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The Aims of Newton’s Natural Philosophy

Kirsten Walsh writes…

In a previous post I discussed the aim of absolute certainty in Newton’s early optical papers.  I argued that this aim provides the link between Newton’s mathematical and experimental methods.  This quest for certainty is an enduring feature of Newton’s natural philosophy, leading to a modest natural philosophical agenda.  For example, in the General Scholium to the Principia (1713), Newton writes:

    “I have not as yet been able to discover the reason for these properties of gravity from phenomena, and I do not feign hypotheses … And it is enough 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.”

But is this really enough, for Newton?  Apparently it’s not.  In the very next paragraph, Newton begins to speculate on the “subtle spirit” that permeates bodies and might be operative in various phenomena.  It looks like he is proposing a causal explanation of universal gravitation.  However, these speculations end before they really begin, when Newton concludes that “there is not a sufficient number of experiments to determine and demonstrate accurately the laws governing the actions of this spirit.”

This is the final line of Principia.  And, for such a controversial book, this is a rather inauspicious ending.  But I think we can glean something about the aims of Newton’s natural philosophy from this.

To begin, we need to distinguish between what Newton wants to achieve, and what he thinks he can achieve.  Newton wants to give a complete, true theory of the world – including an account of the motions of the planets, the cause of gravity, and even God’s relation to the natural world.  But, in the trade-off between completeness and truth, Newton sides with truth.  For, as he writes in an unpublished Preface to Principia (mid-1710s), “still it is better to add something to our knowledge day by day than to fill up men’s minds in advance with the preconceptions of hypotheses.”

Newton’s modesty and restraint should not be misinterpreted as lack of epistemic ambition.  The surest way to achieve absolute certainty would be to keep his domain of inquiry as narrow as possible.  But Newton doesn’t do this.  Instead, he pushes at the boundaries of what can be known with certainty.  This is demonstrated by his use of Baconian Induction to make increasingly general claims about gravity.  Newton ambitiously generalises from pendulums, to terrestrial bodies, to all bodies.  In an unpublished Preface to Principia, he writes:

    “But it has also been shown in the Principia that the precession of the equinoxes and the ebb and flow of the sea and the unequal motions of the moon and the orbits of comets and the perturbation of the orbit of Saturn by its gravity toward Jupiter follow from the same principles and what follows from these principles plainly agrees with the phenomena.”

So what do those final two paragraphs of the General Scholium tell us about the aims of Newton’s natural philosophy?  I. Bernard Cohen says that the General Scholium is similar to the discussions that are found in scientific papers today: Newton is discussing the implications of his results and suggesting areas of further research.  On this reading, Newton is saying that there are two jobs ahead:

  1. To give a causal explanation of gravity; and
  2. To apply the theory of gravity to other phenomena in order to solve other problems.

Importantly, Newton thinks that we can begin on (2) without waiting to complete (1).  This is why Newton says it is enough that he has established that gravity exists and acts according to certain laws.

Related Posts: Newton on Certainty, Newton’s 4th Rule for Natural Philosophy.

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