Avicenna, Aquinas, and Leibniz on the argument from contingency

Avicenna,Aquinas, and Leibniz all present versions of what would today be called the argument from contingency for theexistence of a divine necessary being. Their versions are interestingly different, despite Aquinas’s havingbeen deeply influenced by Avicenna and Leibniz’s having been familiar with Aquinas.  I think all three of them are good arguments,though I won’t defend them here.  Idiscussed Avicenna’s argument in anearlier post.  I defendAquinas’s in my book Aquinas ,at pp. 90-99.  I defend Leibniz’s inchapter 5 of my book FiveProofs of the Existence of God . Here I merely want to compare and contrast the arguments.

Because Iwant to focus on what I take to be the main thrust of each of the argumentsrather get bogged down in exegetical details, I will offer my own paraphrasesof the arguments rather than quote directly from any of these thinkers’ texts.

Here are thethree arguments.  Avicenna’s is from the Najāt, and can be paraphrased asfollows:

At least one thing exists.  It has to be either necessary orcontingent.  If it’s necessary, thenthere’s a necessary being, and our conclusion is established.  But suppose it is contingent.  Then it requires a cause.  Suppose that cause is a further contingentthing, and that that further contingent thing has yet another contingent thingas its own cause, and so on to infinity. Then we have a collection of contingent things.  That collection will itself be eithernecessary or contingent.  But it can’t benecessary, since its existence is contingent on the existence of itsmembers.  So, the collection must becontingent, and in that case it too must have a cause.  That cause is either itself a part of thecollection, or it is outside it.  But itcan’t be part of the collection, because if it were, then as cause of the wholecollection, it would be the cause of itself.  And nothing can cause itself.  So, the cause of the collection of contingentthings must be outside the collection. But if it is outside that collection, it must be necessary.  So, there is a necessary being.

Aquinas’sversion is the third of his famous Five Ways in the Summa Theologiae.  It can beparaphrased as follows:

Some things are contingent in nature,as is evident from the fact that they come into existence and go out ofexistence.  Such things can’t existforever, since whatever is contingent, and thus is capable of failing to exist,will in fact at some point fail to exist. So, if everything was contingent, then at some point nothing would haveexisted.  But if there was ever a timewhen nothing existed, then nothing would exist now, since there would in thatcase be nothing around to cause new things to come into existence.  But things do exist now.  So, not everything can be contingent, and theremust be a necessary being.  Now, such athing might derive its necessity from another thing, or it might have itsnecessity of its own nature.  But therecouldn’t be a regress of things deriving their necessity from something elseunless it terminates in something having its necessity of its own nature.  So, there must be something which has itsnecessity of its own nature.

Leibniz’sversion is in the Monadology.  It might be paraphrased as follows:

For anything that exists, there mustbe a sufficient reason for its existence. In the case of the contingent things that make up the universe, thiscannot be found by appealing merely to other contingent things, even if theseries of contingent things being caused by other contingent things extendedbackward into the past without beginning. For in that case, we would still need a sufficient reason why the seriesas a whole exists.  But the series as awhole is no less contingent than the things that make it up.  So, the explanation cannot lie in the seriesitself.  A complete explanation orsufficient reason can be found only if there is a necessary being that is thesource of the world of contingent things. So, there must be such a necessary being.

Each of thesethinkers goes on to argue that, on analysis, it can be shown that the necessarybeing must have the key divine attributes, and therefore is God.  But I want to focus here just on thereasoning each argument gives for the existence of a necessary being.  And again, I won’t be defending the argumentshere, but just comparing them.  So Iwon’t say anything about how the arguments might be fleshed out or thereasoning made tighter, how various objections would be answered, and so on.

What do thearguments have in common?  First, they allrest, of course, on the distinction between contingent beings and necessarybeings, and argue that it cannot be that everything falls into the formerclass.  Second, they all reason causally to the necessary being as thesource of everything other than itself. Third, for that reason, they all have at least a minimal empirical component insofar as theyappeal to the contingent things we know through experience and argue from theirexistence to that of a necessary being.

A fourthsimilarity is that all three thinkers cash out the nature of the necessarybeing’s necessity in terms of the distinction between essence and existence.  Though, as my paraphrases indicate, this is adistinct step that does not and need not be stated in the argumentsthemselves.  Furthermore, our threephilosophers do not do this in quite the same way.  Avicenna, and Aquinas following him, holdthat the cause of things in which essence and existence are distinct must be anecessary being in which they are notdistinct.  Leibniz, however, does not saythat God’s essence is his existence,but that his essence includesexistence.  (This way of putting thingshas influenced much contemporary discussion – and not for the better, because itobscures the implications for divine simplicity that Avicenna's and Aquinas’s wayof speaking makes clear.)

A fifthsimilarity is that none of the three arguments either presupposes or assertsthat the universe had a beginning.  Allof them hold that the existence of a necessary being can be established even ifwe were to suppose that the world of contingent things has always been here.

A sixthsimilarity is that each of the arguments moves from a claim about contingentthings considered individually to a claim about contingent things consideredcollectively, albeit in different ways. For Avicenna, just as an individual contingent thing requires a cause,so too does the totality of contingent things require a cause.  For Aquinas, just as individual contingentthings must fail to exist at some point, so too must the collection ofcontingent things fail to exist at some point, at least if there were nonecessary being.  For Leibniz, just asindividual contingent things require an explanation outside them, so too doesthe collection of contingent things require an explanation outside it.

How do thearguments differ?  First some background.  Scholastic philosophers often distinguish physical from metaphysical arguments for God’s existence.  Physical arguments are those that proceedfrom facts about the concrete physical world as interpreted in light of thephilosophy of nature.  For example, Aquinas’sFirst Way is commonly interpreted as a physical argument because it begins withthe reality of motion, understood along Aristotelian lines.  Metaphysical arguments are those that proceedfrom more abstract considerations that are not tied to the physical world perse.  For example, Aquinas’s proof for God’sexistence in De Ente et Essentiabegins with the fact that there are things whose essence and existence aredistinct and argues that such things require a cause whose essence just issubsistent existence itself.  Since thereis an essence/existence distinction in angels no less than in material things,the argument does not depend on facts about the physical per se. 

Of the threearguments we’re considering here, Aquinas’s has a more physical cast than thoseof Avicenna and Leibniz.  For theobservation that material things come into being and pass away, and the claimthat material things individually and collectively would go out of existence givenenough time, play a big role in the argument, and these are points about thephysical qua physical. 

By contrast,Avicenna’s and Leibniz’s arguments have a more metaphysical cast.  Even if we take them at least to refer tophysical things, what they focus on is the contingencyof these things rather than anything specifically physical.  And angels, which are immaterial, are in asense contingent too, insofar as there is in them an essence/existence distinctionand thus the need for a cause which imparts existence to them.  (To be sure, there is for Aquinas also a sense in which angels are necessarybeings, since once they exist, there is nothing in the created order that candestroy them.  Still, they have to becreated by God, who could also annihilate them if he wills to.  Hence angels have only a derivative necessity rather than a strict necessity.  For that reason, they also have a kind ofcontingency.)

Hence, itseems that one could remove any reference to the physical as such from Avicenna’sand Leibniz’s arguments without altering their basic thrust.  Indeed, one could even remove any referenceto any actual specific contingent things and argue simply that if there are contingent things (whetheror not there really are any), they couldn’t be the only things that exist, for the reasons Avicenna and Leibnizgive.  Aquinas’s Third Way, by contrast,would be a very different sort of argument if the physical claims it makes wereremoved from it.

A seconddifference is that the notion of explanation,and with it the Principle of Sufficient Reason (PSR), play an explicit role inLeibniz’s argument but not in Avicenna’s and Aquinas’s.  That is not to deny that Avicenna and Aquinasare at least implicitly committed to PSR, and that it lurks in the backgroundof their arguments, which are of course offering explanations.  But this is not thematized in theirarguments, the way it is in Leibniz. This reflects Leibniz’s distinctively rationalist approach to metaphysics.

Here’s oneway to understand the difference.  TheScholastics distinguish several “transcendentals,” attributes that apply to allthings of whatever category – being, truth, goodness, andso on.  These are taken to be “convertible,”the same thing looked at from different points of view.  For example, truth is being considered asintelligible, and goodness is being considered as desirable.  (I say a lot more about the transcendentalsin this article.) 

Avicenna’sand Aquinas’s arguments essentially consider reality under the guise of the transcendentalattribute of being.  The being of contingent things, they argue,must derive causally from the being of something that exists in an absolutely necessaryway.  Leibniz’s argument, by contrast,essentially considers reality under the transcendental attribute of truth. The intelligibility of contingentthings, he argues, presupposes a necessary being which is intelligible in itself rather than by reference to something else.

A thirddifference is that the impossibility of an infinite regress of a certain kindplays a role in Aquinas’s Third Way that finds no parallel in Avicenna’s andLeibniz’s arguments.  To be sure, and asI have said, none of the three arguments rules out the possibility of aninfinite temporal regress – a regressof what Aquinas would call “accidentally ordered” causes extending backwardinto the past.  None of them supposes ortries to establish that the world had a beginning.  But Aquinas’s argument does include thepremise that a series of beings that derive their necessity from something elsewould have to terminate in something that has its necessity of its own natureor built into it.  And here he isappealing to the impossibly of an infinite series of causes of what he calls an“essentially ordered” kind, also known as a hierarchicalcausal series.  (I discuss thedifference between these two kinds of causal series in many places, including Aquinas and Five Proofs.)

Any furtherdifferences between the three arguments seem to me to reflect these threefundamental differences.  And thedifferences are important, both because they capture different aspects ofreality, and because they entail that some objections that might seem to haveforce against one version of the argument from contingency will not necessarilyapply to other versions.  (Though, as Ihave indicated, I think each version can successfully be defended againstobjections.)