Disputationes metaphysicae

1. Since this name of efficient cause extends most widely, and is attributed to many things most improperly and by a certain imperfect analogy, and to others, although it is attributed properly, yet according to various modes of causing, therefore, before we proceed further, it is necessary to set out first the various divisions of this cause; to understand which we need to have before our eyes the definition given; for according to the diverse mode of participating in it there will be a difference among causes.

The efficient cause per se and per accidens

2. Various modes of causes per accidens.—The first division of the efficient cause, therefore, can be set up into per se and per accidens, which Aristotle handed down in the places cited. The cause per se is that on which the effect directly depends according to that proper being which it has inasmuch as it is an effect, in which way (says Aristotle) the sculptor is the cause of the statue; and since this alone is properly and simply a cause, almost the whole of the following disputation will be about it alone. The cause per accidens, however, since it is not a true cause, but is so called through some relation or likeness or conjunction with a cause, cannot conveniently be defined by one common description, but is spoken of in various ways. For sometimes a cause is called per accidens on the part of the cause, sometimes on the part of the effect. On the part of the cause, that is said to cause per accidens which is conjoined per accidens to the principle of causing per se; which is sometimes the very subject of the form which is the principle of acting; and in this way water is said to heat per accidens, because it is accidental to water that it be hot and consequently that it heat. And in this way (says Aristotle) Polyclitus is the cause of the statue. Sometimes, however, it is another form conjoined per accidens to the other in the same subject—per accidens (I say) as regards the power of effecting, whatever be the case as to whether they are necessarily conjoined on some other ground. And in this way the musician writes per accidens, and the white heats, etc. Aristotle adds, however, that this latter mode is more improper or more remote than the former, because the concomitant form contributes nothing whatever to the effect of the other, whereas the subject of the form, since it at least sustains the form itself in itself, seems to contribute something. He adds further that these causes per accidens can be assigned both in particular and in common, that is, in species or genus, as this white thing, or the white heats. Likewise, that they can be expressed simply or compositely, as the musician builds, or Polyclitus the musician builds. But these things pertain more to the mode of predicating than of causing, and since they are per accidens, they can be multiplied to infinity according to the various modes of conceiving and speaking.

3. A doubt is satisfied.—One can only doubt, since actions belong per se to supposits, why Aristotle says that action is attributed per accidens to the supposit of the form. I answer first that action is per se from the supposit inasmuch as it is affected with such a form, but not necessarily according to itself; and thus the hot heats per se, but not the water. I add, however, secondly, that action is attributed per accidens to the supposit when the power of acting is also present in it per accidens, as in the example given, because then the power of acting is in no way contained in the supposit itself taken barely, and so the action is conjoined wholly per accidens. But if the power of acting is present in the supposit per se, then even if the supposit itself be named barely, it will not be called a cause per accidens, but per se, of the action; and thus I judge these to be per se: the sun illuminates, fire heats, according to the fourth mode of saying per se, from I Poster., c. 4, because fire radically and virtually includes the proper notion of heating. You will say: then this is per se: a man makes a statue, which is contrary to Aristotle. Some answer that that proposition is not per se because it is not necessary, but contingent. But this has no bearing on causality per se or (what is the same) on the fourth mode of saying per se; for these propositions too are not necessary: The musician sings, the hot heats, and yet they are per se in the fourth mode. Hence, as regards causality, I judge these to be per se: an animal moves itself, a man reasons, speaks, etc.; but that one, a man makes a statue, is rather per accidens, because, although that art cannot exist except in a man (I leave aside the intelligences), yet it is simply accidental to man, whence man taken simply does not include that art except in a very remote potency.

4. The cause per accidens on the part of the effect: what it is.—Sometimes also a cause per accidens is commonly assigned on the part of the effect, that is, with respect to what is accidental to the effect per se; and in this way the very cause per se of some effect is the cause per accidens of what is conjoined to the effect per se, in which way motion is the cause of heat or the hot the cause of the black. In this way also those things which happen by chance or fortuitously are said to have a cause per accidens, as that someone digging should find a treasure. In this genus of causes per accidens, however, it must be noted first that sometimes an effect is called per accidens with respect to the concurrence of certain causes, although, that concurrence being supposed, such an effect flows per se from such a cause; as, for example, that Peter should happen to pass by that way while a stone is falling is accidental; yet, that concurrence being supposed, that the stone by its own force should strike and make such a wound is an effect proceeding per se from such active force of the stone, and therefore the stone will not then be called the cause per accidens of that effect, except with respect to that concurrence or also with respect to the intention of the one moving it. But of effects per accidens in this way we shall say more below, in treating of chance and fortune. Again, it must be noted that when an effect is called per accidens because it is conjoined to the effect per se, it can be called per accidens in two ways: in one way, with respect to the intention of the agent alone; in another way, also with respect to the action itself and the connection of one effect with another. For it happens that one effect is not only beside the intention of the agent, but also is in no way connected with its action, as that one digging the earth should find a treasure. And then there is most properly a cause and effect per accidens. Sometimes, however, one effect is by the nature of the thing connected with another, although it is beside the intention of the agent; as, for example, the corruption of one thing is by the nature of the thing necessarily conjoined with the generation of another, yet it is beside the intention of the natural agent; and for this reason it too is commonly called an effect per accidens, yet not so properly and simply as the former; for inasmuch as it is necessarily connected, it is in some way per se. A sign of which is that there can be science and demonstration of it. Indeed, inasmuch as, in the genus of disposition, the privation of the contrary form is as it were a necessary means for introducing another form, that effect can be said not to be wholly beside the intention of the agent; because, although it is not intended for its own sake, it is yet in some way intended for the sake of the principal end; and therefore, as far as the present matter is concerned, we do not count such effects among those which are from a cause per accidens. Hence all those which are of this kind either consist more in the figure of predication than in causality, as when it is said: the physician heals the musician—of which kind are nearly all the examples given in the former member—or certainly they are reduced to chance and fortune, of which below.

5. Whether the conditions required for acting are causes per accidens.—Besides these two modes, certain conditions necessary for acting are also commonly numbered among causes per accidens, which do not per se inflow into the effect or the action, such as the nearness of the agent to the patient, the removal of some impediment, and any other similar thing, which is commonly called a condition sine qua non. However, this, although it is per accidens with respect to proper and direct causality, yet as regards physical necessity, and inasmuch as it can fall under science, is in a certain manner per se; and therefore of these necessarily required conditions we shall have to speak particularly below. I only note that, since a condition sine qua non of this kind agrees with the principle per se of action in this, that it is required of necessity, it is sometimes not easy to discern in which of the two ways some disposition or property of a thing concurs to an action, namely, whether as a principle per se or only as a condition sine qua non. And therefore sometimes this can be known from the general notion or mode of such a property, as we easily understand that nearness is only a condition, because it is only either a relation or a mode of presence, which of their genus are not active; and in the same way we reason that figure or density are not principles per se of actions, but at most are conditions required for some actions, or for their speed and mode. Sometimes, however, this is more obscure, and must be hunted out from some special notion of the thing in question; as happens in that common question, whether the cognition of the object is only a necessary condition for the appetite to effect its motion, or is also a principle per se co-effecting it; for scarcely can either part be determined by certain reason, and therefore particular conjectures taken from the proper matter must be used. Two things, however, occur to be noted here in general. One is that, whenever it is established by experience that some property is necessary for an action, and from elsewhere no sufficient reason for that necessity can be given except from proper and per se causality, then one must not take refuge in a condition sine qua non, but that is a sufficient sign of some efficiency per se; the best example of which is in the necessity of the impressed species for seeing, which no sufficient one can be understood unless efficiency per se be posited, and therefore it will not rightly be said to be a condition sine qua non. But when a sufficient reason for the necessity of such a condition can be given without efficiency per se, it can easily be understood to be only a condition sine qua non, especially if from elsewhere there is in the cause another power sufficiently proportioned for it to be understood to suffice for inflowing the action; and perhaps so it happens in the said example of the cognition required for the elicited appetite, of which elsewhere. And so much concerning the first division.

The physical and the moral cause

6. What is here meant by the name of physical cause.—The moral cause: which is meant in the present matter.—The twofold mode of acting of the physical cause.—Secondly, the efficient cause can be divided into physical and moral. Physical cause is not taken here for a corporeal or natural cause acting through corporeal and material motion, but is taken more universally for a cause truly and really inflowing into the effect; for, just as we said above that nature sometimes signifies any essence whatever, so a physical influx is sometimes called that which takes place through true causality, real, proper and per se; and in this way God is a physical cause when He creates, and an angel when it effects motion, whether in the heaven or even in itself, and the intellect when it effects intellection, and the will volition, and so of the rest. The morally efficient cause, however, can be spoken of in two ways. For sometimes a cause is called moral only because it effects freely; and in this way a moral cause is not wholly co-distinguished from a physical cause taken generally, as explained by us, but will be distinguished from a physical cause acting naturally and necessarily; for thus the will, when it freely loves, is a true and physical cause of its love, which yet it effects morally or freely. In another way the moral cause is taken as wholly distinguished from the physical, and it is said of that cause which per se does not truly effect, yet morally so conducts itself that the effect is imputed to it; in which way a counselling cause, or a requesting one, or one not impeding when it can and ought, is called a moral cause. And in this latter way we now take the moral cause, so that a physical cause is said to be that which truly effects, a moral one, that which does so only imputatively. Whence it comes about that, considering these things physically or metaphysically, this division is brought back to the former one, of the cause per se and per accidens; for a cause truly effecting physically is said only of the cause per se; but the cause which causes only morally or imputatively, considered physically, is only a cause per accidens, since it does not inflow per se and truly. Hence it is always either a cause not impeding, when it can and ought, or a cause applying or inducing the cause per se, whether through counsel, or through prayers, or by way of merit, or sometimes even through local motion, as when someone applies fire to a house; for although he is the cause per se physical of that motion, yet he is only a cause per accidens of the combustion. But this causality, which physically is per accidens, morally and imputatively is reckoned per se. Of the moral cause taken in this way, therefore, nothing more need be said by us, because inasmuch as it is per accidens physically, it does not fall under science; but inasmuch as it is per se in the genus of morals, its consideration belongs to moral science, not to metaphysics. The physical and true cause, however, can have a twofold mode of acting, namely, either naturally or freely, or necessarily or contingently; and of these two modes of acting we shall speak below, because they pertain especially to explaining both the perfections and natures of the diverse causes, and also the series and connection of the effects of the universe and the mode of inflowing or concurring of the first cause with the second; but of the morality which belongs to free acts, inasmuch as they are free, the consideration does not belong to the present science, but to moral philosophy, or rather to divine theology.

The principal and the instrumental cause: first exposition

7. Thirdly, the true and proper efficient cause is divided into principal and instrumental, the use of which words is commonly very ambiguous and equivocal; wherefore their signification must be diligently explained. The principal cause is commonly said to be that to which the action is properly and simply attributed. But this description does not sufficiently declare the thing; first, because the form of a thing—the soul, for example—is in some way a cause, and not an instrumental one, as is established; therefore, if that division is adequate, it is contained under the principal cause, and yet the action is not properly attributed to it, because the soul, as Aristotle said, does not properly weave; therefore that description does not fit every principal cause. For this principal cause, as the reasoning given declares, is one thing that operates, another that is the principal principle of operating, which are commonly called the principal cause ut quod and ut quo; but the description given fits at most the former cause, not the latter. Indeed, it can further be objected that this description does not fit every cause ut quod, but only the proximate and particular cause, not the universal and first. For, as will be established below, the principal efficient cause is divided into first and second, or into proximate and universal, and each is a principal cause; indeed the first is more principal than the second, and yet the effect is not simply and properly attributed to the first, but to the second; for when a man walks, sees or understands, God is not said to walk, to see or to understand, although He effects those motions more principally than the man. But this objection does not press much; for in truth the effects of secondary causes can be properly and truly attributed to the first, because neither he that planteth is any thing, neither he that watereth; but God that giveth the increase.¹ Hence, although the earth brings forth the green herb, yet it is attributed to God that He bring forth grass for cattle, and herb for the service of men,² and that He forms the foetus in the womb, and finally that He works all our works, as we shall say more at length below. That those denominations of seeing, etc., are not attributed to God, therefore, is because they are not taken from pure efficiency, but rather from the information and reception of the motion or act effected. For one is not said to see who in any way whatever makes vision, but who so makes it that he is informed by it; indeed this denomination is taken chiefly from this latter relation, so much so that if it could remain without efficiency, or at least without principal efficiency, it would suffice for that denomination. There is no doubt, therefore, that the effect, as regards pure efficiency, can be truly and properly attributed to every principal cause operating ut quod.

8. Whether the absolute denomination of agent is attributed to an instrument.—It raises a greater difficulty that this does not seem to be proper to the principal cause, for sometimes it is attributed to the instrumental, as is evident from theological examples; for a man is said to consecrate, to absolve, etc., although he does this only instrumentally; and in physical matters the seed is said properly to dispose the matter, and by means of that disposition to effect the generation. And the reason seems to be that this denomination is properly attributed to the operating supposit, and therefore, if it happens that the instrument is a supposit in some way operating per se, the action can also be attributed to it. For if water is properly and simply said to heat, even though it is only a cause per accidens, solely because it is the supposit sustaining the principle of heating, what wonder that the action is attributed to a man or another supposit, even though he have only an instrumental power of operating? But to this it can be said that this denomination is never attributed to a cause except by reason of some principality (so to speak) which it has in such an operation; as it is attributed to a man that he consecrates or absolves, etc., because in exercising the external actions through which the effects are produced he operates as a principal cause, and because he performs those same actions as master of his actions. The seed, however, is not properly said to generate, nor even properly to organize or dispose the matter, except inasmuch as by its own power it can effect something preliminary, in which it behaves as a principal agent. Finally, water is not said to heat per se, but per accidens, and therefore it is not to the point, because here we are dealing with attribution per se. And that same denomination per accidens has its foundation in this, that water, in the notion of a subsistent and of what sustains the principle of heating, behaves as the principal operator. Thus, therefore, speaking simply, it is proper to the principal cause that the action and the effect be attributed to it. And this could be fitted with proportion to the agent quod and to the principle of acting quo: to the former as operating, to the latter as the principal notion of operating. And it can also be added that principal cause is said absolutely of the cause operating ut quod, but of the principle only reductively and in a qualified sense; and therefore it suffices that the description be fitted in the same way. In this manner, therefore, that definition can be upheld, although the proper reason why the effect is simply attributed to the principal cause is not explained in it, though the proper notion of the principal cause seems to consist in that.

Second mode

9. Otherwise, therefore, the principal cause is commonly said to be that which inflows into the effect by its own power; or otherwise, that which inflows by sufficient power, or which, with the concurrence due to it, is sufficient to produce the effect. By none of these modes, however, does the proper and adequate notion of this cause seem to be sufficiently declared. For to inflow into the effect by its own power belongs not only to the principal cause, but also to the instrumental, especially in natural instruments, whatever be the case with others. For natural heat, which is the instrument for converting nourishment into the substance of the one nourished, has a natural and innate power for that action; and similarly the heat of fire, if it is the natural instrument for introducing the substantial form of fire, has a proper and inborn power for that instrumental action. For neither can it act without proportioned power, nor does it have that power as extrinsic or adventitious; therefore it has it as innate and proper. But if it be said that it has a power indeed, but an instrumental one, this is what we are inquiring, namely, what is lacking to the notion of a principal principle, since it has been shown that it operates by its own power. Nor does what was added in the second place, about sufficient power, seem necessary. First, because just as heat alone does not suffice to introduce the form of fire, so neither does the form alone without heat; therefore by the same reasoning the form will not be a sufficient principle. Then, because the principal cause is to be further divided into total and partial, and each is distinguished from the instrumental; but the partial cause does not operate by sufficient power; therefore it is not rightly separated from the instrumental in this. Moreover, when the power of the principal cause is said to have to be sufficient, either sufficiency in every genus or order of efficient cause is understood, and this is false, otherwise no secondary cause would be principal, because none is absolutely sufficient; indeed not even the first cause, when it concurred with the second, would act as a principal cause, because then it does not operate as sufficient by itself alone. But if sufficiency in its own genus is understood, the instrumental cause too is sufficient in its own order; therefore by this the proper notion of the principal cause is not sufficiently declared. Finally, what is added about due concurrence declares the matter no further, both because the necessity of concurrence does not belong to every principal cause, but only to the second, and also because concurrence is due not only to the principal cause, but also to the instrumental; for concurrence is due to heat both for heating and for producing fire, if the heat is sufficient; by none of these modes, therefore, does the notion of the principal cause seem to be sufficiently declared and distinguished from the instrumental.

Third mode

10. On account of these things there can be another way of speaking: that the principal cause is that which proximately and by its own influx inflows into the effect or into the form constituting the effect. This opinion is better declared by adding at the same time the notion of the instrumental cause and the difference between the two. For the instrumental cause will be that which by its action does not immediately attain the effect or form, but something preliminary from which such a form results, which the instrument does not attain proximately and in itself. Whence it comes about that every instrumental cause is principal with respect to some effect, namely, that which it attains proximately and per se, but instrumental with respect to another which results from it. This is declared by examples, first in artificial things; for a saw, for example, is an instrument for making such an artefact, which instrument proximately effects only the cutting, by impelling and locally moving the intermediate parts, and to this effect it is rather compared as a principal cause; but from it results the artificial form, to which it is compared as an instrumental cause, because it does not attain it immediately by its own action, but it only results from the prior effect. Next, in natural things the heat of fire is the instrument for the form of fire, because it immediately attains heat, with respect to which it behaves as a principal cause; but from that results the substantial form, which it does not immediately attain, according to the probable opinion of Scotus and others. Finally, in supernatural things, according to the opinion of Paludanus, Capreolus and others, the sacraments are called instruments of grace because they attain not grace itself, but something preliminary to it. Hence this way of speaking can not undeservedly be attributed to these authors cited in the second and third examples, whom St. Thomas seems to favour not a little, I, q. 45, a. 5, saying that a secondary instrumental cause does not participate in the action of the superior cause except inasmuch as through something proper to itself it operates dispositively towards the effect of the principal agent. And the reason can be given that the concurrence of an instrument is always imperfect, and therefore it does not belong to it to attain immediately the induction of that form for which it is the instrument, but only to prepare for it by effecting. Hence, conversely, every cause which immediately attains the form will be principal and not instrumental.

11. Instruments attain the form intended by the principal agent.—This opinion, however, cannot be proved, because in the first place it is false that the instrumental cause as instrumental never immediately and in itself attains the form intended by the principal agent; which is evident first by contrary induction. For in supernatural things the sacraments are instruments for effecting grace, attaining it in itself; and similarly Christ, through the actions of His humanity, immediately attained supernatural works, of which much has been said in tomes I and III of the Third Part. Again, in natural things it is more probable that accidents, as instruments of substances, immediately attain the eduction of the substantial form. It is also a probable opinion that the phantasm instrumentally attains the production of the intelligible species, and yet it does not attain something preliminary, but immediately the species itself in itself. Finally, in the instruments of art the form of the effect sometimes seems to be attained proximately through the form of the instrument, as when a like figure is impressed with a seal upon wax or a coin, although even then in truth the figure is not made per se, but the Ubi, whatever Scotus, to be cited below, may signify. Therefore it is said more generally that it is indeed true that these instruments do not per se attain the figure or artificial form; yet this is not because they concur instrumentally, but because that form is not otherwise producible per se. Hence neither does the principal agent itself induce it otherwise; for per se it makes only the local terminus of the motion, which is some Ubi, and because it effects that motion in a certain mode and order, such a figure results from it; therefore in that action the principal and the instrumental agent cannot be distinguished in attaining mediately or immediately. Indeed, if we consider the terminus produced per se, which is the Ubi, the instrument seems to attain it more immediately, because the painter does not move the colour except by means of the brush, and so of the others. The ultimate form or figure, however, is not attained per se by another action, but results from the prior terminus.

12. Moreover, it is declared by reason, because the instrument can be understood to concur to the effect of the principal agent in three ways, namely, either only remotely, by attaining, that is, something preliminary, or only proximately, namely, by attaining immediately the form intended by the principal agent, or in both ways at once; but the notion of an instrument is not limited to the first mode, granted that sometimes it can so concur; therefore the instrumental agent is not rightly distinguished from the principal in that mode of attaining. The minor has been sufficiently declared by the examples adduced. And the reason can be given that, although instrumental concurrence is of an inferior notion, nevertheless, because the instrument does not attain by itself alone, but as aided and strengthened by the principal agent, there is therefore no repugnance in the instrument's immediately attaining the effect of the principal agent. Hence St. Thomas, III, q. 77, a. 3, ad 3, and q. 79, a. 2, ad 3, says: Nothing prevents an instrumental cause from producing a greater effect, and one beyond its species. By which reasoning the foundation of the contrary opinion remains solved; nor does St. Thomas in the other place oppose us, but rather favours this opinion; for he does not say that the instrument concurs only dispositively to the effect of the principal agent, but that it does not participate in the action of the principal agent except by means of its own action. Hence the Thomists gather that every instrument has two actions: one by which it does something proper to itself, in which it does not completely exercise the office of an instrument; the other properly instrumental, by which it immediately inflows into the effect of the principal agent. Which actions Ferrariensis, II Cont. Gent., c. 21, contends are always really distinct. Cajetan, however, I, q. 45, a. 5, more rightly says that this is not necessary, for sometimes there is only one action, in which according to reason or accommodation we distinguish something which the instrument can attain from its own resources, and something which it attains as an instrument. Both, however, agree in this, which pertains to the present matter, namely, that the instrument as such, and through the action which it exercises as an instrument, attains immediately and per se the effect of the principal agent, so far as it is attainable or producible per se. And in truth the notion of a natural instrument is scarcely saved otherwise, except improperly and by extrinsic denomination, as I shall declare below. And Scotus also inclines to this opinion, In IV, dist. 1, q. 1.

Fourth exposition of the principal and instrumental cause

13. A fourth way of speaking can be that the instrumental cause is that which acts only as moved by another; conversely, the principal cause is that which per se and without the motion of another has the power of operating. This explanation seems to be taken from what we experience in the instruments of art; for these are better known to us, and in them there is no other mode of operating. Hence St. Thomas, II Cont. Gent., c. 21, reason 4, says that an instrument is never employed to cause anything except by way of motion, for it is of the notion of an instrument that it be a moved mover; and in the fifth reason he says that the instrument is a medium between the principal cause and the effect, so that the efficacy of the principal agent reaches the effect by means of the motion of the instrument. From which some, dissenting extremely from the third mode treated above, infer that, when the principal and the instrumental cause concur together, the effect is attained more immediately by the instrumental than by the principal—more immediately (I say) by immediacy (as they say) of supposit, because the principal agent does not act except through the instrument; but by immediacy of power the principal agent is said to act more immediately, because the instrument does not act except in its power. And hence this opinion is confirmed by reason, for the instrument as such does not act except in the power of the principal agent; but the instrument cannot rely in operating on the power of the principal agent except because it is moved by it; therefore the notion of the instrumental cause is rightly explained by this, that it acts as moved by another, and consequently the principal cause will be that which does not need the motion of another in order to act.

14. Whether the instrument claims for itself some motion from the principal cause.—But neither does this opinion fully declare the matter or satisfy. For when the instrument is said to act as moved by another, this can be understood in two ways. In one way, of a real motion or change received in the instrument itself and preliminary to its action; and in this way Ferrariensis, II Cont. Gent., c. 21, understands that proposition. In that sense, however, the proposition cannot be universally true; indeed it is scarcely found true except in the instruments of art, which are applied to operating through local motion; but in the instruments of nature, by no means. For if the phantasm is an instrument, what action does it first receive in itself in order to act? Does it receive in itself some spiritual light? But that is fictitious and plainly repugnant to a material subject. Likewise, the heat of fire is its instrument for generating fire, and vital heat or the nutritive power for producing flesh, in which no true antecedent motion can be devised. Likewise, the seed is called the instrument of the generator, which, after it has been separated, receives no new motion from the principal agent. I omit supernatural instruments, of which I have spoken at length in III, q. 13 and 62. I add, next, that not all things which move when moved act as instruments, properly and strictly, but as principal agents, at least partial ones. For certain things do not alter unless altered, as pepper does not heat unless heated, and yet it does not heat only through the received power, but also through its own and innate power, which is an eminent power for heating; whence in this it behaves as a principal agent. Likewise, a man does not move a stick except by moving his hand. But although for this reason the hand is commonly called an instrument, or rather the instrument of instruments, or the instrument before instruments, as Aristotle said, IV De Partib. Anim., c. 10, yet in truth it is a partial principal mover, for it has in itself a partial power of drawing, or impelling, or lifting. Hence in these things the motion of one thing is often only a necessary condition for drawing or impelling another, which is not enough to take away the notion of a principal agent and to induce the proper notion of an instrument. Indeed, even the heaven (as many hold) does not move unless moved, and yet it does not move as an instrument, of which below. Therefore, from the notion or condition of a moved mover through a real change received in itself, the instrumental cause is not rightly distinguished from the principal, because neither are all instruments so moved in order to act, and in almost the same way in which some instruments happen to be moved, some principal causes also happen to be moved. A sign of which is that the instruments of art, even for doing what they can by their own power, need some motion, and through art it comes about only that that motion takes place in an ordered way.

15. Whether creatures are to be called instruments of God.—In another sense, therefore, the instrument can be said to act as moved by another, that is, as subordinated to another, or as taking its place, or also as aided by the principal agent, according to the various kinds of instruments to be declared below. And in this sense it is indeed true that every instrument acts as moved by another (if one may speak so); yet this is not proper to the instrument strictly taken, as we are now dealing with it and distinguishing it from the principal cause; but rather it is common to every inferior principle of acting and to every cause besides the first. For in the first place all secondary causes act as moved by the first, that is, with the concurrence and aid of the first, as we shall declare below, and yet they are not properly instruments, but principal causes in their own genus. I know that some of the ancient authors sometimes call secondary causes instruments of the first, as is evident in St. Thomas, q. 3 De Potent., a. 7, where he concludes thus: Thus, therefore, God is the cause of every action, inasmuch as every agent is an instrument of the divine power operating. The same is taken from I, q. 105, a. 5. And Damascene speaks in the same way in his Physics, c. 9, where he says that this is a doctrine of Plato, whom in this he judges is to be preferred to Aristotle. The same is said by Simplicius, I Phys., text 29. Hence Scotus, In IV, dist. 1, q. 1, § Ad quaestionem ergo, says that a cause can be said to operate principally in two ways. First, principally, that is, independently; and in this way secondary causes are not principal, but instrumental. Secondly, principally, that is, through a proper and intrinsic form; and in this way he says secondary causes are principal. In which, although he does not sufficiently explain the notion of the principal cause, he yet truly says that secondary causes are principal; and in the doctrine of Aristotle it is certain that they are so to be called, as is gathered from II Phys. and V Metaph., in the places cited. And from the very propriety of the word it is established that to operate principally and to operate independently are not the same; therefore, for a cause to be principal, it is not necessary that it be wholly independent. And conversely, that a cause be dependent and in need of the aid of another, superior one, is not enough for it to be an instrumental cause. Moreover, the intellect is not commonly reckoned an instrument, but the principal principle of its natural acts, nor is the will and other similar faculties, although in acting they are subordinated to their forms. It is true that all these faculties, and indeed all accidents, are called instruments by some, and rightly so, if by the name of instrument they understand every principle quo of operating which is not for its own sake, but in order that some supposit may use it; but not if we speak rigorously of instrumental cause and action; for thus heat is not properly an instrument for producing heat, since the effect does not exceed the perfection of the principle or cause.

What the true notion of the instrumental and the principal cause is

16. Various acceptations of instrument.—Since, therefore, the difficulty of this matter seems to depend in great part on the use of terms, it is necessary, lest we labour in equivocation, to distinguish their significations. And beginning with the instrument or instrumental cause: in one way it can be distinguished from the principal cause by antonomasia, or that which operates wholly independently, as Scotus said; and thus the distinction of these members in general is easy; but I do not now speak so, because I think it an improper manner of speech. Secondly, every cause or power which is given to someone that he may operate through it can be called an instrumental cause; in which way the form too can be called the instrument of the supposit, although this is not in use, because the form essentially constitutes the supposit itself; it is said, however, of every other power superadded to the supposit; indeed sometimes even of the integral parts, especially the organic ones, as of the hand and the arm; in which way theologians call the humanity of Christ the conjoined instrument of the Word, which is said more properly in the order to miraculous and supernatural actions than in the order to actions connatural to the humanity. And taken in this second way, any power or principle of acting given to an operating supposit that it may operate through it is called an instrument, even if such power be sufficient and equally perfect, or even nobler than the effect. But the principal cause divided against the instrumental taken in this way will be the operating supposit itself, inasmuch as through itself, or through its substance, or through its form, it has the power of operating. In another way, again, that is called an instrument which is substituted in place of another to execute an action, even if it happens that such an instrument is nobler than the action or effect to which it is destined; in which way gravity is commonly called the instrument of the generator, and impetus the instrument of the thrower; and thus the principal cause respectively opposed to this instrument will be that whose place the instrument takes. And in moral matters this kind of instrumental or ministerial cause is more frequently used.

17. In the last and most proper way, that is called an instrumental cause which concurs or is elevated to effect something nobler than itself, or beyond the measure of its own perfection and action, as heat, inasmuch as it concurs to produce flesh, and in general an accident inasmuch as it concurs to produce a substance. And of this kind of instruments many doubt, not only in natural things, but also in supernatural ones—speaking, as we are speaking, of instruments truly effecting and physically attaining the superior effects themselves—as may be seen in Scotus and others, In IV, dist. 1; and we shall touch on it more at length in the following disputation, in treating of the efficient cause of substances. We, however, suppose that in every order of things instruments of this kind can be given, as we shall show of natural ones in the place cited, and we have said of supernatural ones in the III Part, and of artificial ones all admit it, although in them, as I said, the form chiefly intended by the artificer is not attained per se and immediately by the instrument, but results; yet in whatever way it comes about, it in some way exceeds the power of the instrument considered according to itself, and is made by it as it is directed and moved by art. In this proper and rigorous way, therefore, we take the instrument when we distinguish it from the principal cause.

18. And thus the principal cause will be that which, through principal power, that is, either nobler than or at least equally noble with the effect, inflows into the action through which such an effect is produced. In which way not only the first cause, but also the second, both univocal and equivocal, is comprehended under the name and notion of principal cause. Nor does that notion belong only to the total cause, but also to the partial, if it operates by similar power, because a partial cause properly so called bears a relation to a co-partial cause of the same notion and order, such that neither is subordinated to the other, but from both coalesces one total principal cause of the same order, which is partially from such causes; and therefore each is principal, although partial. And a principal cause of this kind is rightly said to operate by its own power, not only because it has that power of acting as intrinsic and innate, but also because it has it per se proportioned to the effect and not needing any elevation; and although sometimes it needs the concurrence of a superior cause on account of the general notion of participated being, yet not on account of a special disproportion with the effect; and therefore it needs only the concurrence due to it by reason of its perfection.

19. How the instrument is said to operate in the power of the principal agent.—The instrumental cause, however, co-divided against the principal so taken, will be that which inflows into the effect through power of an inferior notion or perfection, as has been declared above by reason and examples. And it is said to operate in the power of the principal agent, not because it does not need some intrinsic power, that is, existing in it, for its instrumental action—whether that power be innate or superadded to the thing, and whether it be permanent and durable when the action ceases, or be fluent (as they say) and consisting in motion; for in various instruments it can be found in various ways; but some instrumental force is necessary, otherwise no action could proceed from such a thing, as Scotus rightly noted above, and I declared at length in tome I of the III Part, disp. XXXI, sect. 6. These instruments, therefore, are said to operate in the power of the principal agent because the force which is in them is disproportioned and insufficient, and has the power of operating only according to the measure of the power and elevation of the principal agent. Hence such concurrence or elevation is not properly or for its own sake due to it, but is due only to the principal agent, or by reason of it. And according to this interpretation all the difficulties touched upon in the other modes cease; indeed many things which are said in them, except in the third mode, are brought back to a fitting sense.

The first and the second cause

20. And from these things the other divisions of efficient causes, which are subdivisions of the said members, are easily understood. For, fourthly, the efficient cause is divided into first and second; where, if we speak broadly, the efficient cause in general can be divided, because every cause depending on another, whether it be principal or instrumental, can be called second; speaking more strictly, however, the principal cause is subdivided there; for one is wholly independent in operating, and this is called first; another, however, is dependent, even though it operates through principal and proportioned power, and this is called the second cause. That both kinds of cause exist in the nature of things is most evident. For either there is given in things some principal cause dependent on another in operating, and from this member the other is necessarily inferred, because one cannot proceed to infinity in dependent causes, and therefore one must stop at some independent one, which we shall pursue more at length below in demonstrating that God exists. But once an independent cause is admitted, the other member is also necessarily inferred, because all causes cannot be independent; indeed such a cause cannot be other than unique, as we shall demonstrate more at length below in treating of the actions of the first cause, and further on in disputing of the unity of God. And therefore of this division we shall say nothing more, because almost the whole of our disputation on the efficient cause will be concerned with these two members.

The univocal and the equivocal cause

21. The instrumental is not an equivocal cause.—Fifthly, the cause is divided into univocal and equivocal; in which partition the cause in general can also be divided, if we call the instrumental cause equivocal. This, however, is said improperly and beyond common usage, as St. Thomas noted, In IV, dist. 1, q. 1, a. 4, q. 1, ad 4, from Alexander and the Commentator, XI Metaph., com. 24. There, therefore, the principal cause is divided, and indeed it can be divided in general, because the first cause too is equivocal. But since the first cause is supposed to be not only equivocal, but also of a wholly different order, that division is therefore more fittingly given of the proximate principal cause. For one effects an effect of the same notion, and this is called univocal, as fire when it generates fire; and universally, the cause which, operating through the power of its form, renders a like effect, is a univocal cause and principal in its order, as St. Thomas rightly notes, III, q. 62, a. 1. Another, however, is the cause producing an effect of another notion, which must be nobler than the effect, otherwise it would not be principal, but instrumental; and this is called an equivocal cause, because it does not agree formally with the effect in the same form, but contains it eminently. Again, the principal cause too can be divided into that which [operates] from necessity of nature, or with freedom. Likewise, into that which acts through motion or change from a presupposed subject, and that which acts without motion and change and without a subject. But these divisions are contained in the foregoing, and only declare determinate actions or modes of operating of the said causes, of all which we must dispute in particular below.

The conjoined and the separated instrument

22. Sixthly, the division of the instrumental cause or instrument into conjoined and separated is to be noted, in which terms there is commonly great equivocation; and therefore it must be diligently observed that an instrument can be understood to be conjoined to the principal agent (for it is with respect to that that an instrument is called conjoined or separated) in two ways, namely, either according to being or according to causality. I call conjoined according to being that which is united in some way, whether by contact, or by some presence, or by some real union, to the principal agent, in which way the pen is a conjoined instrument, etc.; the separated, however, opposed to this, will be that which is in no way conjoined to the principal agent, as is the seed once separated; and according to the various modes of conjunction and separation there can be latitude and variety in these terms. According to causality, I call conjoined that instrument which, in order to cause, needs the actual and proper influx and causality of the principal agent, in which way the pen too is a conjoined instrument; whence, conversely, the separated instrument will be that which in its action does not require a special influx and causality of the principal agent. And in this way, if heat be called the instrument of fire for heating, although it is conjoined according to being, it can be called separated according to causality, because it needs no other influx in order to heat besides its own proper power. Whence it sometimes comes about that an instrument can be conjoined in one way and separated in another, as is evident in the said example. And conversely, the sacraments are called separated instruments of the humanity of Christ existing in heaven, namely according to being, and yet according to causality they are perhaps conjoined, because they can effect nothing except with Christ actually inflowing together with them through His humanity. Sometimes an instrument can be conjoined in both ways, as sight or the impressed species; sometimes separated in both ways, as the gravity of the thing generated with respect to the generator. All these things are clear, if the terms are understood. One thing only remains difficult in the matter: how there can be an instrument separated according to causality, speaking properly and strictly of an instrument according to the last mode declared above; for if it be taken more broadly for a power per se sufficient for the effect, which takes the place of that cause which impressed it, then the matter is clear, as in the impetus of projectiles, gravity, etc. Yet in a proper instrument, which must be elevated to act for a superior effect, as is the seed, for example, this is most difficult to understand. But the exposition of this matter requires many things to be explained first, which will be handed down in the following disputations.