries.  He equates hypothetical queries with begging the question.

In his later work, Newton’s queries become increasingly speculative, suggesting that they function as de facto hypotheses.  Does Newton ultimately reject his early ‘method of queries’?

Next Monday we’ll have a guest post from Greg Dawes on Galileo and the Experimental Philosophy.

Newton’s ‘Crucial Experiment’

Kirsten Walsh writes…

In his first optical paper, Newton claims that he has performed an Experimentum Crucis, which proves that refrangibility is an original property of the light, not an effect of the prism:

    …the true cause of the length of that Image was detected to be no other, then that Light consists of Rays differently refrangible, which, without any respect to a difference in their incidence, were, according to their degrees of refrangibility, transmitted towards divers parts of the wall.

This experiment and its role in Newton’s theory of colours raises some questions that I’m not really sure how to answer.  I hope you can help me.

Firstly, let’s have a closer look at this Experimentum Crucis:

Newton's Experimentum Crucis

White light travels from the Sun (S), through the first aperture (F), through the first prism (ABC), where it is refracted for the first time, producing an image on the first board (DE).  A small amount of light passes through the second aperture (G), producing an image on the second board (de).  A small amount of light passes through the third aperture (g), through the prism (abc), where it is refracted for the second time, producing an image on the screen (MN).  Newton “took the first Prisme in [his] hand, and turned it to and fro slowly about its Axis”, so that different parts of the refracted image could pass through the apertures to the second prism.  He took careful note of where each image appeared on the board MN.

Newton finds that each time a particular ray passes through a prism it refracts to precisely the same degree.  For example, light that refracts to 50 degrees at the first prism refracts to 50 degrees at the second prism as well.  Newton argues that this shows that refrangibility is an original and constant property of light.

Newton’s Experimentum Crucis was heavily criticised by his contemporaries.  Hooke, for example, argued that this experiment is not a crucial experiment, because it does not prove that colour is an original property of light.  Hooke believes that light becomes coloured as it passes through the prism, and Newton’s experiment does not convince him otherwise.

While colour is conspicuously absent from Newton’s discussion of this experiment, this line of criticism is extremely common.  For example, Newton’s contemporaries, Hooke, Huygens and Pardies, and more recently, writers such as Sabra and Bechler have all made criticisms along these lines.  As I have previously discussed, Newton used mathematics and measurement in order to achieve absolute certainty.  So it is no accident that Newton only discusses refrangibility and not colour in this experiment.

Newton concludes that white light is composed of rays of every colour in equal amounts, but he argues for this in two steps:

1)      Light is a “Heterogeneous mixture of differently refrangible Rays”; and

2)      There is a one-to-one correspondence between refrangibility and colour.

So, while the Experimentum Crucis only supports step (1), it is often mistaken as an argument for Newton’s conclusion.  Newton takes a great deal of care to establish (1) experimentally, but he seems to take little care at all to establish (2), and hence, the conclusion.  In his first optical paper he simply asserts it as proposition 2; in his reply to Huygens he asserts it as a note to his definitions.

This raises two questions.  Why did Newton take so little care over step (2)?  How did Newton’s main opponents miss this lack of care?