Hypotheses versus Queries in Newton’s Opticks

Posted on February 20, 2012 by Kirsten Walsh

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, 2^nd 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!

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