I. Introduction

Causes are those things or principles that are responsible for something else’s being. For example, the cause of the spill on my desk is that which was responsible for knocking over my coffee cup. In ordinary life, we try to explain things we observe by giving an account of their causes. Natural scientists likewise aim to explain events in nature by uncovering their causes. Once we know the causes of certain types of effects, we can predict their occurrence in the future. Sometimes we can even manipulate entities of the same type as those causes to bring about similar effects. For example, once we know that sunlight causes plants to grow, we can cause future growth in plants by placing them in a sunny area. Causation is an important concept both in ordinary life and in scientific inquiry, and it is also a concept crucial to theology. Theologians have engaged in longstanding debates about topics regarding causation, such as how the sacraments cause grace and how divine causal agency relates to the causal agency of creatures. Accordingly, it is important for Catholic theologians to have familiarity with the history of thinking about causation and especially the most significant developments in the Catholic intellectual tradition. This article provides an overview of ancient, medieval, modern, and contemporary approaches to causation. Special attention is given to the causal theories of Thomas Aquinas, given his importance to the Catholic intellectual tradition and the development of Catholic theology.

II. Ancient Approaches to Causation: Presocratics, Plato, and Aristotle

In the Greek world of epic poetry, events in nature were explained by appeal to the actions of deities who wielded power over different domains of the cosmos. Natural phenomena, such as eclipses and lightning strikes, could not be predicted in advance and their explanations could not be generalized since they were thought to depend on the free behavior of gods who were subject to fleeting emotions and whims. In context of this historical background, the Presocratic philosophers’ development of natural theories to account for and even predict natural occurrences marked a great achievement.

Presocratic theories focused primarily on the material principles of natural things. The material principle is that out of which natural things are made and that into which they will resolve. Thales (c. 585 BC) conceived of this principle as water, while Anaximander (c. 560 BC) held it was a certain unlimited nature not to be identified with any particular thing or element, and Anaximenes (c. 586/585–c. 526/525 BC) thought it to be air. The Presocratics appealed to their theories about material principles to explain observable natural phenomena. Thales, for example, explained earthquakes by appealing to his theory that the earth rests on water (Hankinson, “Reason, Cause, and Explanation in Presocratic Philosophy,” 454). Though the Presocratics did not focus on the full range of causes which Plato and Aristotle later developed, they are nevertheless important in the history of thinking about causes because they tried to explain physical phenomena in terms of regular, predictable patterns in the natural world. As Hankinson puts it: “…everything was the manifestation of the fundamental physical natures of things” (see Hankinson, “Reason, Cause, and Explanation in Presocratic Philosophy,” 454). This approach laid the foundation for later developments in natural philosophy and thinking about causation.

Plato’s (428/427–348/347 BC) approach to explaining the natural world marked a shift away from material causes in favor of emphasizing the patterns or structures exemplified by material things. Plato posited that apart from the material world there was a realm of eternal, immutable exemplars called Forms. The explanation for why imperfect, changing material objects have certain central characteristics is that they participate in specific Forms. For example, a sunset has the characteristic of beauty because it participates in the Form of Beauty and a triangle has three angles because it participates in the Form of Triangle. Plato thought that the object of true knowledge was the Forms, rather than material particulars which were always subject to change (see Hankinson, Cause and Explanation in Ancient Greek Thought, 84–124).

Aristotle (384–322 BC) systematized the different types of causal explanations offered by earlier thinkers into four types of causes: formal, material, efficient and final (Physics II.3, Metaphysics V.2). Unlike Plato, Aristotle did not see forms as extrinsic exemplars that material substances imitated. In Aristotle’s view, each material substance is composed of form and matter as intrinsic constituents. Matter is a characteristic-less receptive principle, while form is that which actualizes matter to compose a determinate type of natural thing. For example, an individual tree is a composite of matter and form, and it is a tree, rather than another type of material substance, in virtue of its form. For Aristotle, change involves matter gaining and losing forms. Efficient causes are the types of causes that initiate change or cause a form to be in matter. The final cause is the end or goal for the sake of which the efficient cause acts. Aristotle thought that explanations of natural phenomena make reference to each of these types of causes (see Falcon, “Aristotle on Causality”).

III. Medieval Approaches to Causation: Occasionalism and Scholastic Aristotelianism

During the medieval period some figures denied that creatures were genuine causes of effects and instead defended a view known as occasionalism. According to occasionalism, God is the only active efficient cause and thus, created substances in the material world do not actively produce any effects. Created substances function merely as passive occasions for God to act. For example, when fire comes into contact with cotton, the fire does not truly burn the cotton. Rather, fire’s contact with cotton is merely an occasion for God to produce burning in the cotton. Occasionalism was defended by many Islamic figures, with Abū Ḥāmid al-Ghazālī (d. 505/1111) perhaps being the most well-known today (see Perler and Rudolph, Occasionalismus: Theorien der Kausalität im arabisch-islamischen und im europäischen Denken). The view was less popular among Christian thinkers, though there were some Christians who also denied that creatures were active causes of created effects. For example, it has been contended that William of Auvergne held such views (Reilly, “Thomas of York on the Efficacy of Secondary Causes”). Thomas Aquinas (1224/5–1274) knew of the occasionalist position and argued against it in several places in his works (On Aquinas’s critique of occasionalism see Fakhry, Islamic Occasionalism and Its Critique by Averroës and Aquinas). 

Medieval Christian approaches to causation drew heavily on ancient sources. Plato’s theory of Forms especially influenced medieval approaches to God’s causation. Augustine (354–430) and other Christian thinkers reconceived of Platonic Forms as divine ideas in the mind of God (On the reconception of Plato’s forms as divine ideas see McIntosh, The Divine Ideas Tradition in Christian Mystical Theology). Divine ideas functioned as exemplars used by God in his creation of particular creatures.

Aristotle’s theory of the four causes and especially his views on efficient causation heavily influenced medieval views on the causality of creatures. In what follows, Aquinas’s key views on causation will be discussed in depth to illustrate how medieval figures both adopted and transformed Aristotelian ideas about causation.

IV. Thomas Aquinas’s Key Views on Causation and Scholastic Debates about Causation

Thomas Aquinas is widely regarded as the most influential figure in the Catholic philosophical tradition. Though he wrote no separate treatise devoted exclusively to the topic, Aquinas nevertheless had well-developed and significant views about causation. His approach to causation is important not only for understanding Catholic thought, but also for appreciating the trajectory of historical thinking about causation in Western philosophy in general.{1} While Aristotle is the source who had the most influence on Aquinas’s views on causation, Aquinas’s theories nevertheless cannot be reduced to Aristotle’s (For a summary of differences between Aristotle’s and Aquinas’s views on efficient causation, see Meehan, Efficient Causality in Aristotle and St. Thomas, 399–405. For an overview of Aquinas’s theories on efficient causation, see Frost, Aquinas on Efficient Causation and Causal Powers.) At times Aquinas departs from Aristotelian views to accommodate theological doctrines unknown to Aristotle. Aquinas also develops aspects of Aristotle’s views in new directions in light of the neo-Platonic and Islamic sources he also draws upon. Some notable examples of Aquinas’s creative synthesis of Aristotelian ideas with Christian theology and other philosophical sources will be discussed below.

Aquinas on the Nature of Causation, and Causes vs. Principles

Aquinas conceives of the notion of a “cause” as a species of a wider genus, namely “principle.” A principle is a beginning or that from which anything else follows. For example, one thing can follow from another according to number, as nine follows from ten, or according to time, as a future time follows from the present moment. The lower number and earlier times are considered principles since there is an order of numbers or times in which they precede another term which follows them. Causes are a specific type of principle. The distinctive characteristic of causes is that existence follows from them (De principiis naturae c. 3).{2}

In Aquinas’s view, the relationship between causes and their effects is one of ontological dependency. He writes regarding causes in general: “[T]he name cause implies a certain influence on the being of the thing caused” (Super Metaphys. V lect. 1, n. 751). Elsewhere he writes: “It is necessary for an effect to depend on its cause. This is part of the notion of effect and cause” (De pot. q. 5, a. 1, ed. Pession, 35). In Aquinas’s view, causation is a two-way ontological relationship: Effects depend on their causes for their existence, and causes give rise to the existence of their effects. Causes need not bring their effects into being simpliciter. A cause may be responsible for something which already exists coming to exist in a new way. For example, the sun might cause a pre-existent strawberry to exist as ripened or reddened.

Aquinas’s conception of the cause-effect relationship is noteworthy because, as will be explained below, competing theories of causation developed in the modern and contemporary periods deny that the causal relationship involves real ontological dependence. These theories instead conceive of causation as a logical relationship between events, such as entailment or counterfactual dependence. From Aquinas’s perspective, it is the dependence in being of an effect upon its cause that unifies the multitude of different instances of causation as instances of causation. Aquinas follows Aristotle in adopting the four-cause schema. The different species of causes are distinguished from one another according to the different way each cause influences the being of its effect. A substance depends on its matter, for example, by being composed of it, while it depends on its efficient cause for introducing its form into its matter. The substance genuinely depends for its being on each of its four causes, yet it depends on each in a different way.

Aquinas’s Conception of Efficient Causation and Divine Activity

In defining efficient causation Aquinas, unlike Aristotle, sought to make room for God’s creation and conservation of creatures as types of efficient causation. When natural created agents act as efficient causes, they act upon pre-existing matter to actualize new forms in it. Creation and conservation, however, are not acts of changing something pre-existing. Through creation, God produces both the form and matter of material substances ex nihilo. Through conservation, he preserves created substances as they are without changing them. In Avicenna’s texts, Aquinas encountered the idea that, despite these differences, divine activity should nevertheless be conceived of as a type of efficient causation alongside the efficient causation exercised by creatures, rather than a sui generis type of causation.{3} Since Aristotle did not explicitly accept the doctrines of creation and conservation, he was content to define an efficient cause as a source of motion or change (Meehan, Efficient Causality in Aristotle and St. Thomas, 185–188). By contrast, Aquinas defines efficient causes as those causes which cause their effects by action or operation since this definition does not exclude creation and conservation. Aquinas writes: “That which causes something through its operation causes it as an efficient cause” (De Verit., q. 28, a. 8, ed. Leon., vol. 23 3/1, 843). Elsewhere he states: “An efficient cause is a cause in so far as it acts” (Super Metaphys. V, lect. 2, n. 775 [ed. Marietti, 213]). Action, also called operation, is the unique way in which efficient causes influence the being of their effects. In Aquinas’s view, actions are exercises of active power. He writes: “Action is properly the actuality of a power…” (ST I, q. 54, a. 1, ed. Leon., vol. 5, 39). Elsewhere he states: “[N]othing is able to act except through an active potentiality existing in it…” (ScG II, c. 60, n. 1375). Accordingly, efficient causes are set apart from the other types of causes in so far as they cause by exercising powers. Fire, for example, is the efficient cause of heating water when it exercises its power to heat upon the water. God’s act of creation is an act of efficient causation because God causes creatures by exercising an active power. God’s creation and conservation of creatures and creatures’ causation of changes in one another are all the same type of causation, that is, efficient causation, because they involve causing an effect by exercising an active power in action.{4}

Aquinas on Active Power and Passive Power

Aquinas thinks that paradigmatic instances of efficient causation in the natural world involve one substance, referred to as an agent, causing a motion or change in another substance, referred to as a patient. As we saw above, Aquinas thinks that agents act by exercising an active power. He writes: “For every action, there are two things to consider, namely the thing itself which acts, and the power (virtutem) by which it acts, just as fire heats through heat” (ScG III, c. 70, n. 2464). It is likewise through a passive power that a patient is able to undergo a specific type of action. For example, not every type of substance is able to be burned, but rather only those which are combustible. Powers are the intrinsic features by which substances are able to cause and undergo specific types of action. Each type of material substance has specific active and passive powers.

Aquinas identifies the active powers of material substances with their forms. He writes: “[T]he potentiality for being is from the part of matter, which is a being in potentiality, but the potentiality for acting is from the part of form” (ST I-II, q. 55, a. 2, ed. Leon., vol. 6, 351).{5} Aquinas thinks that forms are principles of activity because they are the principles of actuality, that is, principles by which substances are actualized in determinate ways. And he sees actuality as connected with activity because actuality is a principle of perfection and completion. Following the neo-Platonic tradition, Aquinas thinks that whatever is good or perfect necessarily seeks to communicate its goodness to another. He writes: “Communication of being and goodness arises from the good. This is evident from the very nature of the good and from its definition (ratione)” (ScG I, c. 37, n. 307). It is part of the very nature of goodness to make others good as well and being in act is a type of goodness.{6} So, Aquinas thinks that beings which are actual act through the principle by which they are in act—that is, their forms—to make other beings actual in the same way. He writes: “To act towards some effect is proper to a being-in-act: for each one acts in so far as it is in act. Therefore, every being in act is apt to make something existing in act” (ScG II, c. 6, n. 881). For example, existing as actually hot is a certain good that fire has. Since that which is good is necessarily diffusive, it is by the same principle according to which fire is actually hot—that is, its form of heat—that it causes heat in another.

Aquinas’s conception of form as a diffusive good explains both why agents are active through their forms and what sort of effect each agent is apt to produce.{7} Aquinas believes that every agent makes an effect like itself. He writes: “The proximate end of each agent is to induce a similitude of its form in another, just as the end of fire’s heating is to induce a similitude of its heat in the patient” (ST II-II, q. 123, a. 7, ed. Leon., vol. 10, 16). Good things do not make other things good in any random way, rather they seek to make other things good in the same way that they are good. This explains why natural agents act toward a single, determinate effect. Aquinas writes: “… every natural agent acts toward a determinate species because every agent makes its like” (Super Metaphys. X lect. 11, n. 2134 [ed. Marietti]).

Aquinas conceives of an agent’s action as its communication of its form to another. He writes in his De potentia: “To act is nothing other than to communicate that by which the agent is in act in so far as it is possible” (De pot. q. 2, a. 1 [ed. Pession, 25]). In his Commentary on the Sentences, he similarly writes: “In any alteration or generation the agent multiplies its form in matter” (Super Sent. II, d. 30, q. 2, a. 1 [ed. Mandonnet-Moos, vol. 2, 780]). He explicitly states that the agent does not hand over its own form to the patient or deposit something extrinsic into the patient (ScG III, c. 69, n. 2458; De pot. q. 3, a. 8 ad 10 [ed. Pession, 62]). Rather, the agent forms the patient’s matter to be actualized with a form like its own.

In Aquinas’s view, matter also has an important role in causal interactions between material substances. Matter is the principle through which material substances undergo action. He writes: “No thing, whatever its degree of materiality, receives anything according to that which is form in it, but only according to that which is material in it ... Corporeal things do not receive any impression in virtue of form, but only in virtue of matter” (De verit., q. 9, a. 1 ad 12 [ed. Leon., 22 2/1, 282]). Patients are able to be acted upon by agents because they have a component that is able to be formed by the agent, namely matter. The complementarity between form and matter is what allows agents to act and patients to suffer. Aquinas writes: “There is mutual action in corporeal substances because the matter of one is in potentiality to the form of the other and vice versa” (De pot. q. 3, a. 7 [ed. Pession, 57]).

Aquinas makes clear in certain passages that it is not prime matter (i.e. matter devoid of any form) through which substances undergo change. Rather, substances undergo determinate types of change through their matter taken together with specific forms that actualize it. For example, Aquinas states that through being actualized by the form of dryness, a substance is combustible (Q. D. De anima, q. 12 [ed. Leon., vol. 24/1, 108]). It is through its matter that a substance can receive form in general, but in order to receive the specific forms proper to the change of combustion, a substance’s matter must be prepared by the form of dryness. Wet substances cannot undergo combustion. Aquinas believes that for every active power, there is a determinate type of passive power (Super Metaphys. IX, lect. 6, n. 1834). This is to say that for every form which can be communicated, there is a determinate potentiality for receiving that form. It is through its matter and specific forms which actualize it, that a substance has determinate passive potentialities.

Aquinas on the Causality of Final Causes

Aquinas thinks that the end or final cause is the most fundamental of the four causes. He claims that the final cause is “the cause of all causality” since if it were not for the sake of its purpose or goal, the agent would not act (De prin. natur., ch. 4 [ed. Leon., vol. 43, 44]; ST I, q. 105 a. 5 [ed. Leon., vol. 5, 476]). As we saw above, natural efficient causes cause form to be in matter and Aquinas thinks that efficient causes only operate in so far as they intend to achieve some goal through their action.{8} For example, the cause of a person’s taking a walk is the health they hope to achieve through walking. Likewise in the natural realm, the cause of fire’s act of heating is the heat fire intends to communicate to another substance. Final causes have been subject to many objections in the history of philosophy.

It is a standard assumption that causes must be prior to their effects. Final causes, however, do not exist prior to the actions which they cause. For example, the health that a person hopes to achieve through walking does not exist until after the person has engaged in much walking. So, how can that health be a cause of the walking if it does not yet exist until after the walking has already happened? Likewise, how can the heat that fire seeks to communicate to another cause the fire’s act of heating if that heat does not exist until after the heating has occurred? This worry led early modern figures to reject final causes. (For a brief overview of early modern debates about final causality, see section 8 in Nadler, “Doctrines of Explanation in Late Scholasticism and Mechanical Philosophy.”) Spinoza expresses this apparent problem with final causes when he writes: “This doctrine concerning the end turns nature completely upside down. For what is really a cause, it considers an effect, and what is an effect it considers as a cause. What is by nature prior it makes posterior” (Spinoza, Ethics, Part I, Appendix, [trans. E. Curley, 28]). Since the efficient cause’s action is that which causes the end to exist it seems backwards to claim that the end is a cause of the agent’s action. How can an end be the cause of an action if it depends on the action for its being?

Aquinas himself considered this objection.{9} In response, he writes: “Even if the end is last in the order of execution, it is nevertheless first in the intention of the agent. In this way, it has the nature of a cause” (ST I-II, q. 1, a. 1, ad 1 [ed. Leon., vol. 6, 6]; De pot. q. 3, a. 16 [ed. Pession, 87]). As the response implies, Aquinas draws a distinction between two different ways an end can be considered: as the end of execution and as the end of intention. The end of execution is the actual terminus of the agent’s action, while the end of intention is the object which the agent intends to reach. The end of execution is the agent’s effect. The end of intention, however, is the end which functions as a final cause of the agent’s action. It is through being intended by an agent that the end causes the agent’s action. The end which the agent intends, not the effect which it actually produces, is that on account of which it acts. For example, it is the health that a person intends and not the health that later exists in her body that causes her to walk. Aquinas’s approach to this issue is another example of the influence of his Islamic sources. Prior to Aquinas, Avicenna had claimed that the end functioned as a cause in so far as it exists in the soul of the agent.{10}

Aquinas applied this conception of the causality of final causes to non-human causes as well. In his view, even non-intellectual natural causes intend goals. He writes: “to intend is nothing other than to have a natural inclination toward something” (De prin. natur., c. 3 [ed. Leon., vol. 43, 42]). In Aquinas’s view, intention does not require an agent’s cognizing and consciously desiring a goal. Intention only requires an inclination toward a further state. Because goodness is self-diffusive, all beings that are good in a particular way are inclined in virtue of their goodness toward producing that same manner of goodness in another. Fire, through its form of heat, has a natural inclination toward heating other things and fire’s action of heating is caused by this inclination.

Per se vs. Per accidens Efficient Causation     

Aquinas realized that both human causes and natural causes often cause effects that they did not intend. Accordingly, he followed Aristotle in distinguishing between two different types of causes: per se causes vs. per accidens causes. Aquinas writes that a per se cause “produces its effect through the power of its nature or form, thus it follows that the effect is in itself intended by the cause” (ST I-II, q. 85, a. 5 [ed. Leon., vol. 7, 115]). Per se efficient causes intend the effects they produce and they produce these effects through a power that they have through their nature to produce such an effect. An example of per se efficient causation is fire that heats something through the inherent power it has to heat. Per accidens causes, by contrast, produce unintended effects which are not the natural result of the exercise of one of their inherent active powers. Aquinas writes that a cause is “called a per accidens cause of an effect because that effect which follows is outside of the intention of such a cause...” (De pot. q. 3, a. 6 ad 6, ed. Pession, 19). Efficient causes sometimes produce unintended effects because the qualities of the patient upon which the agent acts, as well as the influence of other agents, impact the final effect that follows from an agent’s power. Aquinas gives an example. He says that fire can sometimes cool a substance if through heating, the substance’s pores are opened, and an inner heat is lost (Super Metaphys. IX, lect. 2, n. 1789). Fire has no active power for cooling, and thus, it has no inclination toward cooling. Yet, the fire nevertheless causes coolness by its action because of how its causality is received in the patient. It is important to note that per accidens causation derives its efficacy from per se causation. Aquinas writes that: “Per se causes are prior to per accidens causes” (ScG II, c. 39, n. 1155). Agents cause their per accidens effects in virtue of causing a prior effect according to per se causation. For example, the fire which cools a substance by opening its pores, causes the cooling in virtue of causing the heating by which it opens the pores.

Types of Efficient Causes: Perfecting, Preparing, Advising and Assisting (Instrumental) Causes

In addition to distinguishing between per se and per accidens causes, Aquinas also adopted from Avicenna a four-fold division between types of efficient causes. Aquinas recognized that efficient causes often act together in networks and influence each other’s actions and effects. To capture the different ways in which the action and effect of one efficient cause might also be attributed to the agency of another efficient cause, Aquinas followed Avicenna in positing four different types of efficient causes: perfecting, preparing, advising, and assisting (Super Metaphys. V, lect. 2, n. 766–769; Super Phys. II, lect. 5, ed. Leon., vol. 2, 70, n. 5).

The nature of the first two causes can be illustrated with an example. Consider a carpenter who makes a bench. In addition to himself, the final effect of the bench can be attributed to the people who did tasks such as cutting and sanding the boards out of which the carpenter made the bench. The carpenter is the perfecting cause because he induces the form of the bench in the materials. Aquinas writes: “The efficient cause which is said to be a perfecting cause is the one which causes the ultimate perfection of a thing…” (Super Metaphys. V, lect. 2, n. 766 [ed. Marietti, 212]). The causes that prepared the materials so that they can take on the form of the bench are the preparing or disposing causes of the bench. Aquinas writes: “The disposing cause, however, does not induce the final perfecting form, but rather only prepares the matter for that form…” (Super Metaphys. V, lect. 2, n. 767). Put otherwise, the preparing or disposing cause efficiently causes the potentiality for the final effect.

An advisory cause is the cause which gives the perfecting cause its form and therefore, its end or goal. In human affairs, an advising cause is the one who proposes a form to the human agent’s intellect (Super Phys. II, lect. 5 [ed. Leon., vol. 2, 70, no. 5]). By proposing a form to the intellect, the advisor gives the advisee the goal for which they act. The designer who drafted a building plan for the bench would be the advising cause for the carpenter in the earlier example. In the realm of nature, Aquinas claims that the agent which generates a natural substance is its advising cause. By inducing a substance’s substantial form, the generator causes the substance it generates to have certain ends and thus, to act in a certain way. Thus, by generating a substance, the generator becomes an advising cause of the generated substance’s future actions and effects.

Assisting, or instrumental, causes are causes that help another efficient cause, known as a principal cause, to reach its end. The principal cause determines the end for which the instrumental cause acts, but the instrumental cause operates toward the end through its own power and action. (For a more detailed discussion of instrumental causality, see Frost, Aquinas on Efficient Causation and Causal Powers, ch. 8). For example, when a carpenter uses a saw to make a bench, the saw cuts through its sharpness for the sake of making a bench. The saw only acts towards this end of bench making, however, in so far as it is moved by the carpenter.{11} The key difference between an instrumental cause and a principal cause is whether or not the end for which the cause acts belongs to the cause through itself. The saw, for example, does not operate toward the end of making a bench through itself, but only in so far as it is moved by the carpenter. The carpenter, by contrast, acts toward the end of making the bench in virtue of himself.

Aquinas on Instrumental Causes

Aquinas developed several other important views about instrumental causes. These views are worth considering in more detail since his theory of instrumental causation is especially important to his theology. Aquinas maintains that the sacraments function as instrumental causes of grace and that created causes in general relate to God’s causality as instruments employed by a principal cause.{12}

One defining aspect of instrumental causes in Aquinas’s view is that instrumental causes have two actions, one of which is the instrument’s proper action (e.g. cutting) and another which goes beyond the instrument's native power (e.g. making a bed). Aquinas writes:

[A]n instrument has two actions, one instrumental, in accordance with which it acts not by its own power but by the principal agent’s power; however, it has another proper action which belongs to it according to its own form; just as it belongs to an axe to cut on account of its sharpness, but to make a bed in so far as it is the instrument of an art. But it does not accomplish the instrumental action unless it exercises its proper action; for it makes a bed by cutting. (ST III, q. 62, a. 1, ad 2 [ed. Leon., vol. 12, 20])

When the axe acts upon the wood, it both cuts and makes a bed. It cuts due to the sharpness it has of itself. Yet, the axe also makes a bed through its cutting even though it has no native power to make beds. The axe only works towards making a bed in so far as it is being used by a person who has the art of carpentry. However, the axe really contributes to the act of bedmaking and it does so in virtue of the action which arises from its native power, that is, cutting. Without the use of a tool that can split wood, the carpenter cannot make a bed. The act of bedmaking only occurs through the act of cutting and splitting wood. The carpenter acts through the axe’s power to cut wood. Yet, Aquinas thinks that the axe also acts through the carpenter’s power to make beds. It is only by acting through the power of the principal cause that instruments achieve the end that goes beyond their native powers.

Aquinas thinks that principal causes move the instrumental causes they use and by doing so, the principal transfers a transitory power to the instrumental causes. Aquinas writes:

[A]n instrument achieves an instrumental action in so far as it is moved by the principal agent and through that motion it participates in some way in the power of the principal agent, but not in such a way that the power is in the instrument according to perfected being, since motion is an imperfect act. (De verit., q. 26 a. 1 ad 8 [ed. Leon., vol. 23 3/1, 750])

While the axe is being wielded by the carpenter, it has within it the power to craft a bed. But once the carpenter stops moving the axe that power disappears from it. The principal cause’s power is not received in the instrument in a stable way as a permanent form. Nevertheless, it is through this received power that the instrument works toward the action that transcends its native powers.

Aquinas believes that this transfusion of power from principal cause to instrument is one of the defining elements of instrumental causality. He writes: “[S]omething is constituted a principal agent from the fact that it has some form which it is able to transfuse into another” (Super Sent. IV, d. 19 q. 1, a. 2, qc. 1 [ed. Mandonnet-Moos, vol. 4, 978]). Without the principal cause’s transfusion of its power into the instrument by its motion of the instrument, the instrument is not able to operate toward an effect that lies beyond its own native power. Nevertheless, it must be remembered that the instrument is not merely a passive medium through which the principal cause’s power passes. The instrument truly contributes to the principal cause’s action by its own native powers. This is evidenced by the fact that principal causes choose specific instruments to carry out specific actions due to the type of native powers the instrument has. The carpenter, for example, chooses the saw rather than the hammer to make a bed because the saw can cut through its sharpness.

Since the principal cause acts through the instrumental cause’s powers and the instrument likewise acts through the power received from the principal cause, Aquinas sees correlative principal and instrumental causes as joining together to form a unified cause of their joint effect.

Aquinas writes: “The principal and instrumental agent are as if one cause since one acts through the other” (Super Sent. IV, d. 19 q. 1, a. 2, qc. 1 [ed. Mandonnet-Moos, vol. 4, 978]). Moreover, he thinks that correlative principal and an instrumental causes exercise their causality through a single, unified action. He writes: “The action of an instrument, in so far as it is an instrument, is not other than the action of the principal agent” (Super Sent. IV, d. 19 q. 1, a. 2, qc. 1 [ed. Mandonnet-Moos, vol. 4, 978]). The act of bedmaking that the saw performs is the very same act of bedmaking that the carpenter performs. Each performs this action by acting through the other’s power. The carpenter makes the bed through his power of carpentry and the saw’s sharpness which he employs to reach his end, and the saw makes a bed through the carpenter’s power of carpentry received in it and its own native sharpness.

Debates about Causation in Medieval Scholastic Philosophy

Scholastic figures generally agreed with the broad contours of the Aristotelian approach to causation which was exemplified in Aquinas’s views presented in the sections above. However, there were many debates in the later medieval period about how to develop the precise details of the Aristotelian approach to causation. Some important debates pertained to the nature of active powers and actions, the conditions required for causal activity and the validity of positing final causation in nature.

One major debate about the nature of causal powers focused on the relationship between a substance’s active powers and its substantial form. More specifically, the debate centered upon whether the human substantial form was the active power through which human beings engaged in their characteristic activities of knowing, willing and sensing, or whether these human powers were accidental forms distinct from the human substantial form. While this debate focused specifically on the human being and its powers, some of the arguments proceeded from general claims which had implications for how all substantial forms relate to the powers by which substances operate. (For an overview of the debate and discussion of the critique of Aquinas’s position, see Wood, “The Faculties of the Soul and Some Medieval Mind–Body Problems.”) Aquinas argues that no substance acts immediately through its substantial form. The powers by which substances immediately act are accidental forms distinct from their substantial form. Against this position, other scholastic figures, such as Henry of Ghent (c. 1217–1293) and John Buridan (c. 1300–c. 1359), held that powers, such as the human intellect and will, were identical with the human substantial form, that is, the soul. 

Another related debate concerned how active powers are individuated. In Aquinas’s view each action that immediately arises from a substance entails a distinct active power through which the action is performed. For example, the acts of heating and illuminating entail two separate active powers in fire, namely a power to heat and a power to illuminate. John Buridan critiqued the position that powers are individuated by actions and their unique objects. In his view, it is not necessary to posit a distinct power to account for each type of action which a substance can perform. He thought that there are “multi-track” powers which enable various types of actions. He was among those who held that the human soul was identical to its powers. Thus, he saw the soul as an example of a single power from which many different types of actions immediately arose (see Löwe, “Aristotle and John Buridan on the Individuation of Causal Powers”). 

Medieval scholastic thinkers also debated about how to analyze the ontological relationship between an agent’s action, the motion it causes in its patient and the patient’s passion or undergoing of that motion. Aristotle claimed that one and the same motion was both the agent’s action and the patient’s passion (Physics, 3.3). Medieval figures, however, developed different accounts of the nature of the sameness and the difference which obtains between these realities. On one end of the spectrum, thinkers such as William of Ockham (c. 1287-1347) argued that action and passion differ only conceptually from the motion involved in efficient causation. Action and passion are merely two different concepts of the same reality (see Brower-Toland, “Causation and Mental Content: Against the Externalist Reading of Ockham,” especially section three). On the other end of the spectrum, thinkers such as Peter Auriol (c. 1280-1322) argued that an agent’s action must be an entity which exists prior to and is separable from the motion which is caused through it. (On Auriol’s critique of Aquinas’s views on action, see Frost, “What Is an Action? Peter Auriol vs. Thomas Aquinas on the Metaphysics of Causality” and Löwe, “Peter Auriol on the Metaphysics of Efficient Causation.” On Scotus’s views in comparison with Aquinas’s see Löwe, “John Duns Scotus versus Thomas Aquinas on Action–Passion Identity.”) Aquinas’s own position, which has been subject to interpretive debate, lies somewhere between these two extremes (see Frost, “Aquinas’s Ontology of Transeunt Causal Activity”).

In addition to debates about the nature of active powers and the agent’s action, there were disagreements among medieval scholastics about the conditions which needed to be met for efficient causation to occur. One important debate regarded whether an agent could move itself or whether the agent and patient in a causal interaction must be distinct. Aquinas held that the agent and the patient must be distinct in order for the agent to communicate a form to the patient in action. (On Aquinas’s rejection of self-motion, see Frost, Aquinas on Efficient Causation and Causal Powers, 71–76.) By contrast, other medieval figures believed that self-motion was possible. Many believed that the human will could move itself to act, yet some medieval figures, such as John Duns Scotus (1265/6–1308), adopted the stronger position that even natural agents (i.e. those lacking intellect and will) could engage in self-motion. Scotus thinks that growth is an example of self-motion in which a substance acts through one of its qualitative forms, such as heat, to induce a new quantitative form in itself. (On Scotus’s views on self-motion, see King, “Duns Scotus on the Reality of Self-Change” and Kim, “Why Does the Wood Not Ignite Itself? Duns Scotus’s Defense of the Will’s Self-Motion.”)

Another condition for efficient causal interactions which was debated was the “contact” condition. Aquinas held fast to the Aristotelian view that the agent must be in physical contact with the patient in order to act upon it. Contact with the patient could be immediate, as in cases where the agent and the patient were touching one another, or contact could be mediated by an intermediary, known as a “medium,” which touches both the agent and the patient. Aquinas supported the contact condition with a claim that an agent can only act where its power is physically present (see Frost, Aquinas on Efficient Causation and Causal Powers, 71–76). Thus, an agent must be in physical contact with something in order to act upon it. However, other scholastic figures rejected the contact condition and maintained that action at a distance does occur in the physical world (see Kovach, “Aquinas’s Theory of Action at a Distance—A Critical Analysis”). Thinkers such as Ockham supported this view by educing cases in which an agent appears to act at distance, such as magnetism and the action of a sensible object on the power of vision (see Goddu, “William of Ockham’s Arguments for Action at a Distance”).

One final debate that was especially important was the debate about whether natural causes act for the sake of ends or goals. As discussed above, Aquinas thought that even natural causes which lack cognition act for the sake of ends since they have natural inclinations toward certain effects. This position was rejected by Ockham, Buridan, Walter Chatton (c. 1290-1343) and others. They relegate final causality to the realm of voluntary activity since only voluntary agents can know and choose to act for the sake of goals (see Kovach, “Aquinas’s Theory of Action at a Distance—A Critical Analysis”). This trend continued in modern philosophy.

V. Causation in Modern and Contemporary Philosophy

In the early modern period, there were many developments in natural philosophy that impacted approaches to causation. The Aristotelian idea of form was called into question and hylomorphism, and the related theory of the four causes, was replaced by a mechanical approach to nature. Modern thinkers found it difficult to explain how physical substances could impact one another. Occasionalism saw a resurgence in Christian thinkers such as Nicolas Malebranche and Louis de La Forge (see Nadler [ed.], Causation in Early Modern Philosophy: Cartesianism, Occasionalism, and Preestablished Harmony).

David Hume (1711–1776) is the modern philosopher who most influenced contemporary assumptions about causation. (For a summary of the various aspects of Hume’s views on causation, as well as a very useful annotated bibliography of secondary literature, see Lorkowski, “David Hume: Causation.”) Hume rejects the validity of our concepts of “power” and “production” since they cannot be experienced by the senses. Hume thinks that the necessary connection between a cause and its effect is likewise a mental projection rather than a genuine feature of reality. In his view, causes and effects are ontologically separate from one another. Causes do not genuinely produce effects by acting through powers. Rather, causes and effects are merely logically related to one another due to their constant conjunction. For example, on Hume’s view fire does not genuinely produce burning in cotton. Rather, fire and burning are ontologically separate and unconnected realities. They merely appear to our mind as being necessarily connected since burning always occurs together with fire.

Mainstream discussions of causation in contemporary analytic philosophy accept the basic assumptions of Humeanism, namely that cause and effect is a logical rather than an ontological relationship. Much of the contemporary discussion of causation in the analytic tradition has focused on how to specify the precise nature of the logical relationship between causes and effects.

Another contemporary approach to causation which has its roots in modernity is nomicism. On this view, laws of nature are central to understanding causation. Particular entities in the natural world are related to each other as cause and effect because of laws of nature that govern the universe. Statements about what universally happens in the world express necessary laws that govern the universe. (For an important contemporary account of laws of nature, see Armstrong, What Is a Law of Nature?) In the early modern period laws of nature were traced to the divine will, but in contemporary philosophy some philosophers take laws of nature to be brute, basic features of reality. (On Descartes’s view of laws of nature as established by the divine will, see Ott, Causation and Laws of Nature in Early Modern Philosophy, 51–60.)

Recently in contemporary analytic philosophy there has been a resurgence of power theories of causation, similar to the theories defended by Aristotle and his medieval followers. (For an overview of contemporary work on causal powers, see Jacobs, Causal Powers.) Some contemporary philosophers find that this approach offers a more intuitive and plausible conception of the project of natural science. For Humeanism, the project of natural science turns out to be about uncovering which arbitrary connections obtain between types of events in the world. Similarly, for nomicism, natural science is about uncovering what abstract laws of nature happen to obtain. For powers-based theories of causation, by contrast, the project of natural science is to come to a deeper understanding of the powers of observable material substances. The object of natural science is not regularities or abstract laws. Rather, it is the real features of material substances. This view of natural science fits better with how scientists, and even ordinary people, conceive of what they are doing when they seek to understand the material world.

Furthermore, on powers-based theories of causation, generalizations, such as fire burns cotton, are rooted in the active and passive powers of material substances. The real features of things in the world explain why certain regularities obtain and certain natural laws are true. Humeanism, by contrast, maintains that the regular connection between certain events in nature, for example, fire coming into contact with cotton and burning, is merely a brute fact. There is no real explanation for why certain events are connected as cause and effect. Nomicism, on the other hand, postulates laws of nature to explain why certain events are connected. Yet, the laws of nature are themselves without explanation. Theists trace them to an arbitrary choice of the divine will, while non-theists claim they are brute facts. Accordingly, some contemporary philosophers find a powers-based theory of causation attractive because it offers a more satisfying explanation for why certain generalizations hold true in natural causation. It is true, for example, that fire burns cotton because fire by its nature has an active power to burn and cotton by its nature has a passive power to be burned.