According to Aquinas but not originally, I'm not exactly sure whose theory it was that Aquinas reworded.

  • By "explain" do you mean restate? Commented Mar 20, 2021 at 10:39
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    The key is to identify contingent truths and necessary truths. Efficient causes are used to explain contingent truths, which can be also described as Principle of Sufficient Reason (PSR). While most theology's proof of existence of god hinges on the "demand" for a necessary (ontological) truth (god) to bear our contingent truth. However, this demand for necessity is not a contingent truth itself which can be scientifically measured and validated in our contingent world as a sufficient reason. Commented Mar 20, 2021 at 20:17

1 Answer 1


From https://philosophy.lander.edu/intro/cause.shtml :

  1. There is an efficient cause for everything; nothing can be the efficient cause of itself.
  2. It is not possible to regress to infinity in efficient causes.
  3. To take away the cause is to take away the effect.
  4. If there be no first cause then there will be no others.
  5. Therefore, a First Cause exists (and this is God).

Premise 1 is unjustified and also inconsistent with there being a First Cause, because a First Cause by definition would not be caused by anything else.

Premise 2 is unjustified.

Premise 3 is the fallacy of denying the antecedent. From Cause -> Effect it is fallacious to conclude ~Cause -> ~Effect. For example, suppose an executioner caused a prisoner's death. From this we cannot conclude that had that executioner not been there, the prisoner would not have died; another executioner could have done the job instead.

Edit: Here's an alternative presentation of it that makes a bit more sense. From https://www3.nd.edu/~jspeaks/courses/mcgill/201/aquinas-cosmological-argument.html

  1. Everything which has come to exist has been caused to come to exist.
  2. Nothing which has come to exist can be the cause of its own existence.
  3. Everything which has come to exist is caused to exist by something other than itself. (follows from 1,2)
  4. It is impossible for a chain of causes of this kind to go on to infinity.
  5. There must be a first cause, which causes other things to come into existence but did not itself come into existence. (follows from 3,4)

The conclusion 5 would almost follow from the (unjustified) premises, if we interpret premise 2 to rule out causal cycles of any size (time flowing in a circle). However, more than one first cause (a thing that did not itself come into existence but causes other things to do so) are also possible from these premises.

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    It looks to me that Premise 1 is stated in order to show that it reduces to absurdity, and that the opposite (first cause must exist) is therefore true. Commented Mar 20, 2021 at 5:31
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    @MarkAndrews it's not that but your comment did cause me to look up other presentations of it. See the edit. Premise 1 should properly not be that "everything" has a cause, but only that everything that "has come to exist" has a cause, allowing the possibility of something that has not "come to exist" to have no cause.
    – causative
    Commented Mar 20, 2021 at 5:45

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