It is obvious that all the members of any series of causes essentially subordinate the one to the other must exist simultaneously. Whether such a series could be infinite depends, therefore, on the question whether an actually infinite multitude is intrinsically possible. This difficulty cannot be urged with such force against an infinite regress in causes accidentally subordinate to one another; for here such a regress would not involve an actually infinite multitude of things existing simultaneously. In the case of essentially subordinate causes, moreover, the series, whatever about its infinity, must contain, or rather imply above it, one cause which is first in the sense of being independent, or exempt from the subordination characteristic of all the others. And the reason is obvious: Since no one of them can exist or act except dependently on another, and this on another, and so on, it is manifest that the series cannot exist at all unless there is some one cause which, unlike all the others, exists and acts without such subordination or dependence. Hence, in essentially subordinate causes an infinite regress is impossible.[456] In Natural Theology these considerations are of supreme importance.
(b) An efficient cause may be described as immanent or transitive according as the term of its action remains within the cause itself, or is produced in something else. The action of the First Cause is formally immanent, being identical with the Divine Nature itself; it is virtually transitive when it is creative, or operative among creatures.
(c) An efficient cause is either a principal or an instrumental cause. When two causes so combine to produce an effect that [pg 374] one of them uses the other the former is called the principal and the latter the instrumental cause. Thus I am the principal cause of the words I am writing; my pen is the instrumental cause of them. Such an effect is always attributed to the principal cause, not to the instrumental. The notion of an instrument is quite a familiar notion. An instrument helps the principal agent to do what the latter could not otherwise do, or at least not so easily. An instrument therefore is really a cause. It contributes positively to the production of the effect. How does it do so? By reason of its nature or structure it influences, modifies, and directs in a particular way, the efficiency of the principal cause. But this property of the instrumental cause comes into play only when the latter is being actually used by a principal cause. A pen, a saw, a hammer, a spade, have each its own instrumentality. The pen will not cut, nor the saw mould iron, nor the hammer dig, nor the spade write, for the agent that uses them. Each will produce its own kind of effect when used; but none of them will produce any effect except when used: though each has in itself permanently and inherently the power to produce its own proper effect in use.[457] We have instanced the use of artificial instruments. But nature itself provides some agencies with what may be called natural instruments. The semen whereby living organisms propagate their kind is an instance. In a less proper sense the various members of the body are called instruments of the human person as principal cause, “instrumenta conjuncta”.
The notion of an instrumental cause involves then (a) subordination of the latter, in its instrumental activity, to a principal cause, (b) incapacity to produce the effect otherwise than by modifying and directing the influence of the principal cause. This property whereby the instrumental cause modifies or determines in a particular way the influence of the principal cause, is called by St. Thomas an actio or operatio of the former; the distinction between the principal and the instrumental cause being that whereas the former acts by virtue of a power permanently inherent in it as a natural perfection, the latter acts as an instrument [pg 375] only by virtue of the transient motion which it derives from the principal cause which utilizes it.[458]
We may, therefore, define an instrumental cause as one which, when acting as an instrument, produces the effect not by virtue of its inherent power alone, but by virtue of a power communicated to it by some principal cause which acts through it. A principal cause, on the other hand, is one which produces its effect by virtue of an active power permanently inherent in itself.
The designations principal and instrumental are obviously correlative. Moreover, all created causes may be called instrumental in relation to the First Cause. For, not only are they dependent on the latter for the conservation of their nature and active powers; they are also dependent, in their action, in their actual exercise of these powers, on the First Cause (for the concursus of the latter).[459] Yet some created causes have these powers [pg 376] permanently, and can exercise them without subordination to other creatures; while others need, for the exercise of their proper functions, not only the Divine concursus, but also the motion of other creatures. Hence the former are rightly called principal created causes, and the latter instrumental created causes.
(d) Efficient causes are divided into free causes and necessary causes. A free or self-determining cause is one which is not determined by its nature to one line of action, but has the power of choosing, or determining itself, to act or abstain, when all the conditions requisite for acting are present. Man is a free agent, or free cause, of his deliberate actions. A necessary cause, or natural cause as it is sometimes called, is one which is determined by its nature to one invariable line of action, so that, granted the conditions requisite for action, it cannot naturally abstain from acting in that invariable manner. All the physical agencies of the inorganic world, all plant and animal organisms beneath man himself, are necessary causes.
The freedom of the human will is established against determinism in Psychology.[460] The difficulties of determinists against this doctrine are for the most part based on misconceptions, or on erroneous and gratuitous assumptions. We may mention two of them here.[461] Free activity, they say, would be causeless activity: it would violate the “law of universal causation”. We reply that free activity is by no means causeless activity. The free agent himself is in the fullest and truest sense the efficient cause of his free acts. It is by his causal, efficient influence that the act of free choice is determined and elicited. Free causality evidently does not violate the necessary, a priori principle set forth above under the title of the Principle of Causality. But—they urge in the second place—it violates the “law of universal causation,” i.e. the law that every event in nature must be the result of some set of phenomenal antecedents which necessitate it, and which, therefore, whenever verified, must produce this result and no other; and by violating this law it removes all supposed “free” activities from the domain of that regularity [pg 377] and uniformity without which no scientific knowledge of such phenomena would be possible. To this we reply, firstly, that the law of uniform causation in nature, the law which is known as the “Law of the Uniformity of Nature,” and which, under the title of the “Law of Universal Causation” is confounded by determinists and phenomenists with the entirely distinct “Principle of Causality”—is not by any means a law of necessary causation.[462] The statement that Nature is uniform in its activities is not the expression of an a priori, necessary truth, like the Principle of Causality. It is a generalization from experience. And experience testifies to the existence of grades in this all-prevailing uniformity. In the domain of physical nature it is the expression of the Free Will of the Author of Nature, who may miraculously derogate from this physical uniformity for higher, moral ends. In the domain of deliberate human activities it is the expression of that less rigorous but no less real uniformity which is dependent on the free will of man. And just as the possibility of miracles in the former domain does not destroy the regularity on which the generalizations of the physical sciences are based, so neither does the fact of human free will render worthless or unreliable the generalizations of the human sciences (ethical, social, political, economic, etc.) about human conduct. Were the appearance of miracles in the physical domain, or the ordinary play of free will in the human domain, entirely capricious, motiveless, purposeless, the results would, of course, be chaotic, precarious, unaccountable, unintelligible, and scientific knowledge of them would be impossible: for the assumption that reality is the work of intelligent purpose, and is therefore a regular, orderly expression of law, in other words, the assumption that the universe is intelligible, is a prerequisite condition for scientific knowledge about the universe. But determinists seem to assume that Divine Providence and human free will must necessarily imply that the whole universe of physical phenomena and human activities would be an unintelligible chaos; and having erected this philosophical scarecrow on a gratuitous assumption they think it will gradually exorcise all belief in Divine Providence and human freedom from the “scientific” mind!
(e) Efficient causes are either physical or moral. A physical efficient cause is one which produces its effect by its own proper power and action—whether immediately or by means of an instrument. [pg 378] For instance, the billiard player is the physical cause of the motion he imparts to the balls by means of the cue. A moral cause is one which produces its effect by the representation of something as good or evil to the mind of a free agent; by inducing the latter through example, advice, persuasion, promises, threats, commands, entreaties, etc., to produce the effect in question. For instance, a master is the moral cause of what his servant does in obedience to his commands. The motives set forth by way of inducement to the latter are of course final causes of the latter's action. But the former, by setting them forth, is the moral cause of the action: he is undoubtedly more than a mere condition; he contributes positively and efficiently to the effect. His physical causation, however, does not reach to the effect itself, but only to the effect wrought in the mind of the servant by his command. It is causally connected with the physical action of the servant by means of an intermediate link which we may call mental or psychical causation—actio “intentionalis,”—the action of cognition on the mind of a cognitive agent.
The agent employed by a moral cause to produce an effect physically may be called an instrumental cause in a wide and less proper sense of this term, the instrumentality being moral, not physical. Only free agents can be moral causes; and as a rule they are termed moral causes only when they produce the effect through the physical operation of another free agent. What if they employ not free agents, nor yet inanimate instruments, but agents endowed with sense cognition and sense appetite, to produce effects? If a man set his dog at another, is he the moral or the physical cause of the injuries inflicted by the dog? That he is the principal efficient cause is unquestionable. But is he the principal physical cause and the dog the instrument? We think it is more proper to call the principal efficient cause a moral cause in all cases where there intervenes between his physical action and the effect an intermediate link of “psychical” or “intentional” action, even though, as in the present example, this psychical link is of the sentient, not the intellectual, order.