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Is Newton’s Explanation of Gravity a Hypothesis?

Kirsten Walsh writes…

In the General Scholium to Book 3 of Principia, Newton wrote:

    “Thus far I have explained the phenomena of the heavens and of our sea by the force of gravity, but I have not yet assigned a cause to gravity.”

He went on to explain that such a cause would be a hypothesis,

    “and hypotheses, whether metaphysical or physical, or based on occult qualities, or mechanical, have no place in experimental philosophy.”

It might appear that Newton’s methodological statements don’t reflect his real attitude to causal explanations. He explained all the motions of bodies and the sea by the force of gravity. So in some sense, gravity was the cause of those motions. But if gravity was a cause, then wasn’t it a hypothesis? Was Newton’s famous statement “Hypotheses non fingo” a lie?

In this post, I’ll have a closer look at the role of causal explanations in Newton’s method of natural philosophy.

To begin, consider this statement from Query 28 of Opticks:

    “Whereas the main Business of natural Philosophy is to argue from Phaenomena without feigning Hypotheses, and to deduce Causes from Effects, till we come to the very first Cause, which certainly is not mechanical…”

Here, Newton outlined two central tasks for natural philosophers:

  1. To argue from phenomena without relying on, or giving credence to, hypotheses; and
  2. To infer causes from effects until you arrive at the first cause.

The first task is methodological, and it places a constraint on the kinds of inferences one may make from effect to cause. The second task is epistemological: it tells the philosopher what kind of knowledge to seek, and when to stop. Newton shed a little more light on this second task in Query 31:

    “By this way of Analysis we may proceed from Compounds to Ingredients, and from Motions to the Forces producing them; and in general, from Effects to their Causes, and from particular Causes to more general ones, till the Argument end in the most general.”

Perhaps recognising that, once constrained by task 1, task 2 would be too difficult for any single philosopher to complete, Newton wrote in Query 28:

    “And though every true Step made in this Philosophy brings us not immediately to the Knowledge of the first Cause, yet it brings us nearer to it, and on that account is to be highly valued.”

Furthermore, in Query 31, he writes:

    “And therefore I scruple not to propose the Principles of motion above-mention’d, they being of very general Extent, and leave their Causes to be found out.”

And so, in Principia, Newton inferred causes from effects as far as he was able to, while still following the advice of task 1. He stopped short of assigning a cause for gravity, because he could not deduce it from the phenomena. So as he wrote in the General Scholium of Principia:

    “It is enough [i.e. for the purposes of his argument] that gravity really exists and acts according to the laws that we have set forth and is sufficient to explain all the motions of the heavenly bodies and of our sea.”

To conclude, Newton doesn’t rail against causes per se, only against causes that cannot be proved by, or inferred from, experiment. I have argued that Newton was working with a clear distinction between theories and hypotheses, where a hypothesis is:

H1.   Something that is, at best, only highly probable;
H2.   A conjecture or speculation – something not based on empirical evidence; or
H3.   A causal explanation – something concerning the nature of the phenomenon, rather than its physical properties.

I have changed the wording of H1 slightly from the definition I have given in previous posts. Now it looks like I might need to alter H3. What do you think?

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