the reasons for assigning to it this eminent, and, as it were, sovereign position, these are questions to which there is no answer.
"It will be necessary, therefore, to establish the proposition, as the first principle of the Galileo-Newtonian theory, that in some unknown place of the universe there is an unknown body—a body absolutely rigid and unchangeable for all time in its figure and dimensions. I may be permitted to call this body "The body Alpha." It would then be necessary to add that the motion of a body would import, not its change of place in reference to the earth or sun, but its change of position in reference to the body Alpha.
"From this point of view the law of Galileo is seen to have a definite meaning. This meaning presents itself as a second principle, which is, that a material point left to itself progresses in a straight line proceeds, therefore, in a course which is rectilinear in reference to the body Alpha."
It will be observed that the assumption which underlies all this reasoning of Prof. Neumann is that, to conceive motion as real, it is necessary to conceive it as absolute—an assumption in every respect analogous to that of Prof. Tyndall, according to which the reality of matter implies its constitution from absolute, unvarying elements. The logical parentage of the body Alpha is precisely the same as that of the "atom." And I may add that the assumption of Prof. Neumann is the tacit assumption of almost all the physicists and philosophers of the day, although it is not usually developed to its last consequences. It is one of the tasks of Herbert Spencer, for instance, to exhibit the contradictions involved in the essential relativity of motion. "A body impelled by the hand," says Spencer ("First Principles," chap, iii., §17), "is clearly perceived to move, and to move in a definite direction: there seems at first sight no possibility of doubting that its motion is real, or that it is toward a given point. Yet it is easy to show that we not only may be, but usually are, quite wrong in both these judgments. Here, for instance, is a ship which, for simplicity's sake, we will suppose to be anchored at the equator, with her head to the west. When the captain walks from stem to stern, in what direction does he move? East is the obvious answer—an answer which for the moment may pass without criticism. But now the anchor is heaved, and the vessel sails to the west with a velocity equal to that at which the captain walks. In what direction does he now move when he goes from stem to stern? You cannot say east, for the vessel is carrying him as fast toward the west as he walks to the east; and you cannot say west, for the converse reason. In respect to surrounding space, he is stationary, though to all on board the ship he seems moving. But now are we quite sure of this conclusion? Is he really stationary? "When we take into account the earth's motion round its axis, we find that, instead of being stationary, he is traveling at the rate of 1,000 miles per hour to the east; so that neither the
