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# A question on Causality.

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If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

Edited by Nicko0301
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If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

More precisely, the properties of the first ball (mass, shape, etc.) as well as the properties of the second ball and of the environment in which the collision occurs determines the exact way in which the second ball moves (and also the way in which the first ball moves after the collision).

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If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

According to the Aristotelian view of causality, entities act according to their identity.

As Rand put it (way clearer):

"The law of causality is the law of identity applied to action. All actions are caused by entities. The nature of an action is caused and determined by the nature of the entities that act; a thing cannot act in contradiction to its nature.”

So, when the second billiard ball received a hit of force F from another ball, it reacted the way its nature determines that it would react to that specific outside influence (by moving at speed v2). It is the second billiard ball's nature combined with what the first ball did to it that is the direct cause of its subsequent movement, not the first ball itself, or even the movement of the first ball itself. That is where Aristotelian causality differs from the popular, mitaken notion of causality, according to which actions cause other actions. "An entity acts according to its nature, which in turn may cause another entity to act according to its nature (if it is its nature to react in any way at all to the action of the first entity)" is the correct model. (for instance a magnet moved near a penny might cause it to move, because by nature, a penny reacts to magnets being close to it, but not a toothpick: a toothpick does not, by nature, react to the proximity of magnets)

The "actions causing actions" model of causality could not possibly map any of this behavior (magnet or billiard ball example, it doesn't matter). According to it, the first billiard ball's action should hold the key to what subsequent action it causes, no matter what the object that it hits is. In reality, the second action depends on the nature of the object doing the second acting, not just the first action. So, "actions cause other actions" doesn't explain what happens at all.

Edited by Jake_Ellison
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If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

If the first billiard ball didn't cause the motion of the second billiard ball, then why would the motion of the second billiard ball be different if it was struck by an egg yolk?

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It is the second billiard ball's nature combined with what the first ball did to it that is the direct cause of its subsequent movement

Indeed, what the first ball "did to it", in other words, an action. Neither the "nature" of the first ball nor the "nature" of the second ball can generate motion without the specific action of the first ball striking the second ball. If there is no action, nothing will happen. The "nature" of the second ball may determine what exactly happens after it has been struck (in other words, how it moves), but the cause of the movement is the event of the first ball striking the second one.

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but the cause of the movement is the event of the first ball striking the second one.

Well yes, but that wouldn't explain why it happened, only that there is a correlation between the event and the motion. Even a lizard can, I think, safely determine that yes, the event (the impact) caused the motion. But the law of causality implies more than that, it implies that other events would cause the same motion, as long as the natures of the various entities involved (the first ball, the second ball, the table, even the Earth because of its gravitational pull) are the same, and of course the action of the first ball is the same.

Edited by Jake_Ellison
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If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

Jake_Ellison gave the complete answer. I just want to emphasize that the "act of hitting" is not separate from the first ball (or the second, for that matter).

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• 3 weeks later...
If one billiard ball strikes another, what causes the subsequent movement of the struck billiard ball? I know that only entities act, but surely it wasn't the first billiard ball which engendered the motion of the second? Isn't the act of hitting the second billiard ball what causes the hit billiard ball to move?

The billiard balls interacted according to their respective identities, producing a change in the state of the second ball.

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