Newton's philosophy of nature as revealed through the Principia stands out in its search for an 'order' in the universe. This order that Newton sought was characterized by laws, rules, causes and effects. As he writes, 'Natural philosophy consists in discovering the frame and operation of nature, and reducing them as far as may be to general rules or laws - establishing these rules by observation and experiments and thence deducing the causes and effects of things' (The Principia, 1726). This search for a universal pattern led Newton to find the general rules which could explain the phenomena of 'the heavens and the seas'. Newton's studies on matter, motion, light and gravity shaped the course of science and natural philosophy. Yet certain aspects of this philosophy remained elusive to Newton himself and one of these was the concept of gravity or the problem of 'action at a distance'. This essay deals with this very important and puzzling aspect of Newtonian philosophy.
Newton began the mathematical principles in the Principia with definitions of matter, motion and force, thereby distinguishing types of forces and laying down the foundations for the general laws or axioms according to which all events in the natural world could be described. However, this realist philosophy of Newton was characterized by the fact the phenomena that were described by natural laws were distinct from the laws themselves and the phenomena in the natural world 'obeyed' these physical laws. Newton also distinguished between absolute motion and relative motion, absolute space and absolute, true, mathematical time .
In defining force, Newton writes of the 'vis insita' as the innate force of matter which is the power of resisting a change of state whether it is in uniform motion or at rest and it is only with impressed force that any change of state could be possible. From these laws which explained the state of rest and motion of ordinary physical objects, Newton could also explain the motion of celestial bodies. In Proposition VII he writes, that there is a power of gravity pertaining to all bodies proportional to these several quantities of matter which they contain. In the same section he writes 'the force of gravity towards several equal particles of any body is inversely as the square of the distance of places from the particles'. In Proposition XI of the Principia he goes on to say 'the common centre of gravity of the earth, the sun and all the planets is immovable' (The Principia, 1726)
From these excerpts, it is obvious that Newton considered gravity to be dependent on all matter, distinct from matter and acting 'on' matter. Yet according to Newton, the mathematical expression of gravity was only an empirical law for which he suggested two kinds of explanation - a physical explanation and a possible metaphysical explanation. In some of his writings, Newton mentions this physical aspect of a possible cause for gravitational attraction but preferred to remain silent on any metaphysical explanations. The 'other' metaphysical aspect of thinking was developed by later study which gave a deeper understanding of what Newton was trying to imply. Thus the physical explanation of gravity, although looked ad hoc at times, did form part of Newton's thinking. However, the metaphysical doctrine can be considered as more derived from Newton's personal correspondence.
On the cause of the power of gravity, Newton wrote, '(I)t must proceed from a cause that penetrates to the very centres of the sun and the planets. ..(t)hat operates not according to the quantity of the surfaces of the particles upon which it acts but according to the quantity of solid matter which they contain'.(b)ut hitherto I have not been able to discover the cause of those properties of gravity from phenomena and I frame no hypothesis..and hypothesis whether metaphysical or physical , whether of occult qualities or mechanical , have no place in experimental philosophy'.(emphasis added) (The Principia, 1726)
Newton did not explicitly attempt to give a causal explanation of gravity. There were two lines of thought which intrigued Newton. He envisaged a subtle spirit which pervaded all gross bodies and this spirit was responsible for forces of attraction and repulsion and this same subtle spirit were also present in light corpuscles that gave light its reflecting and refracting properties. This spirit also worked according to Newton along the filaments of the nerves, yet Newton was aware that there was no empirical method to determine how the electric or elastic spirit could operate. So was it ether that Newton was talking about?
A.J. Snow writes 'In Optics 1706, Newton pointed hypothetically to the possibility of the existence of ether' (Snow, 1926). Newton believed heavens are free of all sensible matter and resistance, but some parts may contain very thin vapours, steams or effuvia arising from the atmosphere of the earth, planets and comets'. This might have been a description of an extremely rare and ethereal medium. However, Newton's concept of ether was distinctly different from the Cartesian ether which was considered as uniformly pervading space. Newton considered the ethereal medium to be present in varying degrees of rareness being most rare within dense bodies and becoming gradually denser as it moves away from the bodies. As revealed in his work on Optics , Newton even tries to 'speculate' a cause for gravity as the difference in the density of the medium which made bodies move towards the rarer medium. He also used the concept of ether to explain vision as vibrations of rare medium and the same vibrations produced excitations in the brain and different nerves .
Newton's earlier works emphasized the phenomena of gravity as action at a distance between the two bodies. However, in later editions where Newton does allude to ether, he is dissatisfied with his own explanation. He also mentions that all particles of bodies have forces or powers or virtues in them by which they act at a distance upon one another (Newton on Optics, 1706) He used this theory of action at a distance and applied to several fields, including the study of chemical reactions. Considering Newton's own dissatisfaction for the ether hypothesis, the hypothesis does look as a desperate and reluctant measure on the part of Newton. Why did Newton resort to ether at all?
In 1710, Leibniz criticised Newton's use of forces or power to explain the cause of gravity. Action at a distance, was according to Leibniz 'a resort to occult powers' and not scientific. In the third edition of the optics, Newton denied any occult qualities in principles of gravity emphasizing that they are general rules, laws or axioms by which things are formed or occur.
Leibniz himself thought that every piece of matter moved with its own force. He interpreted Newton's mathematical abstractions as metaphysical explanations of gravity. Yet to Newton although 'action at a distance' was a consequence of mathematical abstractions of the phenomenon of gravity, the existence of ether, a priori, was not necessary as it was neither experimentally proven nor mathematically necessary.
Newton could give a mathematical expression of gravity as centres of force attracting at a distance and so he restrained from giving any further explanation of the cause of gravity in his earlier works. The reason, for which Newton was reluctant to consider the ether hypothesis, was according to Snow, threefold 1. Such a medium would have profound resistance to planetary motion 2. There were no reasons or experimental proof for existence of such a medium 3. It would reduce all forces to movements of matter as the primary causes of such movements and thus contradict Newton's own theory (Snow 1926).
Newton tried to show that force, attraction or gravitation is not an attribute of matter and this leads to a gap in Newton's explanation. What then is the cause of gravity or attraction?
This peculiar Newtonian dilemma of a subtle spirit explanation of gravity that can be a cause, has also been interpreted as what is known as the metaphysical doctrine of Newtonian philosophy. Newton definitely had given some metaphysical explanation of gravity. He mentioned God, Spirit, and Deity in his private correspondence with the bishop Bentley. Newton wrote on the effuvia, something non-corporeal, non-material and Bentley construed this suggestion as a metaphysical cause of gravity and a hypothesis which had implications in physics, metaphysics, and theology. Newton however might have distinguished between a mechanical cause and a divine cause.
Newton was greatly influenced by the teachings of Henry More at Cambridge. More spoke often of a Spirit of nature or immaterial force and God can be considered as an immaterial substance as God is an extended being, He is omnipresent, spiritual substance having the attribute of extension. This is the 'spiritual essence' responsible for change and motion in the universe. According to Snow, More's philosophy must have influenced Newton and probably played a direct role in shaping his metaphysical doctrine of gravity. The notion of a universal subtle essence and the Spirit which is a subtle form of God's own essence and extension must have been a motivating factor in Newton?s philosophy of gravitational attraction. Although Newton being an empirical philosopher did not explicitly voice his opinions on God and maintained 'I have not been able to discover the cause of those properties of gravity -I frame no hypothesis' (The Principia, 1726), the peculiarity of the Newtonian position is that it can be interpreted in two very different ways as examined above.
The Principia is fundamentally a work of mathematics and Newton's mathematical approach was largely a categorization method which he used to describe several apparently diverse natural phenomena under rules of the same category. Newton's great achievement was that he 'defined' physical events and also explained why they could be defined in a particular way. So Newton's philosophy can be broadly described as the first natural philosophy that defined, categorised and explained nearly all physical events of nature. This is a huge achievement in itself and definitely the greatest in natural philosophy.
Yet, as we have examined in this essay, Newton's philosophy underwent a change or evolution during a period of twenty years and sometimes can be even considered as 'fluctuating'. May be that is why explanations of gravity stand out as the first scientific philosophy of nature that is explicitly abstract and mathematical and implicitly metaphysical with even wider spiritual and theological allusions.
Bibliography:
Cohen, I. Bernard, 1914 - Introduction to Newton's "Principia".
Cambridge, Mass.: Harvard University Press, c1971.
Manuel, Frank E - The religion of Isaac Newton / Frank E. Manuel.
Oxford : Clarendon Press, 1974.
Newton, Issac, Sir 1642 -1727 - Newton: texts, backgrounds, commentaries / selected and edited by I. Bernard Cohen and Richard S. Westfall.
New York, NY ; London : W.W. Norton, c1995.
Newton, Issac Sir 1642-1727 - Sir Isaac Newton's Mathematical principles of natural philosophy and his system of the world (The Principia 1726) / translated into English by Andrew Motte in 1729 ; the translations revised, and supplied with an historical and explanatory appendix, by Florian Cajori. Vols 1 & 2.
Berkeley : University of California Press, 1962
Newton, Issac, Sir 1642 -1727 -- Newton's philosophy of nature: selections from his writings / edited and arranged with notes by H.S. Thayer; introduction by John Herman Randall, Jr.
New York : Hafner, 1953.
Snow, Adolph Judah - Matter and gravity in Newton's physical philosophy: a study in the natural philosophy of Newton's time.
London, 1926.