Next to consistency, the great principle of causation constitutes the most essential part of all natural truth, and to deny the operation of this principle in particular cases of mental action, simply because we, with our very finite powers, cannot in the extremely imperfect state of our knowledge, yet fully explain some of the most difficult, complex, transient, and ever-changing phenomena of will and consciousness, is contrary to the most weighty evidence. "The Will" is a conscious mental effort to effect an object, the idea of which is already in the mind, and being a mental "effort" it absorbs the mind and thereby incapacitates it at the moment from observing its own action.
If any phenomenon (such as mental action) is essentially dependent upon another, it must be connected
with it in a never-failing or indissoluble manner, so that when the one occurs the other is always present, otherwise it would not be essentially dependent. The only known connections of this kind are those causation and continuity of phenomena, according to which every phenomenon has a cause, and all phenomena are indissolubly connected in endless series. The evidence of the truth of these principles is so vast, that even all mankind thinking through all ages, and after having made an almost infinite number of definite experiments and observations, have never yet met with a single well verified instance of their failure; and we are therefore justified in inferring that they are universal. There are however instances in the physical and chemical sciences, as well as in mental action, where the dependence of phenomena upon those principles is not very apparent, and has not yet been sufficiently proved, but it is probably in consequence of our imperfect knowledge and limited faculties, that we are unable as yet to fully trace such dependence. The history of science, abundantly proves that we should not assume that a phenomenon arises without a natural cause, simply for the reason that it is very difficult to trace its origin, but wait patiently for more knowledge respecting it. It is unphilosophic and contrary to reason to attribute to occult agencies, effects which may be explicable by ordinary causes, or to refuse to believe in more abstruse causes where the assumption of simple ones is contradicted by some of the evidence.
The principle of causation forms the basis of many minor ones, such as selection, evolution, differentiation, &c. Plurality of causes also is a very common circumstance in all the sciences, and especially in concrete phenomena, and in the complex ones of animal life; the arrival of a ship for example at a distant port, is a result of many conditions. Similarly with most of our mental actions, they are compounds of feeling and intellect, and produced by many causes, such as hereditary tendency, acquired habit, internal and external mental excitants, dogmatic belief, knowledge of empirical rules, and occasionally of verified principles. Several of these causes also frequently conspire to produce a single idea or decision.
Various general principles of lesser magnitude arise from the combined action of two or more of the greater ones, and these also appear to operate in mental actions as well as in physical ones. Thus by the combined influence of causation and of the principle that every phenomenon occupies time, "effects often lag behind their causes;" and in some cases during a long period. The greatest heat of summer for example usually occurs several weeks after mid-summer. The mental effects of early mistakes are often not fully experienced until old age. The decline of a nation also follows a long time behind the period of action of the chief causes which produce it.
Although effects are indissolubly connected with their causes, they frequently do not occur in an active form until a long period after them. In such cases
they are stored up in what is termed a potential or latent state, ready for liberation at a future occasion, when the suitable conditions are present; the storage of chemical power in gunpowder, of solar heat in coal, and its subsequent liberation in our fires, are suitable examples. The principle of deferred activity and storing up of power, occurs also in vital and mental phenomena; potential heat is stored up in our food, and is afterwards evolved by oxidation in our tissues. Muscular power is stored up during sleep, ready to be evolved during labour. The storage also of cerebral impressions, and cerebral energy, ready to call forth ideas, and thereby powerful emotions, by the exciting action of memory, may also be viewed as an instance of similar kind belonging to mental phenomena. A new and striking instance of the storage of energy has been shewn in Faure's improved form of secondary voltaic battery, in which the most powerful voltaic current may be (at least practically) stored up (in a box containing lead plates immersed in dilute sulphuric acid) and conveyed to a distance with little loss, and then liberated.
Exciting causes operate very extensively in mental actions as well as in physical ones, a mere look or word from an eloquent speaker will excite the passions and liberate the muscular power of a multitude. Every part of the human body, especially the muscles and nerve centres, is a store-house of power always ready to be set free by the slightest suitable causes; this is strongly illustrated in the irrepressible activity
of children, and in the excitable passions of young men and women. The more immediate cause of this power is the oxidation of assimilated food; and the source of power in the food is the heat of the Sun stored up in the plants and animals they have eaten.