How does Aristotle or a medieval scholastic commentator like St. Thomas Aquinas explain how one cause can impede the action of another cause? Or, conversely, how does the removal of an impeding cause generate an effect?

For example, consider a dam, which impedes a river's flow of water. Once a gate in the dame is opened, an impediment to the flow of water is removed, and the water gushes through. However, there is nothing in the cause (in this example: "opening a gate" or "lack of gate"*) that is in the effect (in this example: "water gushing through").
*This latter description makes it explicit that it's a privation, and privations are non-beings and thus cannot be causes; cf. De Principiis Naturæ's discussion on privation.

I'm assuming a standard description of the relation between cause and effect which St. Thomas Aquinas frequently expresses like this (Super Sent. lib. 4 d. 1 q. 1 a. 4 qc. 4 co.):

omne agens agit sibi simile, ideo effectus agentis oportet quod aliquo modo sit in agente.

Every agent effects a thing like to itself, so the effect of the agent must be in some mode in the agent.

{English translation from p. 219 of The Thomist article "The Notion of Equivocal Causality in St. Thomas" The Thomist 79 (2015):213-63.}

Clearly, the river causes is the efficient cause of the gushing water. Is the lack of a dam an instrumental cause? Are impeding causes instrumental causes?

  • 1
    Hello. Can you elaborate, give context, motivate the question? Commented Jan 11, 2016 at 23:38
  • @RamTobolski I've very much restructured and deepened the question. I hope what I added helps clarify things.
    – Geremia
    Commented Jan 12, 2016 at 1:12
  • Just a thought; In Spinoza's "Ethics" Part Four- On Human Bondage; Axiom [page 3]. "There is no individual thing in nature, than which there is not another more powerful and strong. Whatsoever thing be given, there is something stronger whereby it can be destroyed." CMS
    – user37981
    Commented Jun 9, 2019 at 16:29

4 Answers 4


Taking your example: It is an aspect of the dam/gate that it is actually holding the water back. The dam is damming the water, preventing it from flowing. That is essential part of its being. Impeding itself already has the effect of impedance, it is negating something and by this has a positive effect.

Only because of that, as it is negating something, its removal can have this effect - or rather an effect different from the one that is caused by its being - at all: by ending of the effect of its being.

Thinking of the lack of a dam as causing something without ever having had any effect, i.e. without ever having been there and actually damming the river, seems to exaggerate the speech of cause and effect.


Indeed, "it's a privation, and privations are non-beings and thus cannot be causes". A privation is a principle, not a "per se cause" (causa per se), but a "per accidens cause" (causa per accidens).

On the Principles of Nature:

  1. […] that primarily from which motion begins cannot really be called a cause, even though it may be called a principle. Because of this, privation is placed among the principles and not among the causes, because privation is that from which generation* begins. But it can also he called a per accidens cause in so far as it is coincident with matter

*such as the change from a stable dam that holds back water to a crumbled dam that does not

St. Thomas explains how the removal of an impediment to motion is a per accidens cause in Physics bk. 8 l. 8:

  1. […] Accordingly, it is clear that what moves, i.e., what removes the obstacle preventing and sustaining does in some sense cause motion and in other senses does not; for example, if a pillar supports something heavy and thus keeps it from descending, the one who casts down the pillar is said somehow to move the heavy object that was supported by the pillar. In like manner, one who removes a stopper that was preventing water from flowing out of a container is said in some sense to move the water; for he is said to move per accidens and not per se. Also when a ball rebounds from a wall, it is moved per accidens by the wall but per se by the one who first threw it. For it was not the wall but the thrower that gave it the impetus for motion; but it was per accidens that, being prevented by the wall from continuing according to its impetus, it rebounded into a contrary motion, the original impetus remaining. In like manner, the one who casts down the pillar did not give the heavy object resting upon it the impetus or inclination to be downward, for it had that from the first generator, which gave it the form upon which that inclination follows. Consequently, the generator is the per se mover of the light and the heavy, whereas the remover of obstacles is a per accidens mover.

The ball bouncing off the wall example is interesting because it seems that could be explained by saying the wall stops the ball (annihilates the original mover's impetus) and then immediately gives it all contrary motion such that the initial projector ceases being the per se cause of the ball's motion and the wall becomes its new per cause cause.


As an answer to the header question, I provide the following example:

1 - lack of food causes hunger,

2 - farming provides food,

3 - therefore farming can impede (the lack of food, which causes) hunger.


How..? By pure rhetoric: a double negation removes itself.

Don Quixote sees a peasant but he knows she is a princess made to look plain by an evil magician. Classical descriptions in terms of causes are strongly anthropomorphic and the corresponding 'explanations' are narrative constructions.

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