The Direction of Motion §74. The Absolute Cause of changes, no matter what may be their specialnatures, is not less incomprehensible in respect of the unity or dualityof its action, than in all other respects. Are phenomena due to the variously-conditionedworkings of a single force, or are they due to the conflict of two forces?
Whether everything is explicable on the hypothesis of universal pressure,whence so-called tension results differentially from inequalities of pressure;or whether things are to be explained on the hypothesis of universal tension,from which pressure is a differential result; or whether, as most physicistshold, pressure and tension everywhere co-exist; are questions which it isimpossible to settle. Each of these three suppositions makes the facts comprehensible,only by postulating an inconceivability. To assume a universal pressure,confessedly requires us to assume an infinite plenum -- an unlimited spacefull of something which is everywhere pressed by something beyond; and thisassumption cannot be mentally realized. That universal tension is the agency,is an idea open to a parallel and equally fatal objection. And verbally intelligibleas is the proposition that pressure and tension everywhere co-exist, yetwe cannot truly represent to ourselves one ultimate unit of matter as drawinganother while resisting it.
Nevertheless, this last belief we are compelled to entertain. Matter cannotbe conceived except as manifesting forces of attraction and repulsion. Inour consciousness, Body is distinguished from Space by its opposition toour muscular energies; and this opposition we feel under the twofold formof a cohesion which hinders our efforts to rend, and a resistance which hindersour efforts to compress. Without resistance there can be nothing but emptyextension. Without cohesion there can be no resistance. Probably this conceptionof antagonistic forces originates from the antagonism of our flexor and extensormuscles. But be this as it may we are obliged to think of all objects asmade up of parts that attract and repel one another, since this is the formof our experience of all objects.
By a higher abstraction results the conception of attractive and repulsiveforces pervading space. We cannot dissociate force from occupied extension,or occupied extension from force, because we have never an immediate consciousnessof either in the absence of the other. Nevertheless, we have abundant proofthat force is exercised through what appears to our senses a vacuity. Mentallyto represent this exercise, we are hence obliged to fill the apparent vacuitywith a species of matter -- an ethereal medium. The constitution we assignto this ethereal medium, however, is necessarily an abstract of the impressionsreceived from tangible bodies. The opposition to pressure which a tangiblebody offers to us, is not shown in one direction only, but in all directions;and so likewise is its tenacity. Suppose countless lines radiating from itscentre, and it resists along each of these lines and coheres along each ofthese lines. Hence the constitution of those ultimate units through the instrumentalityof which phenomena are interpreted. Be they molecules of ponderable matteror molecules of ether, the properties we conceive them to possess are nothingelse than these perceptible properties idealized. Centres of force attractingand repelling one another in all directions, are simply insensible portionsof matter having the endowments common to sensible portions of matter --endowments of which we cannot by any mental effort divest them. In brief,they are the invariable elements of the conception of matter, abstractedfrom its variable elements -- size, form, quality, etc. And so to interpretmanifestations of force which cannot be tactually experienced, we use theterms of thought supplied by our tactual experiences, and this for the sufficientreason that we must use these or none.
