by the stone appears to depend partly on its weight and partly upon the exertion of the thrower.  So that, the weight of the stone remaining the same, it looks as if the motive power communicated to it were measured by the distance to which the stone travels—­as if, in other words, the power needed to send it a hundred yards was twice as great as that needed to send it fifty yards.  These, apparently obvious, conclusions from the everyday appearances of rest and motion fairly represent the state of opinion upon the subject which prevailed among the ancient Greeks, and remained dominant until the age of Galileo.  The publication of the ‘Principia’ of Newton, in 1686-7, marks the epoch at which the progress of mechanical physics had effected a complete revolution of thought on these subjects.  By this time, it had been made clear that the old generalisations were either incomplete or totally erroneous; that a body, once set in motion, will continue to move in a straight line for any conceivable time or distance, unless it is interfered with; that any change of motion is proportional to the ‘force’ which causes it, and takes place in the direction in which that ‘force’ is exerted; and that, when a body in motion acts as a cause of motion on another, the latter gains as much as the former loses, and vice versa.  It is to be noted, however, that while, in contradistinction to the ancient idea of the inherent tendency to motion of bodies, the absence of any such spontaneous power of motion was accepted as a physical axiom by the moderns, the old conception virtually maintained itself is a new shape.  For, in spite of Newton’s well-known warning against the ‘absurdity’ of supposing that one body can act on another at a distance through a vacuum, the ultimate particles of matter were generally assumed to be the seats of perennial causes of motion termed ‘attractive and repulsive forces,’ in virtue of which, any two such particles, without any external impression of motion, or intermediate material agent, were supposed to tend to approach or remove from one another; and this view of the duality of the causes of motion is very widely held at the present day.

Another important result of investigation, attained in the seventeenth century, was the proof and quantitative estimation of physical inertia.  In the old philosophy, a curious conjunction of ethical and physical prejudices had led to the notion that there was something ethically bad and physically obstructive about matter.  Aristotle attributes all irregularities and apparent dysteleologies in nature to the disobedience, or sluggish yielding, of matter to the shaping and guiding influence of those reasons and causes which were hypostatised in his ideal ‘Forms.’  In modern science, the conception of the inertia, or resistance to change, of matter is complex.  In part, it contains a corollary from the law of causation:  A body cannot change its state in respect of rest or motion without