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December 27 2025 17:19 EST

Thomas Aquinas writing Summa Theologica, Watercolor Design Drawing, J&R Lamb Studios Library of Congress

Thomas Aquinas writing
Summa Theologica,
Water Color Design
Drawing,
J&R Lamb Studios,
Library of Congress


Introduction to Philosophy

Part III. Thomas Aquinas,
“The Argument from Necessity”



Abstract: Thomas' argument that since all existent things depend upon other things for their existence, there must exist at least one thing that is a Necessary Being is outlined and explained. Objections to that argument are also briefly discussed.


  1. Thomas' Argument from Necessity begins with the empirical observation that contingent objects in the world come into existence and pass away. Therefore, the argument is an à posteriori argument, and the conclusion cannot claimed to follow with certainty.

    1. The Argument from Necessity:
      1. Since objects in the world come into being and pass away, it is possible for those objects to exist or for those objects not to exist. We know this by experience.
      2. Since objects are countable, the objects in the universe are finite in number.
      3. If, for all existent objects, they do not exist at some time, then, given infinite time, the number of objects would run out and there would be nothing in existence. (And then, of course, nothing can come from nothing — there is no creation ex nihilio) for individual existent objects. Everything comes from something.
      4. But, in fact, many objects exist in the universe.
      5. Therefore, it is necessary that a Necessary Being (i.e., a Being of which it is impossible that it should not exist) exists and is the necessary cause of other things.

    2. The concepts of necessity and contingency are essential concepts in the history of philosophy. Aquinas believed God to be a necessary being and most other things in the world to be contingent (i.e., to be dependent existing things subject to other dependencies). In this argument, however, although Aquinas held that God's essence necessitates existence, this not the kind of necessity meant in this third argument. Here, Aquinas simply means God's existence cannot be that of nonexistence. That is, God's existence is permanent.

      1. One way to view Aquinas' metaphysics in this argument is as follows: Suppose all individual finite things exist on a timeline. We can view these things as existing on a line of infinite length, as time has no end. The infinite length of time far exceeds the coming to be and the passing away of a non-infinite number of finite things, so without the coming to be (by continuous creation), at some time nothing would exist.

      2. Yet, for Aquinas, existing things do not themselves have the power to not exist; only the Necessary being has this control over them.

      3. Interestingly, although not evident in this argument, Aquinas believed other existent things are necessary (i.e. beings whose existence cannot end). Angels, souls, and other kinds of caused things are necessary beings. God,however, is the only necessary thing that is uncaused.

    3. Some of the difficulties with the concepts used in this argument and some of the common objections to Aquinas' Arguments are as follows:

      1. On the one hand, if God exists (is a thing), then by the first premise above, it is possible for God to not exist. On the other hand, if the nature of God is assumed different from other existent things, then the argument presupposes the very point which is claimed to be proved — so the argument seems to be a circular argument.

      2. A common objection from analytic philosophy is that the concept of “necessary” can only be a characteristic of propositions and cannot apply to contingent things. So it does not make sense to say that any specific thing or being is necessary. Things just are. The only meaningful kind of necessity is logical necessity; no kind of thing is necessarily independent of other things or is absolutely permanent.

      3. Does the fact that things constantly come into existence (are generated) imply that God continually creates them? These things cannot generate themselves. Thomas does imply that all existent things are necessarily dependent upon this Necessary Being we call God. Yet it seems metaphysically that this kind of dependence implies something like Russell's Five Minute World Hypothesis:
        Suppose that the universe suddenly came into existence complete with with all the things it now has, complete with the memories everyone now has, complete with the historical records of the past, complete with the geological evidence we now have, and so forth. So the world suddenly appeared with all things already being as they are. Is Aquinas' notion of the relation necessary dependence in this case unconnected with the necessity of those things being time-dependent?
        As Russell says, “This [i.e., his hypothesis] may seem an improbable hypothesis, but it is not logically refutable.” (See Russell's argument in the concluding quotation below.)

      4. If we examine Aquinas' third premise, the assumption that nothing can come from nothing, then how is it possible then that God can create the world ex nihilio?

      5. In the second premise, we also have a problem with a criterion of counting — is a chair one thing or several things? (viz., legs, back, and seat). But is the seat one thing or many things? … and of what are those things made? The second premise implies that existing things (corruptible things) cannot exist forever. But this premise does not prove that because things exist now that a Necessary Being exists. Obviously, the only time that one can contemplate this argument is a time in which things must exist.

      6. Aquinas argues that it is impossible for all things which come into being and pass away always to exist, and therefore it is impossible that all things exist like this (because existing things will end). Thus, there must be some thing that doesn't have the possibility of not being — i.e., there must be a Necessary being. He cannot establish that since individual existing things are corruptible, at the present time they would have all been by now exhausted and would have ceased to exist. The most that could be said is that at some future time it might be possible for all things to cease to exist.

      7. Aquinas' reasoning then seems to commit the fallacy of composition. It's like arguing that since all persons will die at some time, some day the human race must come to an end. Moreover, from the fact that something does not exist eternally, it does not follow with necessity that it is impossible for that thing to do so.

      8. Finally, an objection can be made by raising the question of how Aquinas vacillates between treating nature at times being discrete and at other times being continuous. Indeed, Aquinas was opposed to the atomist point of view and for the most part followed Aristotelian physics. He held that time is continuous but that continuous nature holds the principle of “substantial form” which shapes matter into substances. In this “third way,” Aquinas also does not explain that at that time before these individual contingent things or beings began to exist, there would be a necessity for the existence of a Necessary Being.

    Further Reading
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    “Remembering, which occurs now, cannot possibly … prove that what is remembered occurred at some other time, because the world might have sprung into being five minutes ago, exactly as it then was, full of acts of remembering which were entirely misleading. Opponents of Darwin, such as Edmund Gosse's father, urged a very similar argument against evolution. The world, they said, was created in 4004 B.C., complete with fossils, which were inserted to try our faith. The world was created suddenly, but was made such as it would have been if it had evolved. There is no logical impossibility about this view. And similarly there is no logical impossibility in the view that the world was created five minutes ago, complete with memories and records. This may seem an improbable hypothesis, but it is not logically refutable.”

    Bertrand Russell, An Outline of Philosophy (London: G. Allen & Unwin, 1927), 7.



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