Hi Eckard
I'm going to reply to your comments in parts. This is part 1.
In 1632 Galileo published the "Dialogue Concerning the Two Chief WORLD systems". In this work he compared the heliocentric model with the geocentric model by means of a dialogue among the three famous interlocutors: Salviati, Sagredo and Simplicio. The work of Descartes (1596-1650) called "the WORLD" (I think that the NAME IS NOT A COINCIDENCE) in which the aether ideas were presented was divided in several parts, because Descartes was aware of what had happened to Galileo. Descartes also upheld the Copernican view but denied empty space. He first published one part in 1644, but he avoid the publication of the other parts because their content was heretical according to the dogmas of the church. The other parts were published some years later after his dead in 1662, 1664 and the complete text in 1677. By this time Newton was 35 years old and he had already discovered the law of gravitation. So, the rest of the century Descartes' works were widely recognized.
Now, recall that Newton's Principia was first published in 1687, but it took some years to gain wide acceptance in England and some more years to be widely accepted in continental Europe. It was well known in England by the beginning of the XVIII century but not in continental Europe. The reason for this is that in continental Europe Descartes' theory of vortices was well established and more popular. Christian Huygens had improved it immensely from ~1670 to ~1690. This is also why Huygens (Hooke and Leonard Euler too) supported the idea that light was a wave moving through the aether. So, there were basically two beautiful theories competing to gain followers (similar to string theory and loop quantum gravity today). The point in favor of Newton's version is that his theory was a mathematical and elegant model based on the three laws of motion and the law of gravitation (his theory was axiomatic). From these laws he could derived Kepler's laws, whereas Huygens assumed Keppler's laws with no derivation whatsoever from any other law. That is, Descartes' theory had no gravitational law. This was a great disadvantage. If you take a glance at Newton's Principia, the final part is called: "The system of the WORLD". There Newton wrote in the introduction about the cause of gravity:
"...The later philosophers pretend to account for it [gravity] either by the action of certain vortices, as Kepler or Des cartes; or by some other principle of impulse or attraction, as Borelli, Hooke and others of our nation; for, from the laws of motion it is most certain that these effects mots proceed from the action of some force or other.
But our purpose is only to trace out the QUANTITY and properties of his force from the phenomena, and to apply what we discover in some simple cases as principles, by which, in a MATHEMATICAL WAY, we may estimate the effects thereof in more involved cases, for it would be ENDLESS and IMPOSSIBLE to bring every particular to direct and immediate observation... We said, in a mathematical way, to avoid all questions about the nature and the quality of this force..."
So, from here we see that Newton was aware of Descartes work and that he was simplifying his theory to quantify phenomena. However, the philosophical and intuitive conception of gravity was more in agreement with Descartes. To make clear Newton's notion of gravity, I will quote some extracts from correspondences of Newton's contemporaries. Recall the famous motto which was born from a Letter that Newton wrote to his rival Robert Hooke dated 1676:
"What Des-Cartes did was a good step. You have added much several ways, & especially in taking the colours of thin plates into philosophical consideration. If I have seen further it is by standing on the sholders of Giants."
In a 1675 letter to Henry Oldenburg, and later to Robert Boyle, Newton wrote the following:
"Gravity is the result of a condensation causing a flow of ether with a corresponding thinning of the ether density associated with the increased velocity of flow..." [This is actually Descartes idea]"
To make clearer that Newton was actually Cartesian in the philosophical matters of gravity, in the third letter to Bentley in 1692 Newton wrote:
"It is inconceivable that inanimate brute matter should, without the mediation of something else which is not material, operate upon and affect other matter, without mutual contact, as it must do if gravitation in the sense of Epicurus be essential and inherent in it. And this is one reason why I desired you would not ascribe 'innate gravity' to me. That gravity should be innate, inherent, and essential to matter, so that one body may act upon another at a distance, through a vacuum, without the mediation of anything else, by and through which their action and force may be conveyed from one to another, is to me so great an ABSURDITY, that I believe no man who has in philosophical matters a competent faculty of thinking can ever fall into it."
to be continued...
Israel