Types of Efficient Causes
Quotes from Suarez, DM 17, sect. 2
"A per se cause is a cause on which the effect directly depends with respect to that proper esse that it has insofar as it is an effect, in the way in which (says Aristotle) a sculptor is a cause of a statue."
"On the other hand, since a per accidens cause is not a true cause but is instead called a cause because of some relation or similarity to a cause or because it is conjoined with a cause, it cannot be appropriately defined by a single general description; rather, a cause is called per accidens in various senses. For a cause is called per accidens sometimes on the side of the cause and sometimes on the side of the effect.
"That which is said to cause per accidens on the side of the cause is something that is accidentally conjoined to a per se principle of causing. Sometimes this is the very subject of the form that is the principle of acting. It is in this sense that water is said to produce heat per accidens, since it is accidental to water that it should be hot and thus accidental to it that it should produce heat. "And it is in this sense," Aristotle says, "that Polycletus is a cause of a statue" [Metaphysics 5.2.1013b34-1014a4]. In some cases, however, the thing in question is a second form that is accidentally conjoined to the first form in the same subject--I mean that it is accidental that it should be related to the power of effecting, regardless of whether the two forms are conjoined necessarily in some other sense. This is the sense in which a singer writes per accidens and in which what is white produces heat per accidens, etc. ...
"It is also customary in some cases for a cause to be designated as per accidens on the side of the effect, i.e., with respect to that which is accidental to the per se effect. And in this sense a per se cause of a given effect is itself a per accidens cause of that which is conjoined to the per se effect--in the way in which a motion is a cause of heat /584a/ or in which a hot thing is a cause of something black. This is also the sense in which things that happen by chance or by fortune are said to have a per accidens cause--as, for example, that someone who is digging should discover a treasure.
"Again, one should note that when an effect is called per accidens by reason of the fact that it is conjoined to a per se effect, there are two possible ways in which it can be called per accidens: in one way, with respect to the agent's intention alone; in another way, with respect to the action itself, too, and with respect to the connection of the one effect with the other. For it is possible for an effect not only to fall outside the agent's intention but also not to be connected in any way with the agent's action--as, for example, that one who is digging in the ground should discover a treasure. And in such a case the cause and the effect are per accidens in the most proper sense. In some cases, however, the one effect is connected by its nature with the other effect, even though it falls outside the agent's intention. For example, the corruption of one thing is necessarily conjoined by its nature with the generation of another; yet [this corruption] falls outside the natural agent's intention and for this reason it, too, is commonly called a per accidens effect--though not in as proper and absolute a sense as in the previous case. For to the extent that [the corruption] is connected necessarily [with the generation], it is in some sense per se. An indication of this is that there can be scientific knowledge and demonstration with respect to it. Indeed, since within the genus of a [positive] disposition the privation of the contrary form is, as it were, a necessary means for the introduction of the relevant form, one can claim that the effect in question does not fall altogether outside the agent's intention. For even though this effect is not intended for its own sake, it is nonetheless in some sense intended for the sake of the principal end, and so, as far as the present discussion is concerned, we are not counting effects of this sort among those that are per accidens on the side of the cause."
"Second, efficient causes can be divided into physical causes and moral causes. In this context `physical cause' is not being taken for a corporeal or natural cause that acts by means of corporeal and material motion; instead, it is being taken more generally for a cause that has a true and real influence on the effect. For just as we claimed above that in some cases the term `nature' signifies any essence whatsoever, so too sometimes the term `physical influence' is used for that which is effected through true, real, proper, and per se causality. And in this sense God is a physical cause when He creates, and an angel is a physical cause when he effects a motion in the heavens or even within himself, and the intellect is a physical cause when it effects an act of understanding, and the will is a physical cause when it effects a volition, and so on.
"On the other hand, there are two senses in which a cause can be called a morally efficient cause. For* sometimes a cause is called a moral cause solely because it acts freely, and in this sense a moral cause is not altogether distinct from a physical cause when the latter is taken in the general sense that we have explicated; instead, a moral cause will be distinguished from a physical cause that acts naturally and necessarily. For in this sense the will, when it freely loves, is a true and physical cause of its own love, which it nonetheless effects morally, i.e., freely.
"However, `moral cause' is taken in another sense according to which a moral cause is altogether distinct from a physical cause, and it is predicated of a cause which does not truly bring about the effect per se but which behaves morally in such a way that the effect is imputed to it. This is the sense in which an advising cause, or an imploring cause, or a cause that does not prevent something when it can and should prevent it is called a moral cause. And it is in this second sense that we are now taking `moral cause', so that a cause that truly effects something is being called a physical cause, whereas a cause that effects something only by imputation is being called a moral cause. It follows that if we consider these things from a physical or metaphysical point of view, then this division is reduced to the previous division into per se causes and per accidens causes. For `cause which truly effects something physically' is predicated only of a per se cause, whereas a cause which causes only morally or by imputation is, when considered from a physical point of view, /585b/ only a per accidens cause, since it does not have a real and per se influence. Thus a moral cause is always either (i) a cause that does not prevent something when it can and should prevent it or else (ii) a cause that applies or induces a per se cause, whether by means of advice or entreaties or payment or sometimes even by means of local motion, as when someone applies a fire to a house. For even though the individual in question is a per se physical cause of the motion itself, he is nonetheless only a per accidens cause of the burning. But this latter causality, which is per accidens from a physical point of view, is regarded as per se from a moral point of view and by imputation. Therefore, we do not have to say any more about the moral cause taken in this sense. For to the extent that it is per accidens from a physical point of view, it does not fall under scientific knowledge, whereas to the extent that it is per se within the genus of morals, the consideration of it pertains to moral science and not to metaphysics."
A. First explanation
"A principal cause is commonly said to be a cause to which an action is attributed properly and absolutely. However, this description does not sufficiently clarify the matter. For, first of all, a thing's form, e.g., a soul, is in some sense a cause, and, as is obvious, it is not an instrumental cause. Therefore, if the division in question is adequate, then a soul is classified as a principal cause. And yet action is not properly attributed to it, since a soul, as Aristotle said, does not properly act. Therefore, the description in question does not apply to every principal cause. For, as the argument just given makes clear, there is one sort of principal cause which operates and another sort which is a principal principle of operating--they are commonly called, [respectively], a principal cause ut quod and a principal cause ut quo. But the description given above applies at most to the former of these causes and not to the latter."
B. Second explanation
"Second, a principal cause is commonly said to be a cause that influences the effect by its own power or, alternatively, a cause that influences the effect by a sufficient power, i.e., a cause which, given the concurrence that is owed to it, is sufficient to produce the effect."
C. Third explanation
"There is another possible way of speaking, according to which a principal cause is a cause that influences the effect (or the form that constitutes the effect) proximately and by its own proper influence. This position becomes clearer if we immediately add the definition of an instrumental cause and the difference between the two: An instrumental cause will be a cause which does not immediately attain to the effect or form by its own action, but which instead /587b/ attains to antecedent thing from which the form in question results, a form that the instrument does not attain to proximately and in itself. From this it follows that every instrumental cause is a principal cause with respect to some effect, viz., that effect which it attains to proximately and per se, whereas it is an instrumental cause with respect to another effect which results therefrom."
D. Fourth explanation
"There is a fourth possible way of speaking according to which an instrumental cause is a cause that acts only insofar as it is moved by another, whereas, by contrast, a principal cause is a cause that has the power to operate through itself and without the motion of another. This explanation seems to be derived from the things we know by experience in the case of the instruments of a craft. For these things are better known to us, and in their case there is no other mode of operating."
E. Fifth and correct explanation
"Lastly, and in the most proper sense, an instrumental cause is
said to be a cause that concurs in, or is elevated to, the production of
something more noble than itself, i.e., something beyond the measure of
its own proper perfection and action--for example, heat insofar as it concurs
in producing flesh and, in general, an accident insofar as it concurs in
producing a substance."
"Fourth, efficient causes are divided into first causes and secondary causes. Now if we speak loosely, then it is efficient causes in general that can be divided here, since every cause that depends on another--be it a principal cause or an instrumental cause--can be called a secondary cause. However, if we speak more strictly, then it is principal causes that are being divided here. For there is one sort of cause that operates altogether independently, and this is called a first cause, and there is another sort of cause that is dependent, even if it operates by means of a power that is principal and proportionate, and this is called a secondary cause."
"Now there are some [principal] causes that produce an effect of the same type, and these are called univocal causes, e.g., fire when /592a/ it generates fire. And, in general, a cause which, operating through the power of its own form, produces a similar effect is a univocal cause as well as a principal cause with in its own order, as St. Thomas correctly notes in Summa Theologiae 3, q. 62, a. 4. On the other hand, there is another sort of cause that produces an effect of a different type. This sort of cause has to be more noble than the effect--otherwise, it would not be a principal cause, but an instrumental cause--and it is called an equivocal cause, since it does not formally agree with the effect in the same form, but instead contains that form eminently."
"What I am calling a conjoined instrument secundum esse is an instrument that is united to the principal agent in some way or other, whether through contact or through some sort of presence or through some real union--in the way that a writing pen, say, is a conjoined instrument. By contrast, the opposed separated instrument will be an instrument that is not conjoined to the principal agent in any way--as, e.g., semen after it has been separated. And corresponding to the various modes of conjunction and separation there can be some latitude and variation in these terms.
"What I am calling a conjoined instrument secundum causalitatem is an instrument that requires the principal agent's actual and proper influence and causality in order to cause--in the way that, once again, a writing pen is a conjoined instrument. Hence, the corresponding separated instrument will be an instrument that in its action does not require the principal agent's special influence and causality. And in this sense, if heat is called a fire's instrument for producing heat, then even though it is conjoined to the fire secundum esse, it can be called a separated instrument secundum causalitatem, since in order to produce heat it does not require an influence over and beyond its own proper power."