Monday, March 30, 2015

A Priori Arguments and Thomism

I have flirted with the idea of formulating a Thomistic argument that is purely a priori.  More than one comes to mind, but I am interested in exploring a reductio ad absurdum argument that establishes the existence of a necessary being.

1. If nothing exists, then possibility does not exist. (Premise)

2. If possibility does not exist, then it is not possible for nothing to exist. (Premise)

3. Hence, something necessarily exists. (From 1 and 2)

4. Possibly, nothing contingent exists. (Premise)

5. Therefore, something necessary exists. (From 3 and 4)

6. Whatever is necessary has an essence identical to its existence, e.g. it is pure actuality. (Premise)

7. Therefore, pure actuality exists. (From 5 and 6)

The argument is clearly valid, but is it sound?  If it is, then we have a sound a priori argument for the existence of God as being itself subsisting.


  1. Doug

    Glad to see you are still alive.

    I see several problems with your argument.
    One of those is: If possibility does not exist then there is no thing that is possibly necessary.
    If there is no thing that is possibily necessary then there is no necessary thing.
    Hence pure actuality does not exist.

  2. Alive and kicking!

    The point of this experimental argument, I think, hinges on premise (2). When you say that "if possibility does not exist, then there is no thing that is possibly necessary," it's the conditional that is necessarily false. For, if nothing exists, then it must be possible for nothing to exist. Yet in that case, something exists after all, namely possibility.

    That's how the argument goes, anyway. I'm not sure if it's a good argument or not, but I'm trying to think outside the box.

  3. Also, even if the argument is unsound, it doesn't follow that pure actuality does not exist. There are other arguments that bring us to the same conclusion.

  4. How would you defend (1) considering it seems to make a modal claim at odds with contemporary possible-worlds talk?

    Further, coudl you elaborate on your reasoning for (5) and how you got there from (3) and (4)?

    1. Could you explain how (1) is at odds with contemporary possible worlds talk?

      (5) follows from (3) and (4) given that if something necessarily exists, but nothing contingent exists, something necessary exists.

    2. Hi Doug. Could one argue that even if nothing existed, there would be some possible world(s) in which things do exist (like this one)?
      Unless we are counting possibility as a thing (I'd assume not)?

      And got it; thanks.

    3. Ah, nevermind. Missed this from below:
      "We're not talking about nothing as an existing thing, but about possibility. If possibility exists, then it has being. That's true by definition, and anything with being is a thing."

  5. Doug

    I am not trying to prove that pure actuality doesn't exist, but I don't think your argument is sound.
    "Nothing exists" means that no entities exist. Possibility is not an entity, so to say that nothing exists entails that it is possible that nothing exists.
    Anyway, if you want to show that it is impossible that possibily doesn't exist, I think there is a better way to do it, but it's not really an a priori argument.
    A suggestion:

    1 Something currently exists
    2 It is possible that something exists (from 1)
    3 It is necessary possible that something exists (from 2 and S5)
    and then you can get to the rest of your argument.

    I would, of course, challenge premise 6, but that's another matter.

    1. Why assume that possibility doesn't exist? After all, possibilities are dependent on some actuality.

    2. I didn't say that possibility doesn't exist, I said that possibility isn't an entity. Pössibility may well be dependent on some actuality, but "nothing" isn't an actuality. If it were, it would be the purest actuality anyone could imagine.

    3. Okay, I think I get what you're saying now. My claim isn't that "nothing" is an actuality, but that possibility can only be instantiated by some actuality, whether possibility is a "thing" or not.

    4. And that's why your argument doesn't work. If nothing is actualized, meaning that there isn't anything at all that is actualized, then there is no possibility that is actualized.
      So, translated to your argument, you would get

      1 If nothing exists, then no possibility is instantiated.
      2 If no possibility is actualized, there is no actuality.
      3 If nothing is actualized then nothing exists.

      So, you don't get to a reductio ad absurdum.

    5. A possibility is actualized, namely possibility. In order for nothing to exist, it would have to be possible for nothing to exist. If we don't want to call possibility a "thing," you've still said it exists. Would you like to take that back?

    6. No, I don't want to take back anything I said. And I don't want to treat nothing as if it is an existing thing either.

  6. We're not talking about nothing as an existing thing, but about possibility. If possibility exists, then it has being. That's true by definition, and anything with being is a thing.

  7. If nothing is no existing thing, then it is not something that is actualized. Then there is no 'being', no possibility of anything, the complete and utter absense of any being.
    The question is: is there a logical contradiction in the complete absense of being? If there isn't, that world is possible even though 'possibility' is not present in it. Possibility is a way in which we evaluate worlds, not something that is necssarily present in those worlds.

  8. First of all, everyone, thank you for your comments. I truly appreciate the feedback, whether critical or affirmative. For awhile I'm only going to reply to comments on occasion. You can blame graduate school and having to write my thesis on that. So, see you soon. I'm not going anywhere.