Essay Overview

What This Question Is Actually Testing — and Why Definition-Listing Fails

The Core Analytical Demand

The question about the difference between a causal series per se and a causal series per accidens is not a terminology test. Your examiner already knows both definitions. What the question tests is whether you can demonstrate that you understand why the distinction exists — what philosophical problem it was designed to solve, what it commits any thinker who uses it to, and what depends on correctly identifying which type of series is at stake in a given argument. An essay that opens with clean definitions of both terms and then stops has answered roughly 15% of the question. The remaining 85% is: what follows from this distinction, for whom, in which arguments, and with what contestable implications.

The distinction originates in Aristotle’s analysis of motion and causation, is refined substantially by Avicenna and Averroes in the Islamic tradition, and reaches its most precise formulation in thirteenth and fourteenth century Latin Scholasticism — principally in Aquinas and Duns Scotus. Each thinker uses the distinction differently, and each use has different commitments and different vulnerabilities. A strong essay situates the distinction in this tradition, traces the specific philosophical work it does in at least one major thinker’s argument, and evaluates the objections to that use. An essay that treats the distinction as self-contained — as if it can be explained in isolation from the arguments it appears in — will not reach the analytical level the question demands.

There is also a secondary demand that most students underestimate: the question requires engagement with the logic of the distinction, not just its content. Why does the essentially ordered series prohibit an infinite regress while the accidentally ordered series does not? What is the precise metaphysical difference that generates this asymmetry? Getting this right — and showing that you can articulate the logical structure, not just report the conclusion — is what separates a first-class response from a competent one.

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Primary Sources You Must Engage With Directly

The primary texts for this topic are: Aristotle’s Physics (Book VIII) and Metaphysics (Book XII, Lambda), for the foundational account of movers and the unmoved mover; Aquinas’s Summa Theologiae (I, q.2, a.3 — the Five Ways), Summa Contra Gentiles (I, cc.13–15), and De Ente et Essentia; and Duns Scotus’s De Primo Principio and Ordinatio I, d.2, q.1–4. The Stanford Encyclopedia of Philosophy’s entry on cosmological arguments provides a reliable secondary survey and can orient you before you engage the primary texts, but it cannot substitute for direct engagement with Scotus’s formulation of the distinction, which is the most rigorous available. Cite the edition and translator you use.

Philosophical Background

The Aristotelian Background — Why the Distinction Arises

Before you can explain the distinction, you need to explain the problem it was invented to solve. The regress problem in causal theory asks: if every effect has a cause, and every cause is itself an effect of a prior cause, can this series of causes extend infinitely? Aristotle’s answer in the Physics is that it cannot — that an infinite causal regress is impossible and a first cause is required. But the argument he gives for this is not obvious, and it depends crucially on what kind of causal series is in question. Your essay needs to reconstruct why Aristotle thinks the regress is impossible, because that reconstruction is the entry point for understanding what the per se / per accidens distinction actually does.

Aristotle’s argument proceeds from his analysis of motion. Every mover imparts motion to the thing it moves. If the mover is itself being moved, it must be moved by something else. Can this go on infinitely? Aristotle distinguishes between a series of movers that are all simultaneously active — each one moving the next in a chain of real-time efficient causation — and a series of causes in which prior members have already completed their causal role. In the first kind of series, there is a structural dependency: the intermediate movers cannot move anything unless they are themselves being moved right now. Remove the first mover and the whole chain immediately loses its capacity to act. This is the structure that becomes, in Scholastic terminology, the per se series. In the second kind of series, prior causes have already acted and may no longer exist. Whether or not they persist is irrelevant to the causal activity currently taking place. This is the structure that becomes the per accidens series.

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Aristotle’s Own Terminology — and Why the Scholastic Labels Are Clearer

Aristotle himself does not use the precise Latin labels per se and per accidens for the causal series distinction — those formulations belong to the Scholastic reception of his work. Your essay should note this: the terminology is Latin-Scholastic, but the underlying distinction is Aristotelian. Aquinas is the first thinker to give the distinction systematic philosophical work to do in the context of the cosmological argument, and Scotus is the first to formalise it with the precision that makes it analytically tractable. If your essay conflates the Aristotelian original with the Scholastic development, it will misread both the textual evidence and the philosophical stakes.

What makes this background necessary for your essay is not historical completeness — it is philosophical precision. The distinction between the two types of series is grounded in an account of what it means for a cause to be causally active. In a per se series, the middle terms are causally active only in virtue of being caused; they are instruments, not independent sources. In a per accidens series, the middle terms are causally active in their own right — the prior causes have already produced them, and they now act independently. This difference in the mode of causal activity is what drives the different conclusions about whether the series can be infinite. Your essay needs to articulate this precisely, not simply assert that one can be infinite and the other cannot.

The Core Distinction

The Distinction — Its Structure, Its Logic, and What Follows From It

The Four Properties That Distinguish a Per Se Series From a Per Accidens Series

Each property generates a specific philosophical implication. Your essay should identify which properties are doing the most work in the argument you are analysing.

Property 01

Simultaneous Activity

  • In a per se series, all members are simultaneously active — the entire chain is operating at the same moment
  • In a per accidens series, prior causes need not be active; they have already completed their causal role and may no longer exist
  • This is the most commonly cited property, but your essay should not treat it as the most fundamental — it is a consequence of the more basic difference in ontological dependence
  • The father-son example illustrates the per accidens case: the grandfather need not be alive for the father to beget the son
Property 02

Derivative vs. Independent Causal Power

  • In a per se series, intermediate causes possess causal power only derivatively — they cause because they are being caused; they are instruments of the prior cause
  • In a per accidens series, intermediate causes possess causal power in their own right — they act as independent efficient causes, not as conduits for a prior cause’s activity
  • This is the most philosophically fundamental property: it explains why a per se series cannot be infinite — an infinite series of derivative causes, each borrowing its power from the next, never arrives at a source of causal power
  • Scotus identifies this property as the crux of the distinction in De Primo Principio
Property 03

Ontological Dependency at Every Link

  • In a per se series, the existence and causal activity of each link is ontologically dependent on the prior link right now — remove any link and the whole series immediately loses its effect
  • In a per accidens series, the existence of later links does not depend on the continued existence of prior links — prior causes have already produced their effects and dropped out of the picture
  • This property is why Aquinas’s Second Way targets per se causation: the dependency he has in mind is not historical but ontological and present-tense
  • Students who read Aquinas’s Second Way as a temporal argument about the first historical cause misread it by conflating per se and per accidens structure
Property 04

The Regress Result

  • A per se series cannot be infinite: an infinite series of causes each deriving their causal power from the next, with no terminus, would have no source of causal power and could produce no effect — it would be like a chain of instruments with no agent wielding them
  • A per accidens series can in principle be infinite: each member acts independently, so there is no structural reason why the series cannot extend back without limit
  • Note: the claim that a per accidens series can be infinite is a logical claim, not an empirical one — your essay should not conflate the philosophical permission for an infinite series with the factual question of whether the universe is temporally infinite
  • Aquinas explicitly accepts the possibility of an infinite per accidens series — he does not argue against eternal creation on rational grounds
Property 05

The Role of the First Cause

  • In a per se series, the first cause must be causally active right now, contributing causal power that is transmitted through the chain simultaneously
  • In a per accidens series, the first cause is simply the earliest historical antecedent — it has already acted and its continued activity is not required
  • This distinction determines what the cosmological argument for a first cause is actually arguing: it is an argument for a contemporaneously active first cause sustaining the present activity of the series, not for a historically earliest event
  • Misreading this leads to the common objection “what caused God?” — which has more force against a temporally first cause than against an ontologically primary cause in a per se series
Property 06

What Kind of Thing Can Be a First Cause

  • The first cause of a per se series must be a cause that is not itself causally dependent on anything — an unmoved mover, in Aristotle’s terms; a being whose causal power is not derived
  • This imposes substantive constraints on what the first cause can be like: it cannot be a member of the series it grounds, and it cannot be in potentiality with respect to its causal activity
  • Scotus uses these constraints to argue that the first cause must be infinite in power — a finite cause could not sustain an entire order of essentially dependent causes
  • Your essay should specify what each thinker concludes about the nature of the first cause and how those conclusions are generated by the structure of the per se series

The staff moves the stone only because the hand moves the staff. Remove the hand and the staff cannot move anything — it was never the source of the movement, only its instrument.

— The canonical illustration of a per se series, in Aquinas’s analysis of the Second Way

Your essay should use the canonical illustration carefully, not as a definitive proof but as a tool for making the logic of the series vivid. The hand-staff-stone example (or equivalently the hand pushing a stick pushing a stone) makes the per se structure concrete: the staff’s capacity to move the stone is entirely borrowed from the hand that moves it. The staff is not causally active in its own right; it is an instrument. Extend this to any length — a series of instruments, each imparting motion only because it is being moved — and the series still requires a terminus that is not itself an instrument. The question your essay needs to address is whether this illustration generalises in the way Aquinas or Scotus needs it to, and whether real-world causal chains actually have the per se structure these arguments require.

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Do Not Confuse Temporal and Ontological Priority

The single most common error in essays on this distinction is conflating temporal priority (A happened before B) with ontological priority (B’s existence depends on A right now). The per se / per accidens distinction is fundamentally about ontological, not temporal, ordering. When Aquinas argues in the Second Way against an infinite series of efficient causes, he is not arguing that the universe must have had a beginning in time. He explicitly says in other texts that reason alone cannot demonstrate that the world is not eternal. What he is arguing is that the present causal activity sustaining the existence of things cannot be explained by an infinite series of derivatively-powered causes. Get this wrong and the entire argument — and your reconstruction of the distinction — is fundamentally misread.

Primary Thinkers

Key Thinkers — What Each One Argues and What Their Use of the Distinction Commits Them To

The distinction between per se and per accidens causal series does not have a single canonical formulation — it is developed differently by each major thinker who uses it, and those differences matter for what the distinction can and cannot achieve. Your essay should not present a single unified account of the distinction as if it belongs to an undifferentiated Scholastic tradition. It should identify which thinker’s version you are primarily engaging with and specify what that thinker’s use of the distinction commits them to.

ThinkerKey TextsHow They Use the DistinctionCommitments and Vulnerabilities Your Essay Must Address
Aristotle Physics VIII; Metaphysics XII (Lambda) Aristotle establishes that a series of movers in which each member moves the next only because it is simultaneously being moved cannot regress infinitely — there must be an unmoved mover. He does not use the per se / per accidens labels, but the structural distinction is operative in his argument. His unmoved mover is eternal, immaterial, and causally active as a final cause (object of desire) rather than an efficient cause — a point that distinguishes his version significantly from the Scholastic appropriations. Aristotle’s unmoved mover is an efficient cause only in a limited sense — it moves as an object of love and desire, not by direct agency. This creates a gap in the argument: the unmoved mover explains motion, but its causal role is not straightforwardly the kind of active causal sustaining that the per se series seems to require. Your essay should specify whether you think this is a problem for Aristotle’s version of the argument or a feature of his metaphysical framework that the Scholastics misread.
Aquinas Summa Theologiae I, q.2, a.3; Summa Contra Gentiles I, cc.13–15; De Ente et Essentia Aquinas deploys the distinction most influentially in the Second Way, arguing that a series of efficient causes in which each member causes only in virtue of being caused cannot regress infinitely. He explicitly distinguishes this from the question of whether the world is eternal (a question he thinks reason cannot settle), making clear that his target is an essentially ordered, simultaneously active series. His First Way (on motion) and Third Way (on contingency) also implicitly depend on the per se structure, though he does not label them as such. Aquinas is vulnerable to the charge that real-world causal chains may not have the per se structure his argument requires — that the causes of physical events are not instruments of a single prior agent but genuinely independent causes. He is also committed to the view that the first cause must be a being whose essence is identical to its existence (esse), which is a substantial metaphysical claim that his cosmological argument alone does not establish. Your essay should specify what further metaphysical commitments Aquinas needs beyond the causal argument itself.
Duns Scotus De Primo Principio (A Treatise on God as First Principle); Ordinatio I, d.2, q.1–4 Scotus provides the most analytically rigorous formulation of the distinction. He distinguishes an essentially ordered series (per se) from an accidentally ordered series (per accidens) by three explicit marks: (1) in an essentially ordered series, the prior cause must be active simultaneously with the posterior; (2) the prior cause is causally superior, not merely numerically prior; (3) all members of the series simultaneously cause the one effect. He then argues that essentially ordered series cannot regress infinitely, and that there must therefore be a first efficient cause — which he further argues must be infinite in power, unique, and intelligent. His argument is more formally structured than Aquinas’s and makes fewer implicit metaphysical assumptions. Scotus is committed to the claim that the first cause of an essentially ordered series must be infinite — a claim he derives from the argument that a finite cause could not sustain an entire order of essentially dependent causes. This is a significant additional commitment that goes beyond the structural argument about the series alone. Your essay should assess whether this inference is valid and whether it has been successfully challenged. The strongest objection to Scotus’s version is the question of whether any actual causal chain has the strictly essentially ordered structure he describes.
Avicenna (Ibn Sina) The Book of Healing (Kitāb al-Shifāʾ), Metaphysics; The Book of Salvation Avicenna’s version of the distinction — which significantly influenced the Latin Scholastics — is embedded in his analysis of necessary and contingent existence. He argues that a series of contingent beings, each depending for its existence on something external to itself, cannot explain its own existence even if the series is infinite. The infinite series of contingent causes is itself contingent and requires a necessary being to explain why anything in the series exists at all rather than nothing. This shifts the focus from the structure of causal transmission to the modal status of the causes involved. Avicenna’s version is arguably more powerful against the objection of an infinite per accidens series than Aquinas’s or Scotus’s, because it targets the modal status of causes rather than their structural relationship. However, it depends on a contested principle: that an infinite collection of contingent things cannot be self-explanatory. Your essay should assess whether that principle is defensible and how it relates to the structural distinction between per se and per accidens series.
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Choose a Primary Thinker and Analyse Them in Depth — Do Not Survey All Four

A common essay error is to attempt to cover Aristotle, Aquinas, Avicenna, and Scotus in roughly equal depth, producing a survey that adequately represents none of them. The question requires philosophical analysis, and philosophical analysis requires depth over breadth. Choose one thinker as your primary case — almost certainly Aquinas or Scotus, who give the most philosophically developed versions — and treat the others as context, contrast, or complication. Your primary thinker should be engaged at the level of specific texts and specific arguments. The other thinkers can be deployed where they illuminate, complicate, or challenge your primary account.

Philosophical Stakes

The Distinction in the Cosmological Argument — Why It Is the Pivot Point

The reason this distinction matters beyond medieval metaphysics — the reason it appears in examination questions and philosophy curricula — is that it is the load-bearing structure of every version of the cosmological argument that argues from causal regress to a first cause. Without clearly identifying which type of series the argument is about, you cannot evaluate whether the argument succeeds. This is not a peripheral point. It is the analytical centre of the topic, and your essay must address it directly.

Stake 01

Why the Regress Argument Only Works for Per Se Series

The argument that an infinite causal regress is impossible only applies to an essentially ordered series. In such a series, remove any member and the effect immediately ceases — because every intermediate member is causally active only derivatively, and the derivation must terminate in something that causes without itself being a caused cause. In an accidentally ordered series, by contrast, each member acts independently. The series could in principle extend infinitely backward, and the effect at the end of the chain would be just as well-explained by any finite segment of that chain. An essay that conflates the two types of series will misread the regress argument entirely.

Stake 02

What Aquinas Is and Is Not Arguing in the Second Way

Aquinas’s Second Way is standardly misread as an argument about temporal origins — about what started the universe. It is not. The efficient causes he is concerned with are causes of present existence and activity, not historical antecedents. The series he targets is one in which each member currently depends on the prior member for its causal power — not one in which prior members have already discharged their role. Understanding this changes what counts as a successful objection and what the argument actually requires a first cause to be. Essays that treat the Second Way as an argument about the first event in time have not understood the per se / per accidens distinction.

Stake 03

The Objection That Raises the Hardest Problem

The strongest objection to the cosmological argument using the per se / per accidens distinction is not that an infinite regress is possible — it is that real-world causal chains may not have the essentially ordered structure the argument requires. If physical causation works by each cause producing its effect and then dropping out, the chains we actually observe are per accidens, not per se, and the argument for a first cause does not apply. Your essay needs to engage seriously with this objection and assess whether Aquinas or Scotus has the resources to meet it. Simply asserting that the series is essentially ordered is not an answer.

Stake 04

What the First Cause Must Be Like — and Why the Series Type Determines This

The conclusion of the cosmological argument — that there is a first cause — is not the end of the philosophical work. The type of series determines what the first cause must be like. In a per se series, the first cause must be causally active right now, sustaining the dependent activity of the entire chain. It cannot be the kind of cause that has already acted and stopped. It must be an uncaused cause — not merely historically first, but ontologically primary. Scotus goes further and argues it must be infinite. Aquinas argues it must be pure actuality with no admixture of potentiality. Your essay should trace how the structure of the per se series generates these constraints on the nature of the first cause, rather than simply asserting that God is required.

Stake 05

Hume, Kant, and the Modern Objections — How the Distinction Shapes the Debate

Hume’s objection — that even if every member of a causal series is explained, the series as a whole may have no explanation — applies most forcefully to an accidentally ordered series, where members act independently. Kant’s objection — that causality is a category of experience and cannot be projected beyond possible experience to a first cause — challenges the application of the concept of cause outside the series rather than the structure of the series itself. Neither objection directly dismantles the per se argument, because the per se argument’s conclusion is about the present activity of the series, not about the origin of the series or about a cause beyond experience. Your essay should assess how effectively the per se / per accidens distinction insulates the cosmological argument from these modern objections.

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Pre-Writing Checklist — Before You Draft

  • You have read Scotus’s three marks of an essentially ordered series in De Primo Principio and can state them precisely without paraphrase
  • You have identified which thinker you are primarily engaging with and located the specific texts that contain their use of the distinction
  • You can explain why the impossibility of an infinite regress applies to a per se series but not a per accidens series — in your own words, using the logic of the argument, not just asserting the conclusion
  • You can explain why Aquinas’s Second Way is not an argument about temporal origins, and what type of causation it is actually about
  • You have identified the strongest objection to your primary thinker’s use of the distinction and have considered what resources they have to respond
  • You have checked whether you are conflating temporal and ontological priority anywhere in your draft
  • You have a clear thesis that specifies what the distinction does in a specific argument and what you are going to argue about it — not just that the distinction exists
  • You can explain what follows about the nature of the first cause from the structure of the per se series, and which thinker’s account of that nature you find most and least defensible
Objections and Responses

Objections, Counterarguments, and How to Handle Them in Your Essay

A philosophy essay on this topic that does not engage seriously with objections to the distinction and to its deployment in the cosmological argument is not doing philosophy — it is doing exposition. The distinction is contested, the arguments that depend on it are contested, and the most philosophically rigorous essays are the ones that identify the strongest version of the opposing case and assess it carefully. Below are the four objections your essay is most likely to need to address.

How to Structure Your Engagement With Objections

Objections That Challenge the Distinction Itself

  • Are any real causal chains per se? The objection that no actual causal chain in the physical world exhibits the essentially ordered structure the argument requires — that all observable causation is per accidens — is the most empirically grounded challenge. Your essay should assess what the scholastics would say: they would likely argue that the dependence of contingent existence on a sustaining cause at every moment is precisely a per se relation, not visible in the surface features of physical causation
  • Is the distinction coherent? Some critics argue that the notion of derivative causal power — causal power that belongs to an instrument only in virtue of being wielded by an agent — presupposes a distinction between instrumental and principal causation that itself needs defending. Your essay should assess whether this is a fatal objection or whether the scholastics have a principled account of the distinction
  • Does the distinction beg the question? The objection that the per se structure is identified in causal chains precisely because we already assume a first cause, and that the argument is therefore circular, is a serious logical challenge. Scotus’s formal approach in De Primo Principio is the most serious attempt to avoid this charge — your essay should assess how well it succeeds
  • The composition fallacy objection: Even if every member of an essentially ordered series derives its power from the next, it does not obviously follow that the series as a whole requires a terminus outside itself. Hume makes this point most clearly. Your essay should assess whether the argument from derivative causal power to a first cause commits the fallacy of composition

Objections That Challenge the Cosmological Argument’s Conclusions

  • What caused the first cause? This objection has more force against a temporally first cause than against an ontologically primary cause in a per se series. The scholastic response is that the first cause of a per se series is by definition a cause that does not derive its causal power from anything — asking what caused it is a category error, like asking what is north of the North Pole. Your essay should assess whether this response successfully deflects the objection or merely relabels the problem
  • The uniqueness problem: Even if the argument establishes a first cause of the series, it does not obviously establish that there is only one such cause. Scotus argues for uniqueness from the infinite power of the first cause — only an infinite being is sufficient to ground an entire order of essentially dependent causes, and there cannot be two infinite beings. Your essay should assess whether this inference is valid
  • The gap problem: Even if a first cause of the essentially ordered series is established, the argument does not by itself establish that this cause is the God of any religion — omniscient, omnipotent, omnibenevolent, personal. This is a distinct further argument, and your essay should be clear about where the causal argument ends and the theological elaboration begins
  • Kant’s transcendental objection: Kant argues that causality is a category that applies only within possible experience and cannot be legitimately extended to a first cause of the entire series of experience. Assess whether the per se framing — which targets present ontological dependence rather than temporal origins — partially escapes Kant’s objection or whether it remains fully vulnerable to it
Response Quality

Strong vs. Weak Essay Responses — What the Difference Looks Like

✓ Strong Analytical Paragraph
“Scotus’s argument in De Primo Principio does not merely claim that an essentially ordered series must terminate — it specifies why, and that specification is the key analytical move. In an essentially ordered series, intermediate causes are causally active only instrumentally: they transmit causal power they do not possess independently. An infinite series of such instrumental causes would have unlimited conduits for causal power and no source of that power — the effect would be produced by nothing. This is not a contingent claim about the world but a structural claim about what it means for a cause to act derivatively rather than in its own right. The objection that physical causal chains may not have this structure is therefore the strongest challenge: if no actual series of physical causes is per se in Scotus’s sense, the argument has no grip on the world it purports to explain. Scotus’s response — that the dependence of contingent being on a sustaining cause at each moment of its existence is precisely an essentially ordered dependence — is the move your essay needs to evaluate, not simply report.” — This paragraph identifies a specific thinker’s specific argument, extracts the logical structure driving it, and moves directly to the objection that most seriously challenges it. Every sentence advances the analysis.
✗ Weak Analytical Paragraph
“A per se causal series is one where all the causes are happening at the same time, whereas a per accidens series is one where the causes happened one after another in the past. Aquinas used this distinction to prove that God exists. He argued that there cannot be an infinite series of causes so there must be a first cause. This is called the cosmological argument. The distinction between per se and per accidens is important because without it the argument would not work. Aquinas was a very important philosopher in the medieval period and his ideas are still discussed today. The distinction shows that at some point there must be an uncaused cause which started everything.” — This paragraph defines the terms incorrectly (the distinction is not about temporal versus simultaneous succession in the way described), attributes Aquinas’s argument as a generic proof of God’s existence without specifying its logical structure, and ends with a circular statement. There is no engagement with specific texts, no reconstruction of the argument’s logic, and no awareness of any objection. It could have been written from a two-paragraph internet summary.

The gap between these paragraphs is not primarily about length or vocabulary — it is about whether the writer has understood the logical structure of the argument they are discussing. The strong paragraph could be shorter and still be strong, because every sentence is doing analytical work: identifying a claim, locating its logical foundation, and positioning it against the objection that most seriously threatens it. The weak paragraph is longer than it needs to be precisely because it is filling space rather than making moves. Every sentence in a philosophy essay should advance an argument. If it is describing context, providing background, or explaining who someone is, ask whether that sentence is necessary for the argument you are making. If it is not, cut it.

Quality Control

The Most Common Essay Errors on This Topic — and What Each One Costs You

#The ErrorWhy It Costs MarksThe Fix
1 Defining per se as “happening at the same time” and per accidens as “happening sequentially” Temporal simultaneity is a consequence of the essentially ordered structure, not its definition. The definition is about the mode of causal power — derivative versus independent. Essays that lead with temporal simultaneity and stop there have explained a symptom rather than the disease. An examiner who knows the literature will recognise immediately that the student has not grasped the metaphysical core of the distinction. Define the essentially ordered series by the derivative nature of causal power in intermediate members: each member causes only in virtue of being caused. Then derive the simultaneity requirement as a consequence — because the causal transmission is ongoing and instrumental, the causes must be simultaneously active. This order of explanation demonstrates that you understand why the distinction has the features it does, not just what those features are.
2 Reading Aquinas’s Second Way as an argument about temporal origins This is the single most damaging misreading in essays on this topic. It generates a cascade of further errors: treating the argument as a response to “what started the universe?”, missing the per se structure entirely, and then being unable to respond to the objection that science can ask about what preceded the Big Bang. Aquinas explicitly distinguishes the question of efficient causation from the question of temporal beginning. An essay that does not make this distinction has not understood the Second Way or the per se / per accidens distinction. Explicitly note that Aquinas accepts (in the De Aeternitate Mundi and elsewhere) that reason alone cannot demonstrate that the world is not eternal — he thinks the world’s creation in time is a matter of faith, not demonstration. His causal argument is about the ontological dependence of contingent beings on a sustaining cause right now, not about what event happened first. Ground this claim in the text, not in secondary summary.
3 Treating “essentially ordered” and “per se” as if they are simply Latin labels for “more important causation” Some students write as if the per se series is the “real” kind of causation and the per accidens series is a looser or more superficial kind, without identifying what makes one essentially ordered and the other accidentally ordered. This produces essays that use the terminology correctly as labels but cannot explain the structural difference. An examiner cannot award marks for terminology that is not backed by conceptual understanding. Work through Scotus’s three marks explicitly and assess each one. A series is essentially ordered if and only if: (1) the prior cause must be simultaneously active; (2) the prior cause is causally superior, not merely numerically prior — it causes the posterior cause to be a cause; (3) all members jointly cause the one ultimate effect. Practice identifying whether a given causal scenario satisfies all three conditions before you use the label.
4 Asserting that an infinite per se series is impossible without reconstructing the argument for why The claim that a per se series cannot be infinite is the conclusion of an argument, not a premise. Simply asserting it — as most essays do — is a failure to do the philosophical work the question requires. The argument is that an infinite series of derivatively-powered causes, each borrowing causal power from the next, has no source of causal power and therefore cannot produce any effect. This argument can be challenged, and the essay that merely asserts the conclusion has given up the philosophical ground where all the interesting questions live. Reconstruct the argument in explicit steps. State the principle that causal power cannot be derived without limit from a series that has no source of causal power. Identify this as the logical structure that makes the regress impossible. Then evaluate whether the principle is defensible: is it a conceptual truth, or a contingent claim about causation that opponents can reject? This move — from assertion to argument to evaluation — is what distinguishes philosophical analysis from philosophical narration.
5 Ignoring the texts and using only secondary sources Essays that cite only secondary sources — textbooks, encyclopaedia entries, secondary commentaries — cannot demonstrate close reading of the primary arguments. The question is asking about how specific thinkers use a specific distinction, and the only way to show that you know how they use it is to engage with their words. Secondary sources often simplify or distort the distinction in ways that make essays based on them inaccurate in the specific ways that trained readers immediately notice. Locate and read Scotus’s De Primo Principio in translation (Allan Wolter’s translation is standard in English-speaking contexts), Aquinas’s Second Way in the Summa Theologiae I, q.2, a.3, and Aristotle’s Physics VIII.5–10. The Stanford Encyclopedia of Philosophy’s entry on the cosmological argument is a reliable secondary orientation tool, but it cannot replace primary text engagement. Quote the primary texts and analyse the specific language used.
6 Ending with “this distinction is still relevant today” or “this debate continues in modern philosophy” Conclusions that assert contemporary relevance without specifying which contemporary debate, in which form, with which modifications, are the philosophy equivalent of the literary-analysis error of asserting that a novel is “still relevant today.” They signal that the essay has run out of things to say and is padding. A philosophy essay ends by consolidating the argument it has made and specifying what follows from its conclusions — about the validity of the cosmological argument, the coherence of the distinction, or the specific thinker’s overall metaphysical commitments. Your conclusion should specify: (1) what you have argued the distinction does and does not achieve in the argument you have focused on; (2) which objection you regard as most damaging and why; (3) what remains unresolved and what further argument would be needed to resolve it. This is a philosophical conclusion — it advances the inquiry rather than wrapping it in a ribbon.

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Common Questions

FAQs: Causal Series Per Se and Per Accidens

What is the difference between a causal series per se and per accidens?
In an essentially ordered (per se) causal series, each intermediate member exercises causal power only derivatively — in virtue of being currently moved or caused by the prior member. All members of the series are simultaneously active, and removing any member immediately eliminates the effect. The intermediate causes are instruments of the prior cause, not independent agents. In an accidentally ordered (per accidens) causal series, prior causes have already completed their causal role and need not be simultaneously active. Each member acts in its own right, independently of the continued activity of prior causes. The standard example: a grandfather’s continued existence is not required for a father to beget a son (per accidens); but a hand must currently be moving a stick for the stick to currently move a stone (per se). The key difference is not temporal but ontological: in a per se series, each intermediate cause’s causal activity depends on the concurrent activity of the prior cause. For help building this argument in a full essay, our philosophy writing service works with students on argument structure and primary text engagement.
Why can’t a per se series be infinite, while a per accidens series can?
The impossibility of an infinite per se series follows from the nature of derivative causal power. In an essentially ordered series, intermediate causes do not possess causal power independently — they are causally active only because they are being caused. An infinite series of such derivative causes would have unlimited conduits for causal power and no source of that power. The argument, attributed most precisely to Scotus, is that a series of instruments without an agent wielding them cannot produce any effect — regardless of how many instruments you add. You can multiply instruments indefinitely; without a principal cause, they remain inert. A per accidens series does not face this problem because each member acts independently, with causal power of its own. There is no structural dependency that requires a terminus. Whether a per accidens series can actually be infinite is a separate metaphysical question, but there is no in-principle argument against it of the same kind. Note: Aquinas accepted the logical possibility of an infinite per accidens series, which is why he did not argue that the world necessarily had a beginning in time.
Which thinker gives the most rigorous formulation of the distinction?
Duns Scotus in De Primo Principio provides the most analytically precise formulation, specifying three explicit marks of an essentially ordered series: (1) the prior cause must be simultaneously active with the posterior cause — it causes the posterior cause’s causal activity, not just the posterior cause’s existence; (2) the prior cause is causally superior, not merely numerically prior — it is the cause of the posterior cause’s being a cause; (3) all members of the essentially ordered series simultaneously produce the one ultimate effect. Aquinas uses the distinction but does not give it this level of formal specification. Aristotle has the underlying structural insight but not the Scholastic terminology or systematic development. If your essay is at an advanced undergraduate or postgraduate level, Scotus’s formulation is the one you should engage with most carefully — it is the version that has been most discussed in contemporary analytic philosophy of religion. Allan Wolter’s translation of De Primo Principio is the standard English-language text. For support working through Scotus’s argument directly, our philosophy writing service provides expert guidance on primary text analysis.
How does this distinction relate to Aquinas’s Second Way?
The Second Way in the Summa Theologiae (I, q.2, a.3) is standardly read as an argument from efficient causation to a first efficient cause. The per se / per accidens distinction determines what kind of efficient causation Aquinas is talking about. He is not arguing about temporal origins — about what historically started the causal chain — but about the present ontological dependence of things on an efficient cause sustaining their existence and activity right now. The series of efficient causes he targets is essentially ordered: each member is causally active only in virtue of being currently caused. This is why Aquinas explicitly states that an infinite series of accidentally ordered causes would be possible in principle — the impossibility of a regress he is arguing for applies only to the essentially ordered series. Misreading the Second Way as a temporal argument about the universe’s beginning is the most common serious error in essays on this topic. Aquinas’s De Aeternitate Mundi makes his position on temporal infinity explicit and should be consulted alongside the Second Way.
What is the strongest objection to the cosmological argument that uses this distinction?
The strongest objection is that real-world causal chains may not have the essentially ordered structure the argument requires. If physical causation works by each cause producing its effect and then ceasing to be causally relevant — which is a reasonable description of how most physical processes work — then the observable causal chains are per accidens, not per se. The argument for a first cause as a terminus of an essentially ordered series would then have no application to the physical world. The scholastic response is to shift to a more abstract level: the argument is not about chains of physical events but about the dependence of contingent being on a cause sustaining its existence at every moment. This is the level at which Avicenna’s modal version of the argument and Aquinas’s participation metaphysics operate. Your essay should assess whether this response successfully relocates the argument to a level at which the per se structure does apply, or whether it makes the argument so abstract that its connection to the physical world becomes unclear. For help building and structuring this response, see our research paper writing service or our editing and proofreading service.
What secondary sources should I use for this topic?
The Stanford Encyclopedia of Philosophy’s entry on cosmological arguments (authored by Bruce Reichenbach) is the most reliable online secondary orientation. For book-length secondary treatment of the scholastic tradition, Edward Feser’s Scholastic Metaphysics: A Contemporary Introduction (Editiones Scholasticae, 2014) gives a rigorous and philosophically serious account of the per se / per accidens distinction in its scholastic context. John Wippel’s The Metaphysical Thought of Thomas Aquinas (Catholic University of America Press, 2000) is the standard scholarly treatment of Aquinas’s causal arguments. For Scotus specifically, Richard Cross’s Duns Scotus on God (Ashgate, 2005) provides careful analysis of the De Primo Principio arguments. For the Aristotelian background, Lindsay Judson’s edited collection Aristotle’s Physics: A Collection of Essays (Oxford, 1991) contains relevant specialist articles. Avoid non-peer-reviewed sources and philosophy forums. Your university library’s access to JSTOR and PhilPapers will provide journal articles on contemporary discussions. For help integrating secondary sources into a structured argument, our philosophy writing service is available.
Final Note

What a Strong Submission on This Topic Looks Like When Completed

A strong essay on the distinction between causal series per se and per accidens does three things well. It reconstructs the logical structure of the distinction with precision — specifying not just what the two types of series look like but why the essentially ordered series prohibits an infinite regress and the accidentally ordered series does not. It locates that distinction in the specific argument of a specific thinker — not as a historical fact but as a philosophical move that carries specific commitments and creates specific vulnerabilities. And it evaluates: it identifies the strongest objection to the thinker’s use of the distinction and assesses whether that objection is successfully met.

The most common failure is the substitution of exposition for analysis. An essay that explains what the distinction is, reports that Aquinas and Scotus use it, and concludes that the cosmological argument requires a first cause has produced a competent summary of the relevant material. It has not done philosophy. Philosophy, at the essay level, requires argument — taking a position on a contested question and defending it against the strongest available opposition. On this topic, the contested questions are: whether any real causal chain has the per se structure the argument requires, whether the inference from the structure of the series to a first cause is valid, whether the first cause as characterised by any specific thinker follows from the causal argument alone, and whether the modern objections of Hume and Kant have independent force against the per se version of the argument. A strong essay picks one of these questions, argues a position, and defends it in the space available.

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