Concept

Per Se vs Per Accidens Causation

Intro

There are two very different ways one thing can cause another. Aquinas calls them per accidens (accidentally ordered) and per se (essentially ordered). Getting the difference right is the key to understanding why his cosmological arguments work.

A per accidens series spreads out across time. The classic example: your father caused you, your grandfather caused your father, your great-grandfather caused your grandfather. But you do not need your father to stay alive in order for you to exist right now. Once he started you, you keep going on your own. Each generation hands off and steps away. This kind of chain could, in principle, run backward forever without contradiction.

A per se series is completely different. Picture a hand holding a stick that is pushing a stone. The stick only pushes the stone because the hand is pushing the stick right now. The instant the hand stops, the stick stops, and the stone stops. Every link in the chain is acting only because every earlier link is acting at the same moment. Pull out any link and the whole thing stops at once.

Aquinas argues that a per se chain cannot run backward forever. There has to be a first mover that is doing all the moving without being moved by anything else. If every link in the chain only has power because the previous one gives it, then somewhere there has to be a source that has power on its own. That source is what people call God.

This distinction is the secret behind why his First, Second, and Third Ways do not collapse under the standard objection "What if the chain just goes back forever?" The objection works against a per accidens chain. It does not work against a per se chain. The two are doing different jobs.

This page lays out the two kinds of causation, gives clear examples of each, and shows how they function inside the cosmological arguments.

In full

The crucial scholastic distinction between two kinds of causal series: a per se (essentially-ordered) series, in which intermediate causes operate simultaneously and have causal power only insofar as they are presently empowered by a primary cause; and a per accidens (accidentally-ordered) series, in which causes operate sequentially in time and intermediate causes do not depend on the prior cause for their continuing causal power. The distinction is load-bearing in Aquinas Five Ways (especially the First, Second, and Third Ways): Aquinas argues that per se series cannot regress infinitely, while per accidens series can. Without the distinction, the cosmological arguments are vulnerable to the standard "what about an infinite past?" objection.

The distinction

Per accidens (accidentally-ordered) causal series

A series in which causes operate sequentially across time, and each subsequent cause does not depend on the prior cause for its continued causal power.

Paradigm case: father → son → grandson. The father causes the son to exist; once the son exists, the son does not require the father to remain alive in order to exist himself, or in order to cause his own son. Each generation is causally autonomous once generated.

Aquinas's claim: A per accidens series can in principle extend infinitely backward in time without absurdity. (Aquinas in fact thinks the world had a temporal beginning, but he holds this on the basis of revelation, not philosophy; he denies that philosophy can demonstrate the world had a beginning. See Summa I, q. 46, a. 2.)

Per se (essentially-ordered) causal series

A series in which intermediate causes operate simultaneously and have causal power only insofar as they are presently being empowered by a primary cause.

Paradigm case (Aquinas's): the hand moves the stick which moves the stone. The stick has no causal power on its own to move the stone; it has causal power only because the hand is presently moving it. If the hand stops, the stick stops, and the stone stops.

Aquinas's claim: A per se series cannot regress infinitely. In an infinite per se series with no first member, there is no source of the causal power being transmitted. The intermediate causes are like instruments without a principal user. But effects do occur (the stone does move). Therefore the series terminates in a first member with intrinsic causal power.

Why the distinction matters for the Five Ways

The First, Second, and Third Ways all turn on the impossibility of an infinite per se regress.

First Way (motion): a chain of actualizers that simultaneously actualize the present motion. If no first actualizer, no motion.
Second Way (efficient causation): a chain of efficient causes presently producing the effect. If no first cause, no present causation.
Third Way (contingency): a chain of beings whose existence is presently sustained by another. If no necessary being, no sustained existence.

The standard "infinite past" objection (the universe might be eternal, with each event caused by a prior event back without limit) does not apply to these arguments because the per accidens / temporal series is not what they are concerned with. They are concerned with the per se / hierarchical series of causes operating now, what is presently sustaining the present existence and motion of things.

Modern science analogue (illustrative)

A useful (though not perfect) analogy: the chain of causes producing the light of a working light bulb at this instant.

The bulb glows because of electric current.
The current flows because of voltage from the wall outlet.
The voltage exists because of the power station.
The power station produces voltage because of turbines being driven.
The turbines turn because of (e.g.) burning coal or falling water.

Each of these causes operates simultaneously with the bulb's present glowing; if any link in the chain stops, the bulb goes dark. The chain terminates somewhere, in a first cause that operates intrinsically (no further input required).

This is not literally a per se series in Aquinas's sense (each link in the modern example does have some intrinsic activity), but it illustrates the structural point: simultaneous, hierarchical dependence is different from sequential, temporal causation.

Common objections and replies

"The distinction is artificial / made-up to save the cosmological argument"

Reply: The distinction is grounded in the act-potency metaphysics: a cause that operates only by being actualized by another in the very act of causing is structurally different from a cause that operates under its own power once produced. The first kind requires a contemporaneous source; the second kind does not. The distinction is not an ad hoc save but a consequence of the underlying metaphysics. Critics who reject the distinction typically also reject the metaphysics; the dispute is at the framework level.

"Even simultaneous infinite chains might be coherent"

Reply (Feser, Oderberg): The contemporary defense (Graham Oppy and others) of the coherence of infinite per se chains generally relies on examples that are not actually per se in Aquinas's sense, e.g., the chain of integers with each integer "produced" by its predecessor. These examples involve abstract mathematical structures, not real causal-power transmission. The Thomistic point is specifically about the transmission of intrinsic causal power; in the absence of any source of intrinsic causal power, no causation occurs.

"Modern physics doesn't recognize the distinction"

Reply: Modern physics doesn't deny the distinction either; it operates at a different level of description (quantitative, mathematical, predictive) than the metaphysical level (qualitative, structural, explanatory). The Thomistic claim is that any adequate metaphysics of causation will need to recognize the structural difference between simultaneous-hierarchical and sequential-temporal causal chains.