The central proof for the existence of God that Avicenna puts forth is the proof from contingency (imkain). In line with the Neoplatonic tradition, he attempts to prove an ultimate efficient cause for bringing the world into being, rather than a cause for motion in the world, as Aristotle does. Unlike most other proofs, this proof depicts God as a non-voluntary First Cause, which produces the world from pre-eternity by Its essence. Thus, despite its great influence on later Muslim thought, the proof had to be adjusted to conform to more orthodox conceptions of God.
Avicenna claims to advance a purely metaphysical proof (as opposed to a physical proof), one that rests purely on an analysis of the notion of existence qua existence, without consideration of any attributes of the physical world.53 He writes:
Reflect on how our proof for the existence and oneness of the First and His being free from attributes did not require reflection on anything except existence itself and how it did not require any consideration of His creation and acting even though the latter [provide] evidential proof for Him.
This mode, however, is more reliable and noble, that is, where when we consider the state of existence, we find that existence inasmuch as it is existence bears witness to Him, while He thereafter bears witness to all that comes after Him in existence.54
If true, this characterisation would set the proof apart from all contemporaneous, cosmological and teleological proofs. In contemporary terminology, it would qualify it to be an ontological proof, that is to say, a proof which argues for the existence of God entirely from a priori premises and makes no use of any premises that derive from our observation of the world. Recent studies of Avicenna's proof, however, differ on whether the argument is cosmological or indeed ontological.55 As we will see, doubt with regard to the purported fundamental novelty of Avicenna's proof was expressed centuries ago.
The proof rests on conceptions that, Avicenna contends, are primary in the mind, intuited without need of sensory perception and mental cogitation, namely ''the existent'' and ''the necessary''. The conception ''the possible'', being what is neither necessary nor impossible, is either equally primary, or derived directly from the conception ''the necessary''.
An existent, by virtue of itself, is either possibly existent, or necessarily existent. If we posit an existent that is necessary in itself, then, Avicenna argues, it will have to be uncaused, absolutely simple, one and unique. If we posit an existent that is possible in itself, it will have to depend for its existence on another existent. The latter will be its cause, not in the sense of being an antecedent accidental cause for its temporal generation, but as a coexistent essential cause for its continuous existence. If this cause is itself a possible existent, it will have to exist by virtue of another. The series of actual existents, Avicenna argues, cannot continue ad infinitum, but must terminate in an uncaused existent that is necessary in itself.
But why does a possible existent require a cause to exist? Avicenna proves this using the argument from particularisation, apparently borrowed from kalaam. A possible existent can exist or not exist. It will exist only once ''the scale is tipped'' by an external cause such that its existence becomes preponderant over its non-existence. When this occurs, its existence will be ''necessitated'' by its cause.
Now, the proof for the existence of God runs as follows. There is no doubt that there is existence. Every existent, by virtue of itself, is either possible or necessary. If necessary, then this is the existent being sought, namely God. If possible, then it will ultimately require the necessary existent in order to exist. In either case, God must exist.56
Apparently based entirely on an analysis of a priori conceptions and premises, the proof will appear ontological. However, other considerations suggest that the proof is fundamentally cosmological. For instance, the deliberately abstract and unexplained premise, ''There is no doubt that there is existence'', appears to derive from our knowledge that ''there is no doubt that something exists'', or it may even mean the same as the latter statement.57 When the proof then goes on to appeal to the dichotomy of possible existence and necessary existence, it branches into two hypothetical directions: that this indubitable existence is either possible or necessary. But this then begs the following question: if our indubitable knowledge that there actually is existence is examined, will this existence turn out to be possible or necessary? In other words, will this knowledge derive from our awareness (no matter how primitive and abstract) of possible existents or necessary ones? Of course, we cannot be aware of necessary existents; therefore, our indubitable knowledge of existence must relate to our awareness of possible existence. Inevitably, it seems, the proof reasons on the basis of possible existence using the causal premise, which explains the existence of possible existents by reference to a necessary existent. It hence appears to hinge on the existence of things other than God to prove His existence.
Indeed, eight centuries ago, Razi wrote that all proofs for the existence of God depart from facts about the world, except that Avicenna had claimed to have advanced a fundamentally new proof purportedly based on a consideration of existence qua existence, without consideration of things other than God. He quotes Avicenna's above statement to this effect. This claim, however, invites two objections from Razi. First, this proof depends on a causal premise: the proof in fact ''infers the existence of the necessary [existent] from the [actual] existence of the contingent''. Second, even if it proves a necessary existent, one will still need to demonstrate that it is other than the physical things perceptible in this world (this recalls the series of proofs, already referred to, which Avicenna advances for the simplicity, oneness and uniqueness of the necessary existent).58 In other words, the argument presupposes these different considerations about the world: one should prove that the world is not necessarily existent, but contingent, and that a contingent requires a necessary existent to exist, before concluding that God, therefore, exists. A good proof indeed, Raazai would add, but not an ontological one. Nevertheless, even if such criticisms are accepted, Avicenna should nonetheless be credited with the first attempt ever to advance such a proof.59
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