THE FORCE BEHIND NATURE. 617 



but he can have no complete knowledge of what he investigates, with- 

 out 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 ex- 

 ternal world — by psychological expressions of the modes in which the 

 human ego is affected by its changes. Thus the ordinary metaphys- 

 ical 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 " occupa- 

 tion 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 Avindow 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 trans- 

 parent colorless liquid. And we can take no cognizance by our vision 

 of the atmosphere which surrounds us, unless its transparence is inter- 

 fered with by mist or fog. Clearly, then, our visual sense can not^j^er 

 se furnish us with a satisfactory definition of matter.f 



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 vir- 

 tue of which all unsupported bodies fall to the earth. But what is 



* " Natural Theology of the Doctrine of the Forces." By Professor Benjamin Martin, 

 of the University of the City of New York. 



\ According to Professor Bain, the conception of space is essentially based on the 

 sense of muscular tension which, according to him, we experience in the ordinary move- 

 ments of our eyes. But I am satisfied that this is physiologically erroneous. These 

 movements are ordinarily guided, as Professor Alison long ago contended, and as Profes- 

 sor Helmholtz and I myself have since experimentally proved, by the visual, not by the 

 muscular sense ; and it is only when we put the muscles to an unusual strain — as when 

 our visual axes converge on an object brought nearer and nearer to the eyes, or when we 

 entirely exclude light from the retina — that we experience any sense of tension in their 

 muscles. 



