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Page:Popular Science Monthly Volume 4.djvu/241

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PRIMARY CONCEPTS OF MODERN SCIENCE.

perception of one who looks at him, nor the inference of one who allows for the ship's motion, is anything like the truth. Nor, indeed, on further consideration, shall we find the revised conclusion much better. For we have forgotten to allow for the earth's motion in its orbit. This being some 68,000 miles per hour, it follows that, assuming the time to be mid-day, he is moving, not at the rate of 1,000 miles per hour to the east, but at the rate of 67,000 miles per hour to the west. Nay, not even now have we discovered the true rate and the true direction of his movement. With the earth's progress in its orbit, we have to join that of the whole solar system toward the constellation Hercules; and, when we do this, we perceive that he is moving neither east nor west, but in a line inclined to the plane of the ecliptic, and at a velocity greater or less (according to the time of the year) than that above named. To which let us add that, were the dynamic arrangements of our sidereal system fully known to us, we should probably discover the direction and rate of his actual movement to differ considerably even from these. How illusive are our ideas of motion is thus made sufficiently manifest. That which seems moving proves to be stationary; that which seems stationary proves to be moving; while that which we conclude to be going rapidly in one direction turns out to be going much more rapidly in the opposite direction. And so we are taught that what we are conscious of is not the real motion of any object, either in its rate or direction, but merely its motion as measured from an assigned position—either the position we ourselves occupy or some other. Yet in this very process of concluding that the motions we perceive are not the real motions, we tacitly assume that there are real motions. In revising our successive judgments concerning a body's course or velocity, we take for granted that there is an actual course or an actual velocity—we take for granted that there are fixed points in space with respect to which all motions are absolute; and we find it impossible to rid ourselves of this idea. Nevertheless, absolute motion cannot even be imagined, much less known. Motion, as taking place apart from those limitations of space which we habitually associate with it, is totally unthinkable. For motion is change of place; but, in unlimited space, change of place is inconceivable, because place itself is inconceivable. Place can be conceived only by reference to other places; and, in the absence of objects dispersed through space, a place could be conceived only in relation to the limits of space; whence it follows that in unlimited space place cannot be conceived—all places must be equidistant from boundaries that do not exist. Thus, while we are obliged to think that there is an absolute motion, we find absolute motion incomprehensible."

I have quoted this elaborate exposition from the text of Mr. Spencer, because it most clearly evinces the difficulty experienced even by those who habitually insist upon the relativity, not only of all our actual knowledge, but also of all our possible cognition, in freeing