For just as the seas are absolutely necessary for the constitution of this earth, so that vapors may be abundantly enough aroused from them by the heat of the sun, which vapors either—being gathered into clouds—fall in rains and irrigate and nourish the whole earth for the propagation of vegetables, or—being condensed in the cold peaks of mountains (as some philosophize with good reason)—run down into springs and rivers; so for the conservation of the seas and fluids on the planets, comets seem to be required, so that from the condensation of their exhalations and vapors, there can be a continual supply and renewal of whatever liquid is consumed by vegetation and putrefaction and converted into dry earth. For all vegetables grow entirely from fluids and afterward, in great part, change into dry earth by putrefaction, and slime is continually deposited from putrefied liquids. Hence the bulk of dry earth is increased from day to day, and fluids—if they did not have an outside source of increase—would have to decrease continually and finally to fail. Further, I suspect that that spirit which is the smallest but most subtle and most excellent part of our air, and which is required for the life of all things, comes chiefly from comets.--Newton, Principia, Book III, proposition 41, translated by Cohen & Whitman (1999) p. 926.
The quoted passage is among my most favorite bits of the Principia. It is pretty nearly the close of the first edition of the Principia. And as I note elsewhere, it contributed to the reception of Newton as a kind of modern (mathematically inclined) Epicurean--something the General Scholium attached to the second edition helps correct. To be clear Newton here does not embrace the Epicurean notion of seeds as the source of human life.
In this account comets play a vital role in the economy of the universe. They bring a life-giving spirit from the outer reaches of the universe to our solar system. During the seventeenth century, 'spirit' is a term for a kind of refined fluid (the term is still to be noticed when we use 'spirits' to refer to liquor.) What Newton has in mind is made explicit in the final sentence of the Book III (which I think was itself only added to the second edition): "And the vapors that arise from the sun and the fixed stars and the tails of comets can fall by their gravity into the atmospheres of the planets and there be condensed and converted into water and humid spirits, and then—by a slow heat—be transformed gradually into salts, sulphurs, tinctures, slime, mud, clay, sand, stones, corals, and other earthy substances." (Newton, Principia: 938)
I have to admit that if people didn't keep telling me that Newton was a rather heterodox Christian, and we didn't have abundant manuscript evidence of his interest in true Christianity, I would read him here as offering an entirely naturalistic account of the cosmic origin and sustenance of life of a sort that, say, Ibn Tufayl hints at in his account of the rationalizing intellectuals' reconstruction of the site and mechanisms of creation. In any case, we shouldn't be surprised that Leibniz mischievously, perhaps, reads Newton as a kind of crypto-Spinozist.
My reason for returning to Newton's account of comets was prompted by a remark by Chris Smeenk after my recent post, where, while interpreting Whewell, I treated Herschel's (1802) discovery that binary star systems obey Keplerian motion as analogous to the claim that satellites of Jupiter and Saturn obey Keplerian motion and, thereby, showing that Newtonian's gravitation is indeed a universal quality of bodies. For, despite the existence of Newton's third rule of reasoning, which encourages the researcher to take the law of gravitation as a universal (but not essential) quality of systems of matter, Newton only has evidence about bodies in a small region of the visible universe (our solar system). And, in fact, I called attention to a Query in which Newton allows that there may be regions of space where matter is differently constituted and so obeys different force laws. (So much for quick summary.)
In response, Smeenk noted that Newton shows that comets also obey Keplerian motion (that is the point of Proposition 40 of Book III of the Principia). It is worth spelling out the implications of this a bit because they point to an interesting ambiguity in Newton's theory of comets that connects up to the question of the evidence for the scope of law of gravitation.
First, Newton is pretty clear that comets we can see in the sky have Keplerian motion around our Sun: "Those who banish the comets almost to the region of the fixed stars are, therefore, entirely wrong; certainly in such a situation, they would not be illuminated by our sun any more than the planets in our solar system are illuminated by the fixed stars....likely that the comets descend far below the sphere of Saturn," (1999: 892) So comets are part of the solar system and their Keplerian motion is due, in part, to the attraction of our Sun. Of course, some of the orbits of regularly returning comets go to the furthest reaches of the Sun's gravitational field.
But Newton also recognizes that some "comets are not restricted to the zodiac as planets are, but depart from there and are carried with various motions into all regions of the heavens," (937; from proposition 41; emphasis added). And, in fact, this is what he assumes when comets bring spirits and vapors from the outer cosmos to our little planet.
So, lurking in Newton's argument is an open research question: when comets enter the solar system, before they are captured by the Sun's gravitational field (or the gravitational field of the outer planets, etc.) are they obeying Keplerian motion? For, prior to the kind of work that Hershel (and Bradley) could do with their more powerful telescopes, it is difficult to establish what kind of force laws matter obeys far away in different regions of space. It should be noted that Book I of the Principia, presents us with a whole class of possible force laws and motions in accord with such alternative possibilities; so Newton has already developed the tools to answer that question.
But, and perhaps this is what Smeenk had in mind, the mere fact that cosmic wanderers do, once they enter or interact with the solar's gravitational field, obey Keplerian motion suggests that they would do so elsewhere, too. And that's because on Newton's (more speculative view articulated in the Queries (quoted on Monday) the laws of motion are themselves ground in the nature of matter--so if you obey Keplerian motion here, you do would do so there, too. (That's compatible with the thought that elsewhere in the universe there is also fundamentally different kinds of matter that might obey additional laws.) So, that the existence of intergalactic wanderers is some evidence for a possible fundamental univocity of matter. In the first edition of the Principia Newton had embraced this explicitly (as a hypothesis) -"Every body can be transformed into a body of any other kind and successively take on all the intermediate degrees of qualities," (198)- , but he had officially dropped it in later editions perhaps because he became more enchanted with a dappled universe (or because he recognized that the univocity he needed -- inertial mass -- was a thinner and more abstract quantity than the transformation thesis requires).*
The fact that light also passes among different regions of the universe, also provides evidence for the thought that the universe is unified (as opposed to dappled). I mention this because in the General Scholium, Newton had used the immense distances of solar systems, and their inability to interact with each other (and so fall upon each other catastrophically) as a kind of evidence of God's benevolent design. So, that a dappled universe may well be thought in accord with His wishes.
*I am also addressing Alan Nelson's interesting question.
Kollerstrom mentions Newton has a similar circulation of gravitational ether in his Dec 1675 Royal Society contribution, condensing in earth and sun and then being somehow returned to the aetherial spaces...
Posted by: David Duffy | 09/17/2020 at 09:54 AM