Part V. THE PHILOSOPHY OF THE ENLIGHTENMENT
Sect. 1. British Enlightenment

Chap. IV. Theological and Ethical questions
[1. Natural Philosophy]

The Empiricism of Locke, which was developed on its theoretic side, by Berkeley and Hume, to its natural conclusion, gave rise to various movements of thought, scientific, theological, and ethical, which now demand our attention. The influence of Locke’s mind may be traced in the general view of the world which Newton took as well as in the physiological researches of Hartley and Priestley, while the Deism of Toland and the Moral theory of Shaftesbury were largely affected, on the one hand by his work on the Reasonableness of Christianity, and on the other by his Denial of the Doctrine of Innate Ideas.

1. Natural Philosophy. Though the discoveries of Sir Isaac Newton, the greatest of natural philosophers, do not strictly belong to the history of mental philosophy, a name so illustrious and epoch-making cannot be passed over without mention. Newton was born in 1642, a year memorable in English history for the breaking out of the Civil War, and doubly notable in the history of science for the birth of Newton and the death of Galileo. He was educated at Cambridge, where he ultimately became Professor of Mathematics. Newton represents the culmination of a long series of scientific efforts which, from the time of Aristotle, had been made to explain the mechanical forces of nature and to reduce the movements of the heavenly bodies to a single principle. It is the glory of Newton to have crowned the labours of a Kepler and a Galileo with success, and though in science more perhaps than in any other department of human effort, each new worker builds on the results of his predecessors, Newton’s name is for ever distinguished as that of the discoverer of the great law of gravitation, as the principle by which bodies, terrestrial and celestial, are governed, first casually suggested by the fall of an apple, but ultimately mathematically proved.

While Newton for the most part confined his attention to purely physical subjects, and did not inquire into the inner meaning of the world whose forces he computed, it is noteworthy that he combined with his strictly scientific reasoning a deep and reverent piety, seeing in the wonderful order and arrangement of the universe the surest proof of an intelligent creator. His theory of knowledge was accepted from Locke, but he recognised that God contained all things in Himself, and that He is the ultimate source of all being.

Newton’s Principia profoundly influenced not science only, but every realm of knowledge. It is difficult for us to-day to realize how largely this new conception of the physical world bulked in the thought of the eighteenth century. What the idea of evolution has been to our own generation the law of gravitation was to the age of Newton. it directed and coloured the whole theological outlook of the times, and became a kind of standing illustration of man’s relation to God.

Locke’s theory of the association of ideas gave rise to the associational psychology of which David Hartley (1705-1757) and Joseph Priestley (1733-1804) are the chief representatives. If the mind is passive and dependent on outward things for its sensations, may thought not be explained on physiological and semi-material principles? The connection of the mental processes with certain states of the body, and the mutual relation of psychical functions and nerve-vibrations, suggested a materialistic theory in which the mechanism of the nervous system was regarded as the primary cause of thought and will. At the same time, these writers did not press their theory to its logical conclusion. They strove to reconcile their psychology with a belief in the immortality of the soul and the divine origin of the world. Priestley especially strongly opposed the atheism of Holbach, and is associated with the Deists in his advocacy of natural religion.