Page:Popular Science Monthly Volume 16.djvu/647

 but he can have no complete knowledge of what he investigates, without borrowing from the other department of investigation." Many of the Nature-philosophers who affirm that we have no knowledge of anything but the matter and motion which lie within the range of "experience" show themselves very imperfectly acquainted with what "experience" really means; unhesitatingly ranking as actual objective facts their own mental interpretations of the sensory impressions they receive from external objects. Many metaphysicians, on the other hand, have reasoned as if our concern were with mental operations alone, and as if the abstractions in which they deal had an existence per se, without any relation to the phenomena of nature. But, among the ablest thinkers of the present time, there seems to be now a pretty general recognition of the necessity for the replacement of the abstract definitions of metaphysics—so far, at least, as they relate to the external world—by psychological expressions of the modes in which the human ego is affected by its changes. Thus the ordinary metaphysical definition of "matter" is that which possesses "extension." But, for this definition to convey any definite idea to our minds, we must know what "extension" means; and this, we are told, is the "occupation of space." Now, the conception of "space," in the opinion of most psychologists, is ordinarily derived from our interpretation of visual sensations; and yet these may be altogether deceptive. When we look at a window from a short distance, we can not tell by the use of our eyes alone whether the space included by its frame is void, or is occupied by a perfectly transparent and colorless glass. A glass globe is held up in front of it, and we can not tell by looking at it whether it is empty, or is filled with pure water or some other transparent colorless liquid. And we can take no cognizance by our vision of the atmosphere which surrounds us, unless its transparence is interfered with by mist or fog. Clearly, then, our visual sense can not per se furnish us with a satisfactory definition of matter.

Now that we have got rid of the fiction of "imponderables," we might fall back on a definition of matter—in use before that fiction was invented—as that which possesses "ponderosity" or weight. But what is weight? The downward tendency, it may be replied, in virtue of which all unsupported bodies fall to the earth. But what is