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Should we call Newton a ‘Structural Realist’?

Kirsten Walsh writes…

At our symposium last week, someone wondered if we can characterise Newton as a ‘structural realist’.  It is certainly anachronistic to attempt to interpret Newton’s epistemic stance in light of the present-day scientific realism debate.  But the sin of anachronism may be forgiven, if it advances our understanding.  So let us see what advantages this interpretation may provide.

Briefly, structural realism is the view that epistemically, a scientist should only commit herself to the mathematical or structural content of her theories, and remain sceptical about the unobservable entities posited by those theories.

To characterise Newton as a structural realist, one might make the following argument:

    P1. Newton is a realist about his theories, but not about his hypotheses.
    P2. Newton’s theories make claims about theoretical structures, whereas his hypotheses make claims about unobservable theoretical entities.
    C. Therefore, Newton is a realist about theoretical structures, but not about unobservable theoretical entities.

Firstly, consider Newton’s hypothesis/theory distinction.  In a previous post I argued that Newton claims that his doctrine of light and colours is a theory, not a hypothesis, for three reasons:

    T1. It is certainly true, because it is supported by (or deduced from) experiment;
    T2. It concerns the physical properties of light, rather than the nature of light; and
    T3. It has testable consequences.

In contrast, he attaches no special epistemic merit to his corpuscular hypothesis because:

    H1. It is not certainly true, because it is not supported by experiment;
    H2. It concerns the nature of light; and
    H3. It has no testable consequences.

T1 and H1 support P1.  They tell us that Newton is a realist about theories because they can be shown to be true on the basis of experiment.  Moreover, he is not a realist about hypotheses because they cannot be shown to be true on the basis of experiment.  This highlights an important feature of Newton’s methodology: Newton is only epistemically committed to those things that are demonstrated experimentally.

T2 and H2 appear to support P2, but only if the ‘entity/structure’ distinction maps onto Newton’s ‘nature/physical properties’ distinction.  Prima facie, it does.  While Newton probably wouldn’t have been comfortable with the entity/structure distinction, the structural realist debate is often framed in terms of the nature/physical properties distinction.  For example, here’s how the Stanford Encyclopedia of Philosophy describes the structural realist position:

    Structural realism is often characterised as the view that scientific theories tell us only about the form or structure of the unobservable world and not about its nature.  This leaves open the question as to whether the natures of things are posited to be unknowable for some reason or eliminated altogether.

So it looks like the argument for characterising Newton as a structural realist is well-supported by Newton’s distinction between theory and hypothesis.  But what do we gain by characterising Newton in this way?

Chris Smeenk recently pointed out to me in an email that the structural realist label identifies a distinctive feature of Newton’s methodology.  Namely, that he is epistemically committed to his abstract mathematical structures.  He is not an instrumentalist about his theories, but neither is he a realist about the nature of the phenomena they describe.  This might shed some light on the optical debate of the early 1670s, for unlike his contemporaries, Newton does not think there is a contradiction in believing that his theory of light is true, while not committing himself to any particular doctrine regarding the nature of light.

Is this a large enough pay-off to warrant the offence of anachronism?  What do you think?

In this brief post, I have only considered Newton’s attitudes to his own theories.  There are other questions to be raised in connection with structural realism, for example, is Newton a structural realist about the history of science?  In other words, what is Newton’s epistemic commitment to the theories of his predecessors?  I shall leave this question for another time.

On another note, we were very pleased with how last week’s symposium went.  We look forward to telling you all about it next Monday.

5 thoughts on “Should we call Newton a ‘Structural Realist’?

  1. This sounds like a really interesting idea, but I’m just not sure that the ‘entity/structure’ distinction maps onto Newton’s ‘nature/physical properties’ distinction.

    I’ll use John Worrall’s “Structural Realism: The Best of Both Worlds?” to illustrate why I’m worried about this. Worrall uses Fresnel’s and Maxwell’s theories of light to argue for structural realism, and draws a distinction between what he calls ‘structure’ and ‘nature’. The basic idea is that there is structural continuity over theory change, since Fresnel’s equations find their way into Maxwell’s theory. But on the level of nature, there is discontinuity over theory change, since the ether was replaced by the electromagnetic field.

    First of all, I’m not sure that ‘physical properties’ and ‘structure’ amount to the same thing. It seems to me that the ether had a number of physical properties. To take one example, it was thought to be an elastic solid. But the electromagnetic field, I take it, was not thought to be an elastic solid. If ‘physical properties’ and ‘structure’ amount to the same thing, then this would be a serious problem for the structural realist, since there is a lack of continuity here. As I understand him, Worrall would most likely categorize these physical properties under nature, rather than structure. In this case, I wonder whether Newton is using ‘physical properties’ in a way that’s different from the way I understand that term. As I understand it (but perhaps Newton would disagree), ‘being an elastic solid’ is a physical property of the ether. I’m no Newton scholar, so I’d be curious to know what he means here.

    Secondly, ‘nature’, I take it, is a technical term for Worrall, and it’s a bit difficult to find a definition for that term in his paper. But given that it is a technical term, I’m not sure that it’s enough to simply quote the excerpt from the SEP entry on structural realism. In other words, even if Newton and the structural realists use the same term (‘nature’), it doesn’t follow that they mean the same thing.

  2. Hi Jonathon,

    Thanks for your comments.

    With regard to your first point, you are correct that Worrall would categorise those physical properties of ether as ‘nature’. Newton would categorise those properties as ‘nature’ too. Newton uses ‘physical property’ to talk about those properties that are observable and can be measured precisely. That is, physical properties are those properties that can be used to reason mathematically about the world. Newton prioritises ‘theories’, which describe mathematically the relationships between such physical properties. This sounds very similar to Worrall’s notion of ‘structure’.

    Regarding your second point, in his ‘Structural Realism’ (1989), Worrall uses the term ‘nature’ to describe (a) “basic furniture of the universe” (p.122), (b) “metaphysical frameworks” (p.122) and (c) “theoretical mechanisms” (p.121). This is precisely how Newton uses the term ‘nature’.

    It is possible that Newton and Worrall wouldn’t agree on how best to demarcate between nature and structure in every case, for example, a ray of light. (Worrall has the benefit of hindsight: his understanding of ‘structure’ is greatly influenced by which parts of the theory survived as limited cases of later theories.) But I think it is safe to say that the distinction identified by Worrall is operative in Newton’s methodology.


  3. Hi Kristen,

    Thanks for the response. I think you’re right that Newton and Worrall need not agree on every case in order to employ the structure-nature distinction. And it’s interesting to find out that Newton uses the term ‘nature’ in the same way that Worrall does.

    I just have a few comments on your response to my first point. First of all, it looks like you may have an additional reason for not wanting to claim that physical properties and structure line up (if you were, in fact, ever inclined to make this claim—based on your response, I may have misunderstood your original post). As I understand it, Worrall’s structure-nature distinction is operative at the level of theoretical content, and not at the level of empirical content. So the properties of, say, the ether, would count as nature. And based on your claim that “Newton uses ‘physical property’ to talk about those properties that are observable,” it sounds like Newton’s notion of ‘physical property’ would be operative at the level of empirical content. In that case, physical properties wouldn’t map onto structure or nature, since the latter are operative at the level of theoretical content.

    Secondly, based on what you have to say here, I take it that it’s not physical properties that line up with structure (which is what I thought you had claimed in your post). Instead, it’s Newton’s notion of ‘theory’ that lines up with structure.

    Thirdly, if this is the case, then I have a concern about whether you’re left with structural realism or a form of instrumentalism. You claim that, according to Newton, theories “describe mathematically the relationships between such physical properties,” i.e., “those properties that are observable and can be measured precisely.” But if I’m right to suspect that this entails that the notion of ‘physical property’ is operative at the empirical level, then I’m not sure that you get something like Worrall’s structural realism. If a theory describes the mathematical relationships that obtain among observable properties, then I’m not sure that you get anything more than a form of instrumentalism. Structural realism is different, since theories are thought to describe the actual relationships that obtain among the entities that make up the unobservable, unknowable ‘nature’ of the universe.

    Best of luck with your project. I do think it’s a very interesting idea, and I’m hoping that there may be something in my comments that can help you pursue it further.


  4. I should have said ‘Hi Kirsten’ not ‘Hi Kristin’. Sorry!

  5. Hi Jonathon,

    I think you are right that, for Newton, a ‘theory’ is a statement of the mathematical relationships between physical properties, and that it’s Newton’s notion of ‘theory’ that lines up with structure. But I don’t think this leaves me with a form of instrumentalism, because Newton is epistemically and ontologically committed to this mathematical structure. He thinks his mathematics correctly describes the relationships between physical properties because those physical properties behave in accordance with his mathematical laws. I haven’t followed up all the references yet, but Guicciardini argues that this is one of the reasons why Newton chose to employ geometric diagrams in Principia, rather than the abstract symbols of algebra. For Newton, these diagrams represent real forces, trajectories and accelerations.

    Thanks for helping me ‘nut out’ my position on this!