The theory of physical causation deals with natural causes, powers, actions, forces, movements, and the results of movements. When these terms are properly defined, all relations between agents and patients, between causes and effects, and consequent phenomena, can be easily expressed with philosophical precision; but when the causes, the powers, the actions, and the movements themselves are all confounded under one common name of force, as it is now the fashion in the scientific world to do, no one need be surprised if such a course ends in philosophical inconsistencies. To show what great proportions this evil has taken, innumerable passages of modern writers might be adduced. But, not to perplex the reader with conflicting quotations from different sources, I will give only a few extracts from one of the best representatives of modern science. I have before me the Correlation of Physical Forces, by Mr. Grove. It is a well-known little work, much esteemed by physicists, and one which certainly transcends the average merit of many modern productions of the same kind. Now, what is Mr. Grove's notion of cause, of force, of power, as compared with one another and with the phenomena of nature? The following passages will show. He says:
"In each particular case, where we speak of cause, we habitually refer to some antecedent power or force; we never see motion or any change in matter take effect, without regarding it as produced by some previous change" (p. 13).
Here force, power, and cause are taken as equivalent; moreover, motion, or a change in matter, is considered as "produced" by a previous change; which implies that a previous change or movement is the efficient cause of a subsequent change or movement. Hence, according to such a terminology, movement, force, power, and cause should be accepted as synonymous. But philosophy cannot admit of such a wholesale confusion.
"A force," says he, "cannot originate otherwise than by devolution from some pre-existing force or forces.... The term 'force,' although used in very different senses by different authors, in its limited sense may be defined as that which produces or resists motion.... I use the term 'force' as meaning that active principle inseparable from matter, which is supposed to induce its various changes" (p. 16).
Here force is again confounded with power and with cause, inasmuch as "active principles" are powers, and "that which produces or resists motion" is a cause. We are told at the same time that the active principle is not a primordial and essential constituent of material substances, but an accidental result of devolution from other active principles residing in other substances. Philosophy cannot admit of such a phraseology; for, as the active principle of a substance is a constituent of its nature, if the active principle of any substance were thus communicated to it by accidental devolution, such a substance would have no definite nature of its own, and would be nothing; and, in spite of this, it would also be capable of becoming anything, according as its active principle might originate from different pre-existing forces. Now, we know that the first elements of any given substance have a definite nature, and a definite active principle independently of devolution from other substances; and that, according to the results of a constant and universal experience, they are not liable to exchange their nature for anything else, but keep it permanently and unalterably amidst all the vicissitudes brought about by the interference of surrounding bodies. It is, therefore, plain that the "active principle inseparable from matter" cannot originate in devolution from other pre-existing forces. But let us proceed.
"The position which I seek to establish in this essay," says Mr. Grove, "is, that the various affections of matter which constitute the main object of experimental physics—viz., heat, light, electricity, magnetism, chemical affinity, and motion—are all correlative, or have a reciprocal dependence; that neither, taken abstractedly, can be said to be the essential cause of the others, but that either may produce, or be convertible into, any of the others" (p. 15).
Every one, of course, will admit that heat, light, electricity, etc., are "correlative." I also admit that they are not "essential causes" of one another; but the fact is that they are no causes at all; since heat, light, electricity, etc., are only modes of motion and "affections of matter," as the author acknowledges, and are therefore to be considered as mere phenomena or effects, of which the one can be the condition, but not the cause, of the other. I know that the popular language admits of such expressions as "heat causes dilatation," "light causes an impression on the retina," "chemical affinity causes combination," "movement causes a change of place." But these and other similar expressions, though used by scientific writers, and even by philosophers, are by no means philosophically correct. We shall see presently that substances alone have efficient powers, and therefore no mode of being and no affection of matter can display efficient causality. Hence light, heat, electricity, and the rest, are neither efficient causes nor efficient powers; and, inasmuch as they are affections of matter, they cannot even be called forces in a philosophical, but only in a technical, sense, as we shall explain hereafter.
As to the mutual "convertibility" of these various affections of matter into one another, I would observe that, although the expression may be correctly understood, yet, as interpreted by Mr. Grove and by other physicists, it cannot be admitted. What do we mean when we say that progressive motion, for instance, is converted into heat? We mean that in proportion as the progressive movement of a body is resisted and extinguished, a correspondent amount of heat, or of molecular calorific vibrations, is produced by mutual actions and reactions. In this sense the conversion of one mode of being into another is perfectly admissible, no less indeed than the passage from a state of rest to one of movement. But Mr. Grove does not understand it so. He thinks that the progressive movement of a body is never extinguished, but only transformed by subdivision into molecular calorific vibrations; and, therefore, that the same accidental entity which was to be found in the progressive movement is still to be found identically, though subdivided, in the calorific motion. Let us hear him:
"It is very generally believed that, if the visible and palpable motion of one body be arrested by impact on another body, the motion ceases, and the force which produced it is annihilated. Now, the view which I venture to submit is that force cannot be annihilated, but is merely subdivided or altered in direction or character" (p. 24). "Motion will directly produce heat and electricity, and electricity, being produced by it, will produce magnetism" (p. 34). "Lastly, motion may be again reproduced by the forces which have emanated from motion" (p. 36).
Such is Mr. Grove's theory of the "convertibility of forces." It is nothing but a wrong interpretation of the old theory of the "conservation of vis viva" by the modern conception of "potential energy," which admits "forces stored up" in bodies, and ready to show themselves in the form of velocity, heat, light, or any other kind of movement. This notion and others of a like tendency constitute the marrow of the new physical theories, and are the pride of our men of science. Let us hope that a time will come when these able men will see the vanity of such fashionable doctrines, and blush for their adoption of them in their scientific generalizations.