[2] “Motion, therefore, is a change of rectilinear distance between two points. Allowing the accuracy of this definition, it appears that two points are necessary to constitute motion; that in all cases, when we are inquiring whether or no any body or point is in motion, we must recur to some other point which we can compare with it; and that if a single atom existed alone in the universe, it could neither be said to be in motion nor at rest. “The space which we call quiescent is in general the earth’s surface; yet we well know, from astronomical considerations, that every point of the earth’s surface is perpetually in motion, and that in very various directions: nor are any material objects accessible to our senses which we can consider as absolutely motionless, or even as motionless with regard to each other; since the continual variation of temperature to which all bodies are liable, and the minute agitations arising from the motion of other bodies with which they are connected, will always tend to produce some imperceptible changes in their distances.”--_Lectures on Natural Philosophy, &c._, by Thomas Young, M.D. Edited by the Rev. P. Kelland. 1845. [3] “The position which I seek to establish in this essay is, that the various imponderable agencies, or the affections of matter which constitute the main objects of experimental physics, viz., heat, light, electricity, magnetism, chemical affinity, and motion, are all correlative, or have a reciprocal dependence;--that neither, taken abstractedly, can be said to be the essential or proximate cause of the others; but that either may, as a force, produce, or be convertible into, the other:--thus heat may mediately or immediately produce electricity, electricity may produce heat, and so of the rest.... Although strongly inclined to believe that the five other affections of matter, which I have above named, are, and will ultimately be, resolved into modes of motion, it would be going too far at present to assume their identity with it: I, therefore, use the term force, in reference to them, as meaning that active force inseparable from matter, which induces its various changes.”--_On the Correlation of Physical Forces_, by W. R. GROVE, Esq., M.A., F.R.S. [4] When discussing the hypothesis of Hobbes--_that no body can possibly be moved but by a body contiguous and moved_--Boyle asks:-- “I demand how there comes to be local motion in the world? For either all the portions of matter that compose the universe have motion belonging to their natures, which the Epicureans affirmed for their atoms, or some parts of matter have this motive power, and some have not, or else none of them have it; but all of them are naturally devoid of motion. If it be granted that motion does naturally belong to all parts of matter, the dispute is at an end, the concession quite overthrowing the hypothesis.